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ASME Conference Presenter Attendance Policy and Archival Proceedings

2011;():i. doi:10.1115/ICNMM2011-NS1.
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This online compilation of papers from the ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels (ICNMM2011) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in the ASME Digital Library and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Biomedical

2011;():1-10. doi:10.1115/ICNMM2011-58052.

Pulsatile laminar-turbulent transitional flow in a three-dimensional (3D) constricted channel represents a challenging topic and has many important applications in bio-medical engineering. In this research, we numerically investigate the physics of a physiological pulsatile flow confined within a 3D channel with an idealized stenosis formed eccentrically on the top wall using the method of large-eddy simulation (LES). The advanced dynamic nonlinear subgrid-scale stress (SGS) model of Wang and Bergstrom [1] was implemented in the current LES approach to properly resolve the unrealistic SGS dissipation effects and numerical instabilities that are intrinsic to the Smagorinsky type dynamic models (DM). The Reynolds numbers tested in the simulation are 1700 and 2000 , which are characteristic of human blood flows in large arteries. An in-house 3-D LES code has been modified to conduct our unsteady numerical simulations, and the results obtained have been validated using two different grid arrangements and the experimental results of Ahmed and Giddens [2]. The numerical results have been examined in terms of the resolved mean velocity, turbulence kinetic energy, viscous wall shear stress, resolved and subgrid-scale Reynolds stresses, as well as the local kinetic energy fluxes between the filtered and subgrid scales.

Commentary by Dr. Valentin Fuster
2011;():11-16. doi:10.1115/ICNMM2011-58115.

Development of novel particle carrier methods has led to enhanced advances in targeted drug delivery. This paper has aimed the investigation of targeting drugs via attached magnetic particles into human body. This goal was approached by inducing a magnetic field near a specific part of the human body to target the drug or as it is called magnetic drug targeting (MDT). Blood flow and magnetic particles are simulated under the presence of the specified properties of a magnetic field. In order to demonstrate a more realistic simulation, the flow was considered pulsatile. Finally, the results provided show valuable information on magnetic drug targeting in human body.

Topics: Simulation , Drugs
Commentary by Dr. Valentin Fuster
2011;():17-18. doi:10.1115/ICNMM2011-58217.

Magnetic nanofluids can be remotely heated by alternating magnetic field and have significant potential for cancer hyperthermia therapy. The heat generated by magnetic nanoparticles is typically quantified by the specific absorption rate (SAR), which represents the thermal power per unit mass of magnetic material generated in the presence of an alternating magnetic field. During hyperthermia treatment, heat dosage of tumor tissue correlates with slowing tumor growth. The therapeutic ratios of cancer can be increased with the use of biofunctionalized magnetic nanoparticles that have higher SAR for modest amplitudes of magnetic field[1]. Hence, understanding the factors that control the heat generation of magnetic nanoparticle suspensions is important to design fluids with optimized biocompatibility and functionality. In all biomedical applications, the nanoparticles must be coated on the surface to prevent their agglomeration [2], enhance biocompatibility and allow targeting to a specific area. Existing studies have shown that the SAR of nanoparticles may change in the presence of functional coating[3–5]. However, while these studies show that the coating may affect the heat generation rate, there is a limited understanding on the mechanisms that cause that changes of SAR. Hence, it is important to carry out a systematic investigation of nanoparticles similar in size but with different organic coating relevant to biomedical applications to obtain a more complete picture of the mechanisms contributing to changes in SAR. In this work, we present a review of our efforts in this area. Specifically, in our studies we are investigating the correlation between the magnetic and physical properties of commercially available nanoparticles systems and their heat generation rate. The susceptibility and SAR of suspensions of coated and uncoated iron oxide nanoparticles of similar particle size are measured. The coatings selected are highly relevant to biomedical applications and include amine and carboxyl functionalization as well as bioaffine ligands such as protein and biotin. The particle and cluster size was determined from transmission electron microscope (TEM), X-Ray diffraction (XRD) and Dynamic light scattering (DLS). TEM and DLS studies suggested that clusters exist in samples. A summary of all morphological properties together with pH of each suspension is shown in Table.1. The AC magnetic susceptibility of the suspensions was measured as a function of frequency with an in-house made apparatus. Finally SAR was determined by heating the suspension in a commercial induction system and measuring the temperature rise as function of time with a fiber optic sensor. Following these measurements, the SAR values were predicted in two ways: 1) based on measured AC susceptibility and 2) based on particle physical and magnetic properties, starting from Debye model for susceptibility. The normalized predicted and experimental SAR values for all samples are also shown in Table 1. From Table 1, it was found that pH may influence aggregation as described in Ref [6], which indicated that at pH about 2 nanoparticles are highly charged preventing their aggregation while in pH in 6–10 suspensions aggregations are more significant. Normalized SAR of nanoparticle system with aggregations seems to be not related to concentration, different from the well dispersed system[7]. The carboxyl coated sample has smallest diameter and show the lowest SAR, as reported in Ref[8]. The results of suspensions of uncoated iron oxide nanoparticles as well as particles coated with amine groups show that normalized experimental SAR (NSARE ) agrees relatively well with calculated SAR using experimental susceptibility (NSARC_ χE); poor agreement was found when experimental susceptibility was substituted with calculated one (NSART_ χC) using Debye model, which is developed for non-interacting magnetic particles. These results suggest that the coating do not have a direct effect on SAR. On the other hand, agglomeration, which was present in both samples, may lead to dipolar interaction between nanoparticles and enhancement in magnetic properties and SAR. For carboxyl coated sample which has negligible clustering, showed no temperature increase and zero imaginary part of susceptibility. Therefore, good agreement between Debye-model based predictions of SAR and experimental results were obtained in this sample. However, unexpected results were obtained for bioaffine ligands coated sample, where the experimental SAR values are higher than the SAR values determined based on experimental susceptibility. Protein coated sample, which has the larger clusters among the two samples, has a heat generation rate is 6 times higher than the prediction. Meanwhile, the biotin coated sample which has relatively smaller clusters show only a small increase in heat generation rate. A possible explanation for these results is the loss of superparamagnetic character and an opening in hysteresis loop at test frequency for suspensions with large clusters, which may increase the dissipated power above that produced by the relaxation heat losses [9]. Above results show that coating had little effect on SAR. On the other hand, aggregations and clusters may significantly affect SAR, possibly due to dipolar interaction between nanoparticles in suspensions with relatively small clusters or loss of superparmagnetic characters when very large clusters are present.

Commentary by Dr. Valentin Fuster
2011;():19-26. doi:10.1115/ICNMM2011-58235.

The free vibration and instability of fluid-conveying multi-wall carbon nanotubes (MWCNTs) are studied based on an Euler-Bernoulli beam model. A theory based on the transfer matrix method (TMM) is presented. The validity of the theory was confirmed for MWCNTs with different boundary conditions. The effects of the fluid flow velocity were studied on MWCNTs with simply-supported and clamped boundary conditions. Furthermore, the effects of the CNTs’ thickness, radius and length were investigated on resonance frequencies. The CNT was found to posses certain frequency behaviors at different geometries. The effect of the damping corriolis term was studied in the equation of motion. Finally, a useful simplification is introduced in the equation of motion.

Commentary by Dr. Valentin Fuster
2011;():27-33. doi:10.1115/ICNMM2011-58251.

In the present research, the motion of the nano-drug in the vitreous chamber of human eye due to saccadic movements in post-vitrectomy eyes is investigated. The average radius of the vitreous cavity in human eye is equal to 12 mm. This cavity is filled with a liquid in post-vitrectomy eyes. A dynamic mesh technique was performed to model the eye motion. The unsteady 3-D forms of continuity, Navier-Stokes and concentrations of nano-drug equations were solved numerically. The numerical model was validated comparing the results of the flow field with available analytic solutions and experimental data for a sphere as an ideal model of vitreous chamber which a very close agreement was achieved. Then, the numerical simulation was performed to a real model of vitreous cavity filled with BSS (Balanced salt solution). The convection and diffusion of nano-drug in the filling fluids of post-vitrectomy eyes is computed and the results are compared with the diffusion of the nano-drug in the stagnant vitreous. The comparison depicts that the saccade movements of human eye accelerate the drug motion one to two orders of magnitude higher than that due to diffusion in stagnant vitreous chamber.

Topics: Motion , Cavities , Drugs
Commentary by Dr. Valentin Fuster

Chemical Reactions

2011;():35-43. doi:10.1115/ICNMM2011-58084.

Gas phase reaction effects in the catalytic oxidation of hydrogen on platinum-coated minichannels and microchannels are investigated numerically in planar geometry. The main objective of this work is to identify the relative importance of the gas phase and surface reactions under different operating conditions. A collocated finite-volume method is used to solve the governing equations. Detailed gas phase and surface reaction mechanisms along with a multi-component diffusion model are used. As the channel size is reduced, heat and radical losses to the walls can significantly alter the combustion behavior. While catalytic walls help in sustaining the gas phase reactions at very small length scales by reducing the heat losses to the walls owing to heat release associated with the surface reactions, they may inhibit homogeneous reactions by extracting radicals due to typically high absorption rates of such species at the walls. Thus, the radical chain mechanisms can be significantly altered by the presence of wall reactions, and the build-up of radical pools in the gas phase, which lead to homogeneous ignition, can be suppressed as a consequence. In the present study, the effects of two key parameters, i.e. channel height and the inlet mass flux on the interaction of gas phase and surface reactions will be explored. In each case, the limiting values beyond which the gas-phase reactions become relatively negligible compared to surface reactions will be identified for hydrogen/air mixtures.

Commentary by Dr. Valentin Fuster
2011;():45-53. doi:10.1115/ICNMM2011-58203.

The reduction of effective transfer length on microscale eliminates the external diffusion limitation on reaction rate and makes it possible to realize the non-equilibrium chemical reactions. The peculiarities of methane and carbon monoxide steam reforming in a minichannel reactor with activation of reactions on thin film catalyst prepared by nanotechnology are considered in this paper. Consistent accomplishment of these reactions can increase the hydrogen yield and reduce the concentration of carbon monoxide in the product. Steam reforming of methane was studied on Rh/Al2 O3 nanocatalyst deposited on the inner wall of the annular minichannel. Steam reforming of carbon monoxide was studied at Pt/CeO2 /Al2 O3 nanocatalyst deposited on the walls of the minichannel plate. The procedure of catalyst preparation which makes the nanoparticles of two nanometers in size is developed. The catalyst has uniform fraction of nanoparticles and optimal oxygen mobility in the lattice of carrier. During tests the data on the composition of the reacting gas mixture in temperature range from 200 C to 940 C were obtained including data on conversion in controlled temperature field when hydrogen content in the product reaches 68% and carbon monoxide content reduces to 1%. Methane steam reforming and water gas shift reaction in the minichannel were modeled numerically. The detailed information on the temperature and species concentration fields has been obtained, and kinetics of multistage reactions was defined when the external heat is supplied to proceed the steam reforming. The temperature regimes of high conversion of methane and carbon monoxide were defined and discussed in connection with the experimental data.

Commentary by Dr. Valentin Fuster

Boiling and Condensation

2011;():55-62. doi:10.1115/ICNMM2011-58126.

A numerical study of subcooled onset of nucleate boiling (ONB) in a micro-channel under pulsed heating using the volume of fluids (VOF) model was conducted. The ONB time was determined when the void fraction at the microheater surface starts to exist. A smooth thin Pt heater was located between the water in the channel and the solid material. The theoretical superheat for homogeneous nucleation did not predict the transient ONB results of convective flow of water. Once heat load increases at the heater, transient flow boiling starts to occur. From a parametric study, it was found that the time constant increases with large substrate thermal diffusivity, low Reynolds number, and large channel diameter.

Commentary by Dr. Valentin Fuster
2011;():63-67. doi:10.1115/ICNMM2011-58141.

In this work we describe the manufacturing and characterization of multi-scale patterned heterogeneous wettability surfaces. We find drastic enhancements of the pool boiling performance in water. Compared to a hydrophilic SiO2 surface with a wetting angle of 7°, we find that surfaces combining superhydrophilic and superhydrophobic patterns can increase the heat transfer coefficient (HTC) by 300% and can increase the critical heat flux (CHF) by more than 100%.

Commentary by Dr. Valentin Fuster
2011;():69-77. doi:10.1115/ICNMM2011-58168.

Two-phase flow instability in microchannel flow boiling exhibits pressure fluctuations with different frequencies and amplitudes. One possible way to suppress the dynamic instability is to introduce synthetic jet near the inlet of the channel. An important feature of synthetic jet is that it allows momentum transfer into the microchannel flow without net mass injection into it. The strength and frequency of the jet can be controlled by changing the driving voltage and frequency of the piezoelectric driven jet actuator. The effects of the synthetic jet together with upstream throttling are evaluated as a means of stabilizing the flow boiling instability. The results are compared with the case without synthetic jet. The pressure dynamics of the microchannel flow caused by the synthetic jet are also analyzed.

Commentary by Dr. Valentin Fuster
2011;():79-89. doi:10.1115/ICNMM2011-58180.

The pool boiling characteristics of nanofluids is affected by the interaction between the nanoparticles and the heater surface which forms a sorption layer and this layer increases the surface wettability and thereby enhances the CHF. While deteriorated nucleate boiling has been attributed to the decreased activation of cavities due to the increased wettability, it fails to explain the enhanced performance observed by several researchers, which can be explained only by an increase in surface roughness and hence a direct increase in the number of cavities, thereby compensating for the increase in wettability. Attempts to characterize the roughness of heater surfaces have been restricted to magnified visualizations and intrusive probing. No non-intrusive tests have been reported on flat heaters, which are ideal to conduct surface analyses. The present work is aimed at conducting a non-intrusive experimental study to analyse the surface roughness modification due to the sorption layer on flat plate heaters. Experiments have been carried out using electro-stabilized aluminium oxide water based nanofluids of different concentrations with heaters having varying values of surface roughness. The burn-out heat flux was measured and the effect of sedimentation time was studied. The surface-particle interaction parameter (Ra /dp ) was varied to capture the phenomena of plugging as well as splitting of nucleation sites. An experiment having a high value of the interaction parameter shows enhanced boiling performance and that with a value close to 1 shows deteriorated performance. Further it was seen that this behaviour is dependent on the particle concentration. Detailed surface characterization has been done using an optical measurements setup and atomic force microscopy. Boiling on nano-coated heaters has been investigated and presented as an effective solution to counter the disadvantageous transient boiling behavior of nanofluids.

Commentary by Dr. Valentin Fuster
2011;():91-99. doi:10.1115/ICNMM2011-58202.

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this paper we used the flow patterns of gas-liquid flow in rectangular microchannel to explain the regularities of refrigerants flow boiling heat transfer. The characteristics of the flow such as frequency of elongated bubbles, their length, velocity of liquid and gas phases were determined by dual laser flow scanning for the upward and horizontal nitrogen-water flow in microchannels with the size of 1500×720 μm. The flow pattern boundaries were determined also and compared with extended Mishima and Ishii correlation. Flow boiling heat transfer data were obtained for vertical and horizontal microchannel heat sink with similar channels using refrigerants R21 and R134a. The data on local heat transfer coefficients were obtained in the range of mass flow rate from 33 to 190 kg/m2s, reduced pressure from 0.03 to 0.25 and heat flux from 10 to 160 kW/m2 . The flow boiling modes with nucleate and convective boiling were observed as far as heat transfer deterioration at high vapor quality and high heat flux. It was found that deterioration occurs for the annular flow when nucleate boiling was suppressed in a thin liquid film, and for elongated bubble flow pattern. The mechanism of heat transfer deterioration was discussed. The model of heat transfer deterioration was used to predict the experimental data.

Commentary by Dr. Valentin Fuster
2011;():101-107. doi:10.1115/ICNMM2011-58208.

A series of experiments was conducted to observe nucleate boiling phenomena in horizontal tubes with inner diameters varying from 0.05 mm to 3.0 mm. Diverse behaviors of bubble growth were explored, identified by which tubes were classified into micro, mini and macro scales. In micro tubes (Di ≤ 200 μm), the liquid was emitted instantaneously with extremely fast liquid-vapor interfacial movement, referred as explosive emission boiling phenomenon. It is hard to record bubble growth process with high speed camera. In mini tubes (200 μm < Di < 2.5 mm), though liquid was also emitted outsides, the interface moves relative slow and the whole process of bubble growth can be observed. Two distinct stages, referred as spherical and oblate bubble growth stages, were divided. In macro tubes (Di ≥ 2.5 mm), only spherical bubble growth stage exists and the growth rate is much smaller than that in mini tubes. Furthermore, the mechanism of diverse bubble dynamics was analyzed. In mini/micro tubes, decreasing tube diameter can trigger a transition from spherical to oblate bubble growth and consequently establish a thin film between liquid-vapor interface and heated wall. The thin liquid film evaporates vigorously and accelerates the interfacial movement, which reversely enhances evaporation of thin film. A positive interaction between interfacial movement and thin film evaporation establishes, resulting in the interface moving faster and faster and consequently emitted liquid outsides instantaneously. In macro tubes, as tube diameter increasing, the transition and sequential positive interaction can not be raised. Hence, the bubble maintains growing spherically as that in pool boiling.

Commentary by Dr. Valentin Fuster
2011;():109-115. doi:10.1115/ICNMM2011-58234.

Cavitation-boiling coupling phenomenon of R134a flow in micro-channel was experimentally investigated. The cavitation structure, a rectangular micro-orifice 0.2 mm wide and 4mm long, embedded in a micro-channel 0.8 mm wide and 1 mm deep which was engraved on an oxygen-free copper plate covered with quartz glass. Electric film heater was employed under the copper bottom corresponding cavitation structures. Flow patterns were observed by microscope. In the experiment, a phenomenon was observed that hydrodynamic cavitation occurred easier as the mass flow rate increasing. The microchannel wall temperature increased and then decreased along channel with heat power, and flow regimes were different when heat power changed. The cavitation initial position is the corner behind the cavitation structure but not the main flow region.

Commentary by Dr. Valentin Fuster
2011;():117-126. doi:10.1115/ICNMM2011-58242.

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy-based and was sprayed on heat sink channels while the second coating was formed by sintering copper particles on heat sink channels. Mini-channel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and twelve rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for mini-channel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from 6 °C to slightly more than 1 °C. No increase in pressure drop or pumping power due to the microporous coating was measured. The mini-channel heat sinks with porous coating had in average 1.5-times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had in average 2.5-times higher heat transfer coefficient and the critical heat flux was 1.5 to 2-times higher compared with the uncoated heat sinks.

Commentary by Dr. Valentin Fuster

Digital Microfluidics

2011;():127-134. doi:10.1115/ICNMM2011-58055.

Digital microfluidic architectures have been a source of great enthusiasm for on-chip fluid applications requiring precise control and reconfigurability. Droplet-based systems operating with exceedingly small volumes (pL) can make use of digital microfluidic control systems to direct fluid motion using voltages on cascaded electrode structures. The voltage on these electrodes can be adapted via software, thus the generalized templates offered by digital microfluidic systems can be tailored for numerous end-user applications. The work presented here addresses the two major challenges for implementing these digital microfluidics systems for end-user applications: parallel addressability and reduced input voltages. The challenges are overcome through dual-phase AC voltage routing in a 16×16 digital microfluidic multiplexer using low (10 Vrms ) input voltages. The first challenge, related to parallel addressability, comes about because of the generalized template for digital microfluidics, with underlying square-grid electrodes forming a two-dimensional, M×N, plane. Such a structure cannot be readily scaled up for use in single-layered highly-parallel architectures as external address lines cannot be effectively contacted to internal square electrodes lying within a 2-dimensional. With this in mind, the work here introduces multiplexing with a cross-referenced architecture having only M+N input lines. Microdroplets lie between orthogonal overlying row electrodes and underlying column electrodes, and nonlinear threshold-voltage localization is used to initiate motion of the desired microdroplet in the two-dimensional plane. Microdroplet interference (motion of undesired microdroplets) along the activated row and column is avoided, as the applied voltage initiates motion only at the overlapped electrode region (where the voltage is doubled and above-threshold). A dual-phase AC voltage control system is used to address the above bi-layered cross-referenced electrode structure and simultaneously provides a natural solution to the second, reduced voltage, challenge of practical digital microfluidic architectures. Reduced input voltages can be achieved in the digital microfluidic system through an integrated centre-tap AC transformer (a dielectric layer in the digital microfluidic multiplexer limits the current and power consumption, allowing for step-up voltage transformation). The dual-phase outputs from this voltage transformer are 180° out-of-phase, and the AC signals from these outputs are routed to the appropriate row and column electrodes to bring about above-threshold motion. Controlled switching is demonstrated in this work for input voltages below 10 Vrms . Structural and electrical design issues for this dual-phase AC digital microfluidic integrated chip are addressed in this work, and results are presented for an integrated digital microfluidic multiplexer prototype.

Commentary by Dr. Valentin Fuster
2011;():135-138. doi:10.1115/ICNMM2011-58159.

Digital microfluidics depends on efficient movement of individual drops for a variety of tasks, e.g. reagent delivery, mixing, sampling, etc. Superhydrophobic (SH) coatings generally show high repellency and low adhesion for a variety of liquids. Therefore, SH coatings can provide for an efficient drop delivery and hence low energy requirements for a fluidic chip. However, wide application of such coatings is hampered by fragile nature of such coatings to date. A new SH coating is developed that addresses the fragility challenge of such coatings. It is based on application of nanoparticles to fluoropolymers. The mechanical stability, wear resistance and durability under prolonged liquid exposure of this new coating is discussed. It is shown that the new SH coating can maintain high contact angles, low contact angle hysteresis needed for drop mobility under adverse conditions/application of digital microfluidic devices. The developed SH coating can also be sprayed onto various surfaces, including glass used in traditional lab-on-chip (LOC) devices, or even paper for enabling novel Lap-on-paper (LOP) devices.

Topics: Microfluidics
Commentary by Dr. Valentin Fuster
2011;():139-147. doi:10.1115/ICNMM2011-58166.

In this article, microdroplet motion in the electrocapillary-based digital microfluidic systems is modeled accurately, and the combined effects of the biomolecular adsorption and micro-droplet evaporation on the performance of the device are investigated. An electrohydrodynamic approach is used to model the driving and resisting forces, and Fick’s law and Gibbs equation are used to calculate the microdroplet evaporation and adsorption rate. Effects of the adsorption and evaporation rates are then implemented into the microdroplet dynamics by adding new terms into the force balance equation. It is shown that mass loss due to the evaporation tends to increase the protein concentration, and on the other hand, the increased concentration due to the mass loss increases the biomolecular adsorption rate which has a reverse effect on the concentration. The modeling results indicate that evaporation and adsorption play crucial roles in the microdroplet dynamics.

Commentary by Dr. Valentin Fuster
2011;():149-154. doi:10.1115/ICNMM2011-58183.

Design of a closed-loop droplet position control is an essential step towards the development of fully automated digital microfluidic devices. However, the performance of any closed-loop controller is ultimately limited by the accuracy and precision of the feedback sensors. In this paper, an effective capacitance based droplet sensor was designed and optimized through simulation to reduce the droplet position error. A full factorial design was conducted on the droplet sensor simulation model to observe the behavior of the position error as a function of the parameters of a digital microfluidic device. An empirical model was then fitted to the data obtained from the designed simulations and optimized to reduce the position estimate error. Results suggest that the performance of the capacitance based droplet sensor studied in this work is most dependent on the dielectric thickness, droplet radius, electrode pitch, electrode separation, filler fluid permittivity, and plate gap. Isoperformance curves of the sensor performance were obtained using the empirical model to show the interaction between digital microfluidic parameters, as well as to aid in the design of digital microfluidic devices equipped with a similar capacitance based droplet sensor.

Commentary by Dr. Valentin Fuster
2011;():155-160. doi:10.1115/ICNMM2011-58214.

This paper reports the experimental characterization of a liquid droplet driven by surface acoustic wave (SAW). The SAW device was fabricated on a single-sided polished Y-cut 128° rotated lithium niobate (LiNbO3 ) substrate. The kinematics and deformation of the droplet was investigated at different driving voltages and droplet volumes. The kinematics of the droplet is characterized by four regimes: initial stationary state, acceleration and strong deformation, deceleration and steady motion with constant velocity. The maximum velocity of the droplet is proportional to the square of the applied voltage and does not change significantly with its volume. Bellow a critical volume, the steady velocity increases with the applied voltage. Above this volume, the steady velocity decreases with the applied voltage. In general, a larger droplet volume results in a higher steady velocity. The results from the investigation reported here can be used for optimizing the driving scheme of SAW-driven droplet-based microfuidics.

Commentary by Dr. Valentin Fuster
2011;():161-168. doi:10.1115/ICNMM2011-58260.

Electrowetting-on-dielectric (EWOD) is a highly efficient technology to perform biological and medical analyses through the manipulation of pico- to nano-liter droplets on digital microfluidic systems (DMS). Droplet splitting is one of the basic fluidic operations that play a vital role in microscale mixing and concentration control. This paper presents the results of numerical investigation of unequal droplet splitting. In order to gain insight into the mechanism of droplet splitting, a three-electrode splitting system is simulated in FLOW-3D® for given geometry and material properties. When unequal voltages are applied to the adjacent electrodes on both sides of a droplet the distribution of electric field exerts spatially varying stress causing the deformation of the interface. The resulting unequal fluid flow rates towards the activated electrodes are determined by the coupled electro-hydrodynamics. The results of multiple simulation runs in terms of liquid flow rates with different ratios of the applied voltage will be very useful in developing the open-loop model of droplet splitting that can be later adopted to design a controller for unequal splitting in DMS.

Commentary by Dr. Valentin Fuster

Electrokinetic Flows

2011;():169-176. doi:10.1115/ICNMM2011-58011.

Electroosmosis has many applications in fluid delivery at microscale, sample collection, detection, mixing and separation of various biological and chemical species. In biological applications, most fluids are known to be non-Newtonian. Therefore, the study of thermal features of non-Newtonian electroosmotic flow is of great importance for scientific communities. In the present work, the fully developed electroosmotic flow of power-law fluids in a slit microchannel is investigated. The related equations are transformed into non-dimensional forms and necessary changes are made to adapt them for non-Newtonian fluids of power-law model. Results show that depending on different flow parameters like Debye-Hückel or related viscous dissipation and Joule heating parameters, non-Newtonian characteristics of the flow may lead to significant deviations from Newtonian flow behaviors.

Commentary by Dr. Valentin Fuster
2011;():177-182. doi:10.1115/ICNMM2011-58027.

This paper describes a particle-separation device combining AC electroosmosis and dielectrophoresis under pressure-driven flow. The whole device comprises an initial hydrodynamic-focusing compartment with Y junction and an electrohydrodynamic compartment with interdigitated coplanar ITO electrode arrays. In the electrohydrodynamic compartment, the electrode arrays on the bottom of the microchannel are inclined at a 10 degree angle with regard to the direction of channel. A lateral flow is generated by AC electro-osmosis flow triggered by a low-voltage AC electric field on the surface of the electrode. Superimposed upon the axial pressure-driven flow applied by the external syringe pump, AC electro-osmosis flow induces a depressed vortical flow above the electrodes. We find that when homogeneously suspended micro polystyrene particles with different sizes (0.86 μm and 5 μm) in the KCl solution (0.1 mM) are transported through the vortical flow region, the small particles, 0.86 μm, successfully become trapped in the lateral flow above the electrode arrays under the combination of AC electroosmosis and positive DEP, whereas the large particles, 5 μm, completely pass through the vortices. The effectiveness of this separation is investigated for different axial flow rates and amplitudes of the applied voltage. It is shown that with increasing flow rate, it becomes hard for the small particle to get trapped. The possibility of trapping, however, is enhanced by increasing the amplitude of the applied voltage. In addition, we found that the effectiveness of particle separation is frequency dependent, tending to zero at both low and high frequencies. The peak of the effectiveness happens at a so-called characteristic frequency which depends on the conductivity and geometry of the electrodes. We expect that this electrohydrodynamic method can be used to separate the particles with high effectivity for various applications in microsystems.

Commentary by Dr. Valentin Fuster
2011;():183-191. doi:10.1115/ICNMM2011-58041.

Generally speaking, most micro-fluidic mixing systems are limited to the low Reynolds number regime in which diffusion dominates convection, and consequently the mixing process tends to be slow and it takes a relatively long time to have two fluids completely mixed. Therefore, rapid mixing is essential in micro-fluidic systems. In order to hasten the mixing process in micro scale, in this study we come up with a novel scheme for a two dimensional micro-fluidic mixer which encompasses three pairs of electrodes, one pair embedded in the mixing chamber and two pairs located in the micro-channels before and after the mixing chamber. The width of the middle pair is assumed to be twice of the other pairs. In addition, the fluids enter the device via two different entrances within a T-junction. The width of all micro-channels is equal to 50 micrometer and the whole mixer is less than 1 millimeter in length. While Electrical potentials are applied to three electrodes in the outlet and inlet ports in order to conduct the fluids within the mixer, the chaotic electrical fields applied to the mixing chamber are derived by the Duffing-Holmes nonlinear system. We numerically simulate the performance of our micro-mixer by solving Navier-Stokes and continuity equations for fluid velocity field, Poisson-Boltzmann equation for describing the electrical double layer potential distribution, Laplace equation for the externally induced electrical field distribution and concentration transport equation in order to obtain the concentration distribution of two fluids within the geometry. Then, the mixing efficiency is calculated in the outlet cross section of the mixer and the results indicate that a mixing performance efficiency of up to 98% is obtainable by utilizing this proposed scheme.

Commentary by Dr. Valentin Fuster
2011;():193-199. doi:10.1115/ICNMM2011-58064.

Due to the recent advances in microfabrication techniques, it is possible to produce microchannels with positive, negative, or even neutral surface charges. According to several numerical and experimental investigations, such a combination of charge patterns on the microchannel walls results in complex flow fields with circulation zones that are highly desirable for fluid mixing requirements as in lab-on-a-chip devices. In this paper, the mixing efficiency associated with electro-osmotic flows in heterogeneous microchannels is investigated. The Navier-Stokes equations are solved for the flow field along with species transport equations to obtain the concentration field. The effects of the Electric Double Layer (EDL) on the flow field are considered using the Helmholtz-Smoluchowski model in which the EDL effects on the fluid adjacent to the walls are replaced by velocity slip at walls. Different configurations and profiles for the wall charges can be applied to the microchannel walls. In the present study, heterogeneous patterns consisting of different patches with constant zeta-potentials are considered. The flow pattern of a single patch consists of a single vortex attached to the channel wall, which significantly increases the mixing performance. It is expected that a combination of several patches would increase the mixing performance considerably. Therefore, the effects of the size, number, and locations of multiple patches on the mixing performance are investigated in detail. The results for a single patch indicate that the mixing efficiency increases with the size of the patch and its proximity to the microchannel inlet. It is expected that with a suitable combination of patches, an optimized configuration can be found in which the mixing efficiency is maximized and the length of the mixing section is minimized. The results can be applied to the design of micro-mixers to minimize their size while achieving the desired mixing requirements.

Commentary by Dr. Valentin Fuster
2011;():201-209. doi:10.1115/ICNMM2011-58165.

The effect of electrophoresis (i.e., applying uniform electric field to use the natural charge of particles) on the transport of a sample (like biomolecules) in active microreactors is numerically investigated. Navier-Stokes equations are solved along with the equations of electrostatics, species mass transport in the buffer and chemical reaction kinetics at reactive surfaces. Unlike previous studies, in which the effect of the charge of the sample bulk on the electric field has been neglected (i.e., the assumption of electroneutrality), here space charge density is assumed to be nonzero. As a result, the governing equations become fully coupled. The efficiency of the microreactor device is analyzed for two different geometries commonly used in biomolecule separation (i.e., open channel and microcylinders). It is shown that the electroneutrality assumption can drastically influence the final adsorbed concentration depending on the device configuration. Average adsorbed surface concentration is compared for each case as a measure of the performance of the device. The plots depicting the influence of the electric field and nonzero space charge density on the bulk concentration profile and the velocity field are also presented and discussed.

Topics: Electrophoresis
Commentary by Dr. Valentin Fuster
2011;():211-217. doi:10.1115/ICNMM2011-58232.

The present study describes the electrical tomography sensing and dielectrophoresis (DEP) force for visualize the 3D particle mixing in the microchannel system. In the presence of non-uniform electric fields generated by point microelectrodes, the dynamic distribution behaviors of a polystyrene particle and deionized water had been investigated in this system. Microchannel was fabricated with five cross sections where 12 electrodes were installed for each measurement plane. In this experiment, the relationship between electric field frequency and DEP force of particles are calculated at different electric frequencies and diameter of particles. The applied electric field intensities are E = ±1 V/mm, ±3 V/mm and ±5 V/mm while the electric field frequencies are f = 1 kHz, 10 kHz, 100 kHz and 1 MHz and the diameter of particles are 1.3μm, 1.5μm and 2.0μm are investigated in this experiment. Simultaneously, imaged by manipulating tomography sensing at cross section A, C and D and the coupled DEP forces at cross section B and D, the particles flowing had been visualized and concentrate uniformly at near the outlets. The electrical capacitances and DEP forces between the electrode pairs of the microchannel were measured and the ECT tomograms representing the particle distribution were constructed from the measured capacitance data for each cross section in microchannel.

Commentary by Dr. Valentin Fuster
2011;():219-227. doi:10.1115/ICNMM2011-58264.

Effective and versatile microfluidic pumps can be designed by utilizing various electrokinetic effects, such as electrohydrodynamics (EHD), induced-charge electroosmosis (ICEO) and dielectrophoresis (DEP). Among these, traveling-wave EHD (twEHD) has emerged as a powerful pumping mechanism due to its potential for miniaturization and the ability to pump a variety of liquids. However, when twEHD is used to deliver colloidal suspensions, the simultaneous presence of traveling-wave DEP (twDEP) effect may favorably or adversely influence the overall pumping performance, depending on the particle-fluid combination and the frequency range of the applied electric field. In this paper, the coupled EHD and DEP flows were studied numerically in a microchannel with a three-phase interdigitated microelectrode array fabricated at the bottom surface. In the numerical model, the particle-fluid interaction due to twDEP was solved using an equivalent mixture model, and the resultant velocity field was compared to that induced by either the repulsion type or the attraction type of EHD. The results show, depending on the frequency range of the traveling wave electric field and the applied thermal boundary condition, the EHD-induced flow can significantly enhance or weaken the twDEP-induced flow, vice versa.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2011;():229-237. doi:10.1115/ICNMM2011-58273.

This paper reports an improved technique to enhance microfluidic temperature gradient focusing (TGF) of sample solutes using Joule heating effects induced by a combined AC and DC electric field. By introducing the AC field component, additional Joule heating effects are obtained to generate temperature gradient for concentrating sample solutes, while the electroosmotic flow is suppressed under the high frequency AC electric field. Therefore, the required DC voltages for achieving certain sample concentration by Joule heating induced TGF technique are remarkably reduced. Moreover, the lower DC voltages lead to smaller electroosmotic flow (EOF), thereby reducing the backpressure effects due to the finite reservoir size. Concentration enhancements of sample solutes are improved by using a combined AC and DC electric field.

Commentary by Dr. Valentin Fuster

Micro-Flows in Fuel Cells

2011;():239-248. doi:10.1115/ICNMM2011-58026.

The appropriate blend proportional factor value which combines two kinds of staggered grids used in Lattice Boltzmann Method (LBM) for simulating the multiphase flow phenomena with large density ratio in the Polymer electrolyte fuel cell (PEFC) is fixed on. The shape deformation of the water droplet is found when using the two kinds of staggered grids to prevent the pressure oscillation when solving the Poisson equation of this LBM model and the shape of the water droplet varies with the changes of the blend proportional factor values. Two methods are adopted to find out the two staggered grids’ appropriate blend proportional factor value that can diminish or minimize the deformation of the droplet. The first one is to compare the simulation results of different blend proportional factors with the theoretical value and find the one mostly approaches the theoretical value; the second one is to compare the current velocity divergences of the two staggered grids using the results calculated by different blend proportional factor values. A water droplet resting in a tunnel is simulated with different blend proportional factor values and the appropriate value is decided.

Commentary by Dr. Valentin Fuster
2011;():249-255. doi:10.1115/ICNMM2011-58172.

The polymer electrolyte membrane (PEM) fuel cell is a zero emission power generation system that has long been considered as a replacement for conventional fossil fuel combustion systems. However, before constituting a viable market for commercial use, the fuel cell’s efficiency and reliability need to be improved significantly. It has been shown that water management has a significant effect on the power and reliability of the cell as the electrolyte membrane must be well hydrated to allow for ion transfer while excess water blocks the activation sites on the cathode side. The latter effect is known as flooding which occurs at large current densities and compromises the normal operation of the fuel cell. To enhance water management, a prodigious amount of studies have been conducted to optimize the properties and structures of different layers. One of the key results of these studies has been the design of a flow field pattern on the relatively hydrophobic surface of a graphite plate which is believed to provide a better mechanism for removing water droplets from the cathode flow channel. However, the wettability gradient between the catalyst layer (i.e., hydrophilic) and the flow channel (which is currently more hydrophobic) introduces problems as the water droplets formed at the catalyst layer will not likely detach, and hence create a film of liquid that will block the activation sites. If the flow channel is made out of a material that is more hydrophilic than the catalyst layer, water removal and transport will be enhanced as water naturally moves from low surface energy to high surface energy sites. However, recent numerical studies conducted on simulation of water transport in the channels show that removing the water film formed on the hydrophilic channels is limited due to the pressure of the gas flow in the channels. To resolve this problem, the use of compact aluminum foams in the flow channels is studied in this paper. It is shown that the hydrophilicity of the foam-filled flow channel helps the transport of the water droplets at the catalyst layer to the channel in which a liquid film is formed. This film is then removed due to the increased pressure developed in the porous media of the foam (as opposed to the regular open flow channel). The paper includes the experimental results obtained for the fuel cell performance using the new geometry with and without the gas diffusion layers (GDLs). These results will be compared to a similar flow channel that does not include the compressed aluminum porous structure. This work will result in finding the optimum geometry for achieving maximum performance in the flooding regime.

Commentary by Dr. Valentin Fuster
2011;():257-262. doi:10.1115/ICNMM2011-58181.

Porous transport layers are an integral part of polymer electrolyte fuel cells (PEMFC). In order to optimize the catalyst layer performance and reduce catalyst consumption, a thorough understanding of mass transport through porous media is necessary. Currently, there is a lack of experimental measurements of effective mass transport properties of porous transport layers. Further, mass transport theories in the literature, such as the binary friction model by Kerkhof [1], have not been extensively validated for porous media. In the present study, mass transport measurements have been performed on the porous media of a PEMFC, namely a GDL and an MPL. The experimental setup described by Pant et al. [2] has been used. The setup uses the diffusion bridge/counter-diffusion technique for the mass transport measurements. The experimental setup has the advantage that it can be used to perform studies for pure diffusion and convection-diffusion mass transport. The setup also facilitates measurement of permeability of porous media, which can then be used in convection-diffusion studies. Preliminary permeability measurements of GDL and MPL from the setup show good agreement with values available in literature. In preliminary experimentation, the conventional diffusivity correlations like Bruggeman equation have been found to overpredict the diffusivities.

Commentary by Dr. Valentin Fuster
2011;():263-268. doi:10.1115/ICNMM2011-58209.

This paper investigates numerical simulation of one-dimensional homogeneous adiabatic gas-liquid two-phase flow in a rectangular microchannel with one boundary porous wall under the assumption of hydrophobic condition. Gas enters the microchannel with a uniform velocity and liquid is injected through the porous side wall. The present approach is to simulate water injection effects and developing mechanism of two-phase flow. The modeling and solution of the conservation equations provide pressure drop, vapor quality, void fraction and tow-phase mixture velocity for different water injection rates. The results show that velocity and pressure drop significantly perturbed when the water injection rate exceeds a critical value. Comparison between the results of the present work with the previous experimental work shows a good agreement.

Commentary by Dr. Valentin Fuster

Gas Flow

2011;():269-278. doi:10.1115/ICNMM2011-58010.

For gas flows in micro devices, the molecular mean free path is of the same order as the characteristic scale making the Navier-Stokes equation invalid. Recently, some micro gas flows are simulated by the DS-BGK method, which is convergent to the BGK equation and very efficient for low-velocity cases. As the molecular reflection on the boundary is the dominant effect compared to the intermolecular collisions in micro gas flows, the more realistic boundary condition, namely the CLL reflection model, is employed in the DS-BGK simulation and the influence of the accommodation coefficients used in the molecular reflection model on the results are discussed. The simulation results are verified by comparison with those of the DSMC method as criteria.

Commentary by Dr. Valentin Fuster
2011;():279-288. doi:10.1115/ICNMM2011-58022.

Binary gas flows driven by pressure gradient through short microtubes are studied by using an upgraded version of the Direct Simulation Monte Carlo (DSMC) method. Two types of mixtures, He/Xe and Ne/Ar, are examined. Several values of the channel length to radius ratio, the downstream to upstream pressure ratio and a wide range of the gas rarefaction are considered. Results are presented for the species and total flow rates and for the axial distributions of the macroscopic quantities. There is a pronounced difference of the flow behavior of the two mixtures due to the different molecular mass ratios. The flow rate of the He/Xe mixture for very short channels and large pressure drops is increased with increasing gas rarefaction, while the flow rate of the Ne/Ar mixture shows a different rarefaction dependence. The obtained results can be useful in optimal design of microfluidic or vacuum devices.

Commentary by Dr. Valentin Fuster
2011;():289-294. doi:10.1115/ICNMM2011-58031.

Microfilters are commonly used to block undesirable particles in the fluid flows and to control the flow patterns in MEMS. The main purpose of this study is to understand the effect of gas type on density, pressure, Mach number, and velocity distributions of fluid flows through a microfilter. The Knudsen number is the slip flow regime passing through the microfilter. We use direct simulation Monte Carlo (DSMC) method to simulate the flow of nitrogen, helium, oxygen, air and methane passing through a specific microfilter. The geometry of microfilter is unique in all cases. Our results confirm that every gas performs a different performance passing through a specific microfilter, and that the efficiency of a microfilter varies in different gas type environments. For example, among different gases, nitrogen and air have the lowest the pressure drops and that helium has the maximum pressure loss passing through a microfilter.

Commentary by Dr. Valentin Fuster
2011;():295-303. doi:10.1115/ICNMM2011-58036.

Laminar/turbulent flows of compressible fluid in microtubes were simulated numerically to obtain the effect of compressibility on the local pipe friction factors. For gaseous flows, the effect of compressibility had not been clarified except for laminar flow whose Mach number is less than 0.45, so the present work extended this to handle higher speed flows including choked ones and turbulent flows. The numerical procedure based on arbitrary-Lagrangian-Eulerian method solves two-dimensional compressible momentum and energy equations. The Lam-Bremhorst Low-Reynolds number turbulence model was adopted to calculate eddy viscosity coefficient and turbulence energy. The physical domain of simulation with the back region downstream from the outlet of the micro-tube was used to be able to calculate the case of under-expansion flow in the tube. The orthogonal curvilinear grid was used for the computational mesh to obtain accurate results. The computations were performed for a wide range of Reynolds number and Mach number including laminar/turbulent choked flows. It was found that in laminar regimes the ratio of the Darcy friction factor to its conventional (incompressible flow’s) value is a function of Mach number and the same goes for the Fanning friction factor. On the other hand, in turbulent regimes, the ratio is still a function of Mach number for the Darcy friction factor but is equal to about unity for the Fanning friction factor. Namely, the Fanning friction factor of gaseous flow in micro-tubes coincides with Blasius formula, even when Mach number is not small and compressibility effect appears. This fact can be seen in choked flow.

Topics: Friction , Turbulence
Commentary by Dr. Valentin Fuster
2011;():305-317. doi:10.1115/ICNMM2011-58040.

Gaseous flow in circular and noncircular microchannels has been examined and a simple analytical model with second-order slip boundary conditions for normalized Poiseuille number is proposed. The model is applicable to arbitrary length scale. It extends previous studies to the transition regime by employing the second-order slip boundary conditions. The effects of the second-order slip boundary conditions are analyzed. As in slip and transition regimes, no solutions or graphical and tabulated data exist for most geometries, the developed simple model can be used to predict friction factor, mass flow rate, tangential momentum accommodation coefficient, pressure distribution of gaseous flow in noncircular microchannels by the research community for the practical engineering design of microchannels such as rectangular, trapezoidal, double-trapezoidal, triangular, rhombic, hexagonal, octagonal, elliptical, semielliptical, parabolic, circular sector, circular segment, annular sector, rectangular duct with unilateral elliptical or circular end, annular, and even comparatively complex doubly-connected microducts. The developed second-order models are preferable since the difficulty and “investment” is negligible compared with the cost of alternative methods such as molecular simulations or solutions of Boltzmann equation. Navier-Stokes equations with second-order slip models can be used to predict quantities of engineering interest such as Poiseuille number, tangential momentum accommodation coefficient, mass flow rate, pressure distribution, and pressure drop beyond its typically acknowledged limit of application. The appropriate or effective second-order slip coefficients include the contribution of the Knudsen layers in order to capture the complete solution of the Boltzmann equation for the Poiseuille number, mass flow rate, and pressure distribution. It could be reasonable that various researchers proposed different second-order slip coefficients because the values are naturally different in different Knudsen number regimes. The transition regime is a varying mixture of different transport mechanisms and the mixed degree relies on the magnitude of the Knudsen number. It is analytically shown that the Knudsen’s minimum can be predicted with the second-order model and the Knudsen value of the occurrence of Knudsen’s minimum depends on inlet and outlet pressure ratio. The compressibility and rarefaction effects on mass flow rate and the curvature of the pressure distribution by employing first-order and second-order slip flow models are analyzed and compared. The condition of linear pressure distribution is given. This paper demonstrates that with some relatively simple ideas from knowledge, observation, and intuition, one can predict some fairly complex flows.

Commentary by Dr. Valentin Fuster
2011;():319-326. doi:10.1115/ICNMM2011-58091.

An hybrid method, coupling the direct numerical solution of the Bhatnagar-Gross-Krook (BGK) kinetic equation and a Navier-Stokes model is presented. The computational physical domain is decomposed into kinetic and continuum sub-domains using an appropriate criteria based on the local Knudsen number and proper gradients of macro-parameters, computed via a preliminary Navier-Stokes solution throughout the whole physical domain. The coupling is achieved by matching half fluxes at the interface of the kinetic and Navier-Stokes domains, thus taking care of the conservation of momentum, energy and mass through the interface. The proposed method is used for the simulation of the flow through a micro-slit. Outlet to inlet pressure ratio of 0.1, 0.5 and 0.9 are considered, for a wide range of Knudsen number. The local parameters (density, velocity and temperature) along symmetry axis show satisfactory agreement with those computed by the continuum model.

Commentary by Dr. Valentin Fuster
2011;():327-334. doi:10.1115/ICNMM2011-58100.

The effects of rib-patterned surfaces on laminar, transitional to turbulent gas flow in micro-channels were experimentally investigated in the present study. The experiments were performed for two micro-channels having either smooth or rib-patterned surfaces. The micro-channels were etched into silicon wafers and capped with glass substrates. The micro-ribs were patterned on the microchannel surfaces and oriented perpendicular to the flow direction. The pressure was measured at seven locations along the channel length to determine local values of Mach number and friction factor for a wide range of flow regime from laminar to turbulent flow. The friction factors with the hydraulic diameter based on the rib-to-upper-wall height were compared with that for incompressible theory on Moody chart. The values of the product of friction factor and Reynolds number (f·Re) as a function of Mach number were also compared with those of smooth micro-channels and incompressible theory.

Commentary by Dr. Valentin Fuster
2011;():335-340. doi:10.1115/ICNMM2011-58102.

Several researches dealing with the single-phase forced convection heat transfer inside micro channels have been published in the past decades. The performance of liquid flow has been proved that agree with the conventional correlations very well (Yang and Lin [2007]). However, owing to the low heat transfer coefficient of gaseous flow, it is more difficult to eliminate the effects of thermal shunt and heat loss than water flow while measuring its heat transfer performance. This study provides an experimental investigation on forced convective heat transfer performance of air and gaseous carbon dioxide flowing through two microtube with inner diameter of 920 μm. A non-contacted liquid crystal thermography (LCT) temperature measurement method that proposed by Lin and Yang [2007] was used in this study to measure the surface temperature of microtube. The test results show that the conventional heat transfer correlations for laminar and turbulent flow can be well applied for predicting the fully developed heat transfer performance in microtubes while taking account of the compressibility effect of high pressure gaseous flow in micro tubes. There is no significant difference between CO2 and air in both heat transfer and friction.

Commentary by Dr. Valentin Fuster
2011;():341-350. doi:10.1115/ICNMM2011-58108.

We utilized direct simulation Monte Carlo (DSMC) method to investigate the effectiveness of the NSF equations in the slip and transition regimes. Monatomic argon confined in a micro/nano lid-driven cavity is considered in this study. Full NSF equations accompanied by the first and second order velocity slip and temperature jump boundary conditions are used to investigate non-equilibrium phenomena. It is seen that although velocity profiles are predicted quite accurately by means of proper slip boundary conditions, the NSF equations fail to predict correct shear stress distribution and heat flux direction even in the middle slip regime. It is also seen that applying the second order velocity slip boundary condition in the transition regime reduces the accuracy of the continuum approach. Fourier law, which assumes the heat always fluxes from hotter to colder region, loses its validity in the slip regime and beyond.

Commentary by Dr. Valentin Fuster
2011;():351-357. doi:10.1115/ICNMM2011-58109.

In the present study, the effect of wall in supersonic rarefied gas flow past a square cylinder is numerically studied. Therefore, a supersonic rarefied gas flow over a square cylinder is simulated first. Then, the simulations are repeated for a square cylinder confined between two parallel plates. In both cases, the Mach number distribution in the flow field of supersonic rarefied gas over the square cylinder is obtained using the direct simulation Monte Carlo method. Close inspection of contour lines of Mach number over the square cylinder shows that a discontinuity in the flow field occurs across the shock wave at the slip regime while there is no discontinuity at the transition flow regime. In the present paper, the effect of blockage ratio (defined as the distance between two parallel plates divided by the cylinder length) on the Mach number distribution in the flow field of supersonic rarefied gas over the square cylinder is also studied. Meanwhile, the obtained results from both mentioned cases are compared to each other. It is found that the deviation of two sets of data diminishes gradually as the blockage ratio increases.

Commentary by Dr. Valentin Fuster
2011;():359-367. doi:10.1115/ICNMM2011-58111.

The present study is concerned with the flow characteristics of a microchannel supersonic gas flow. The direct simulation Monte Carlo (DSMC) method is employed for predicting the density, velocity and temperature distributions. For gas flows in micro systems, the continuum hypothesis, which underpins the Navier-Stokes equations, may be inappropriate. This is because the mean free path of the gas molecules may be comparable to the characteristic length scale of the device. The Knudsen number, Kn, which is the ratio of the mean free path of the gas molecules to the characteristic length scale of the device, is a convenient measure of the degree of rarefaction of the flow. In this paper, the effect of Knudsen number on supersonic microchannel flow characteristics is studied by varying the incoming flow pressure or the microchannel height. In addition, the microchannel height and the incoming flow pressure are varied simultaneously to investigate their effects on the flow characteristics. Meanwhile, the results show that until the diffuse reflection model is used throughout the microchannel, the temperature and the Mach number in the microchannel entrance may not be equal to free-stream values and therefore a discontinuity appear in the flow field.

Commentary by Dr. Valentin Fuster
2011;():369-375. doi:10.1115/ICNMM2011-58167.

The mass flow rate through microchannels with rectangular cross section is measured for the wide Knudsen number range (0.0025 –26.2 ) in isothermal steady conditions. The experimental technique called ‘Constant Volume Method’ is used for the measurements. This method consists of measuring the small pressure variations in the tanks upstream and downstream of the microchannel. The measurements of the mass flow rate are carried out for three gases (Helium, Nitrogen and Argon). The microchannel internal surfaces are covered with a thin layer of gold with mean roughness Ra = 0.87 nm (RMS). The continuum approach (Navier-Stokes equations) with first order velocity slip boundary condition was used in the slip regime (Knudsen number varies from 0.0025 to 0.1 ) to obtain the experimental velocity slip and accommodation coefficients associated to the Maxwell kinetic boundary condition. In the transitional and near free molecular regimes the linearized kinetic BGK model was used to calculate numerically the mass flow rate. From the comparison of the numerical and measured values of the mass flow rate the accommodation coefficient was also deduced.

Commentary by Dr. Valentin Fuster
2011;():377-388. doi:10.1115/ICNMM2011-58200.

Miniaturized devices are used currently in many engineering applications. Nonetheless, despite much progress in their fabrication, the fundamental understanding of fluid flow and heat transfer on the microscale is still not satisfactory. In this study, rarefaction effects in pressure-driven gas flows in annular microchannels are investigated. The influence of Knudsen number, aspect ratio of the annulus, and surface accommodation coefficient on wall friction, mass flow rate, and thermal energy flow rate is discussed. For this, the linearized Navier–Stokes–Fourier (NSF) and regularized 13-moment (R13) equations are solved analytically. The results are compared to available solutions of the Boltzmann equation to highlight the advantages of the R13 over the NSF equations in describing rarefaction effects in the process. Moreover, in order to improve the accuracy of the NSF system a second-order slip boundary condition is proposed.

Commentary by Dr. Valentin Fuster
2011;():389-395. doi:10.1115/ICNMM2011-58236.

Microscale flow simulation is considered in this paper for a microchannel flow geometry. Higher order Lattice Boltzmann Model was used as the numerical method for flow simulation, in which an effective mean free path was used in relaxation time appeared in LBM. To accurately describe rarefied gas dynamics beyond the Navier-Stokes level, high-order LB models have been used. One reason the standard lattice BGK model fails to capture the nonlinear constitutive behavior is that it only retains velocity terms up to second order in the Hermite expansion of the equilibrium distribution function. This is not sufficient to accurately describe stresses in isothermal flows. To capture nonequilibrium effects, we should retain up to fourth-order terms in the Hermite expansion. The effective mean free path makes it possible to investigate flow characteristics in slip flow regime, for which Knudsen number varies from 0.1 to 10 while does not change the computational efficiency of standard LBM. Results are obtained for pressure-driven and a shear flow configurations in microchannels. The nonlinear flow characteristics of the Knudsen layer were captured in shear flow regime.

Commentary by Dr. Valentin Fuster
2011;():397-400. doi:10.1115/ICNMM2011-58259.

Pressure-sensitive paint (PSP) is an optical measurement technique based on the photo-chemical reaction between oxygen and luminescent molecules, and has potential as a diagnostic tool for pressure measurement in the high Knudsen number regime. However, the application of PSP to micro flow measurement is not straightforward, because conventional PSPs are too thick owing to their polymer binder. In our previous work, we fabricated pressure-sensitive molecular film (PSMF) by using the Langmuir-Blodgett (LB) technique. In this study, we investigated the temperature dependency of Pt(II) Mesoporphyrin IX (PtMP) based PSMF, and found that the temperature dependency of the pressure sensitivity is very small. Moreover, we have applied PSMF to the pressure measurement of micro gas flows through the 170μm width micro channel and the 100μm width micro nozzle, and the pressure distributions were successfully obtained.

Commentary by Dr. Valentin Fuster

General Papers

2011;():401-405. doi:10.1115/ICNMM2011-58053.

This work presents some new methods in optimizing electrical energy, harvested using a micro piezoelectric cantilever. Both mechanical and electrical aspects have been considered. Mechanically, two items have been considered to maximize the generated voltage: geometry of the cantilever and placement of the electrodes. It has been shown that for given sizes of length and width of the harvester and for a given natural frequency, the output voltage can be increased by adjusting the thickness of the beam and the proof mass and consequently increasing the amplitude of vibration. As well, the placement of the electrodes plays a very important role in optimizing output voltage. It has also been shown that piezoelectric cantilevers with shorter top electrodes induce higher voltage than cantilevers with longer top electrodes. Overall results agree with the analytical equations reported in literature so far. Moreover, distribution of top electrodes along the width of the cantilever has been taken into consideration. It has been shown how output voltage can be approximately doubled by using two narrower top electrodes along the width of the cantilever. All analysis in this work was carried out in ANSYS. In this research, to improve the electrical efficiency, diodes have been considered in the circuit to reduce electrical losses in comparison to rectifiers which have been used in conventional harvesters. Applying these methods to particular test cases, a 71% increase in output voltage was observed for the case of geometry optimization, a 116% increase was observed for the case of shortening the top electrode and losses in the electrical circuit were reduced by approximately 50% by using diodes comparing to using rectifiers. While these results focused on cantilever based harvesters, the ideas contained are equally applicable to other structures.

Commentary by Dr. Valentin Fuster
2011;():407-413. doi:10.1115/ICNMM2011-58130.

In the field of synthetic biology, enzymatic pathways are used to enhance the efficiency of chemical production processes. These pathways consist of micro reactors that are filled with porous media and enzymes that catalyze the partial reactions. In the present study we summarize the requirements of the micro reactors used for these applications and give an overview of the different problems that have to be solved. Furthermore we present the idea of a generic demonstrator as a research tool. In the interdisciplinary field of synthetic biology it can enhance the interaction between different groups. It will serve as a common base for numerically as well as for experimentally working groups. We propose a definition of a generic demonstrator for a micro fluidic reactor as it is typically used in this field. Besides, present work of our group is presented, that analyzes the flow field in the micro channels of the reaction zone that is filled with porous media. We show how computational costs can be saved by studying the entry lengths of these channels.

Commentary by Dr. Valentin Fuster

Heat Pipes

2011;():415-420. doi:10.1115/ICNMM2011-58107.

Research on Pulsating Heat Pipes (PHP) has received substantial attention in the recent past, due to its unique operating characteristics and potential applications in many passive heat transport situations. Reliable design tools can only be formulated if the nuances of its operating principles are well understood; at present, this is rather insufficient for framing comprehensive models. In this context, this paper reports experimental data on self-sustained thermally driven oscillations in a 2.0 mm ID capillary tube sub-system, consisting of only one vapor slug and one liquid plug (‘unit-cell’). Understanding such a sub-system/‘unit-cell’ is vital, as it represents a primary unit of a multi-turn PHP. Experiments have been performed with two fluids, i.e. Pentane (BP = 36.1°C) and Methanol (BP = 64.7°C) at different evaporator (40°C to 65°C) and condenser temperatures (−5°C to 15°C) respectively. High speed videography and spectrum analysis reveals that self-sustained thermally driven flow oscillations are observed for both fluids, albeit the dominant periodicity is different. Oscillation frequencies vary from 1.5 Hz to 4.2 Hz approximately, depending on the fluid, operating pressure and temperature. Increasing the difference of temperature between the evaporator and condenser sections leads to enhanced driving force for creating flow oscillations. The resulting phase velocities cause interfacial instabilities, resulting in the formation of secondary bubbles which break-off from the main meniscus. Results of this study can be compared to numerical models and will be useful to understand the physics of multi-turn PHPs.

Commentary by Dr. Valentin Fuster
2011;():421-426. doi:10.1115/ICNMM2011-58218.

The implementation of high power density, multi-core central and graphic processing units (CPUs and GPUs) coupled with higher clock rates of the high-end computing hardware requires enhanced cooling technologies able to attend high heat fluxes while meeting strict design constrains associated with system volume and weight. Miniature loop heat pipe (mLHP) systems emerge as one of the technologies best suited to meet all these demands. This paper investigates experimentally a mLHP system designed for workstation CPUs. The system incorporates a two-phase flow loop with capillary driving force. Since there is a strong demand for miniaturization in commercial applications, emphasize was also placed on physical size during the design stage of the new system. Hence system weight is reduced to around 450g, significantly smaller than that of commercial coolers consisting of copper heat sinks that weight around 782g. Experimental characterization shows that the system can reach a maximum heat transfer rate of 170W with an overall thermal resistance of 0.12 K/W. The heat flux is 18.9 W/cm2 , approximately 30% higher than that of larger size commercial systems. To further miniaturize the evaporator module while maintaining the same heat flux, a new structure for the porous evaporator is proposed, which consist of a porous bi-layer, with nanopores at the top surface. The role of the nanoporous layer is to provide a larger surface area for phase-change, enhancing the evaporation rate.

Commentary by Dr. Valentin Fuster
2011;():427-433. doi:10.1115/ICNMM2011-58220.

This work experimentally studied the evaporation characteristics in groove-wicked flat-plate heat pipes. The parallel, U-shaped grooves have a width of 0.25 mm and a depth of 0.16 mm. Uniform heating was applied to the copper base plate near one end, and a cooling water jacket was connected at the other end. The evaporation resistance was calculated based on the difference of the plate temperature and the vapor temperature respectively under and above the center of the heated zone. Water was used as the working fluid. With stepwise increase of heat load, the behavior of the working fluid in the grooves was visualized, and the evaporation resistances were measured. Above a certain heat load, longitudinal liquid recession can be visualized with a steep-sloped liquid front. Behind the short liquid front is the accommodation region where the meniscus appeared to anchor on the top corners of the groove walls. Under a thermally stable situation, longitudinal oscillations of the liquid front existed in many grooves. Also, the liquid motion in different grooves seemed independent, forming a constantly varying zigzag front line. With increasing heat load, the liquid fronts gradually left the heated zone, accompanied by increasing plate temperatures. The evaporation resistance data appeared larger and more scattered than those associated with mesh or powder wicks in our published experiments, presumably due to the relatively large groove size and surface roughness from etching. No boiling was observed in all present tests. The evaporation resistances for groove wicks increase monotonically in response to the gradually enlarged dryout region with increasing heat load.

Commentary by Dr. Valentin Fuster
2011;():435-440. doi:10.1115/ICNMM2011-58233.

In this work, a four-turn Pulsating Heat Pipe (PHP) is fabricated and tested experimentally. The novelty of the present PHP is the capability of obtaining various thermal performances at a specific heat input by changing the magnetic field. The effects of working fluid (water and ferrofluid), charging ratio (25%, 40%, and 55%), heat input (25, 35, 45, 55, 65, 75, and 85 W), orientation (vertical and horizontal heat mode), and magnetic field on the thermal performance of PHPs are investigated. The results showed that applying the magnetic field on the water based ferrofluid reduced the thermal resistance of PHP by a factor of 40.5% and 38.3% in comparison with the pure water case for the vertical and horizontal mode, respectively. According to the experimental results, an optimum thermal resistance of 0.38 °C/W was achieved at the following conditions: water-based ferrofluid as the working fluid in the presence of magnetic field, vertical mode, charging ratio of 55%, 7% volumetric concentration, and 85 W heat input. This thermal resistance is 11.5 times better than that of the empty PHP.

Commentary by Dr. Valentin Fuster

Heat Transfer

2011;():441-449. doi:10.1115/ICNMM2011-58002.

A novel micro heat sink applying the jet-impingement and cross flow is proposed to dissipate the heat from the electrical devices. Six hotspots of 2 mm × 2 mm are positioned on a flat plate of 25.4 mm × 25.4 mm. The area of flat plate except the hotspots is provided a constant heat flux of 20 W/cm2 as background heating source among cases. Four heat fluxes from 40 to 100 W/cm2 on the hotspots are tested to simulate the different operation conditions. The cross flow is used to remove the background heat flux and jet flow is supplied into the swirl microchannel, located at the right top of hotspot, to dissipate the large heat flux from hotspots. The channel depth is 0.5 mm and the width of swirl microchannel is 0.38 mm. The cross flow and jet flow velocity vary from 0.1 m/s to 0.5 m/s and from 0.5 m/s to 2 m/s, respectively. The effects of cross flow and jet flow on the cooling performance are investigated by numerical simulation. The local heat transfer coefficient and Nusselt number are calculated to evaluate the cooling performance of proposed micro heat sink for the targets of low maximum temperature, temperature gradient and pressure drop. The results show that the maximum temperature of the proposed design occurred at the outlet is approximately 65 °C among tested cases. The corresponding pressure drop is 5.5 kPa. The overall thermal resistance reaches as small as 0.23 K/W.

Commentary by Dr. Valentin Fuster
2011;():451-454. doi:10.1115/ICNMM2011-58016.

The often used argument that heat transfer in micro-sized devices is superior due to the fact that the transfer area scales like L2 but the volume like L3 with L as a characteristic length is critically analyzed for various heat transfer situations. It turns out that for steady state heat transfer cases the thermal boundary layer behavior is more important. In general, dimensional analysis should be applied to understand how the heat transfer performance changes when scales are reduced from macro- to micro-size.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2011;():455-462. doi:10.1115/ICNMM2011-58037.

For practical microchannel applications involving convective heat transfer, the flows are usually, not only laminar, they are also simultaneously developing in nature. Moreover, flat plate substrates with microchannels engraved/ machined or etched on them are emerging as one of the most popular flow geometries. Not analyzing such situations as conjugate heat transfer problems with multi-dimensional effects, often leads to erroneous estimation of heat transfer coefficients. In this context, we report three dimensional numerical simulations of simultaneously developing internal convective flow through a square microchannel (side = 400 μm), treating the substrate thickness, flow Reynolds number and the thermal conductivity of the substrate and the fluid, in the conjugate formulation. Constant heat flux is applied at the bottom of the substrate, away from the fluid-solid interface, as in real-time situations. The parametric study reveals that depending on geometry considerations, flow parameters and thermo-physical properties of fluid-solid combination, conjugate heat transfer effects must be accounted for, to correctly estimate the local Nusselt number.

Commentary by Dr. Valentin Fuster
2011;():463-469. doi:10.1115/ICNMM2011-58038.

High heat fluxes have been created by the semiconductor devices due to the high power generation and shrank size. The large heat flux causes the circuit to exceed its allowable temperature and may experience both working efficiency loss and irreversible damage due to excess in their temperatures. In this paper, a swirl microchannel heat sink is designed to dissipate the large heat flux from the devices. The numerical simulation is carried out to investigate the cooling performance. Uniform heating boundary condition is applied and single phase water is selected as coolant. The present micro heat sink applies multiple swirl microchannels positioned in a circular flat plate to enhance the heat convection by creating the secondary flow at high Reynolds numbers. Copper is selected as the material of heat sink. The channel depth and width are fixed as 0.5 mm and 0.4 mm, respectively. The heat is injected into the system from the bottom of heat sink at the heat fluxes from 10 to 60 W/cm2 . Flow is supplied from the top of micro heat sink through a jet hole with a diameter of 2 mm and enters swirl microchannels at the volume flow rates varying from 47 to 188 ml/min. The cooling performances of swirl microchannel heat sinks with different curvatures and channel numbers are evaluated based on the targets of low maximum temperature, temperature gradient and pressure drop.

Commentary by Dr. Valentin Fuster
2011;():471-476. doi:10.1115/ICNMM2011-58074.

This study attempts to take advantage of porous media with high conductivity and the latent heat capacity of phase change material together to enhance the overall heat transfer rate. A 3-D laminar model of a rectangular porous channel with high thermal conductivity and constant wall heat flux is chosen to see the enhancement of heat transfer when used in conjunction with the phase change material slurry. Numerical simulations for various wall heat fluxes and inlet velocities are carried out. The heat transfer coefficient along the heated surface of the channel is compared when adding micro-encapsulated phase change material to the flow passing through porous channel. There is a significant heat transfer enhancement when using phase change material slurry through porous channel, only under specific conditions of heat fluxes, inlet velocities and the particle concentrations. This study also investigates the effect of various porous media properties e.g. porosity and permeability on the heat transfer coefficient when applied with slurry of phase change material.

Commentary by Dr. Valentin Fuster
2011;():477-481. doi:10.1115/ICNMM2011-58094.

Deformation of the triple-phase contact line in various sizes of rectangular microgrooves under vertical vibration conditions was studied in this paper. Width of the rectangular microgroove ranges from 0.2 mm to 0.4mm and depth of the microgrooves is 0.2∼0.6mm. The frequency of vibration is 10Hz, and the amplitude of vibration is approximately 3.5mm. The research results show that oscillation of the liquid film in microgrooves becomes more obvious, and the triple-phase contact line is deformed more greatly when the groove width or the groove depth increases. The main reason is that the flow resistance of the liquid film in microgrooves decreases when the groove width or the groove depth increases.

Commentary by Dr. Valentin Fuster
2011;():483-488. doi:10.1115/ICNMM2011-58096.

The mathematical model is established in this article to describe the relationship between the wetting length of working liquid in the triangular wetting region of rectangular capillary microgrooves and the geometric dimension, tilt angle, type of working liquid and heat flux when heating the back of microgrooves heat sink. The model supposes that the vapour-liquid interface of meniscus is quadratic parabola but not arc. The predictions from the theoretical analysis are successfully compared with the experimental results.

Topics: Wetting
Commentary by Dr. Valentin Fuster
2011;():489-493. doi:10.1115/ICNMM2011-58098.

With the help of a high-speed camera (30000 Frames/second) and a wide-field stereo-microscope, the effects of mechanical vibration on the meniscus film and triple-phase contact line in rectangular microgrooves were experimentally investigated. Distilled water was used as working liquid. The images of the oscillated meniscus film in an oscillation period were captured through the high speed camera and they were analyzed using a MATLAB program. The results show that as the vibration table moves upward, the length of contact line increases; as the vibration table moves downward, the length of contact-line decreases. During the oscillation, the axial liquid film spreads upward further along the microgrooves and the deformation of the contact line becomes more obvious. The increase of the triple-phase contact line length caused by the external mechanical vibration is helpful for contact line heat transfer enhancement. Besides, deformation curve of the contact line with and without heat input under different vibration conditions is similar, while the contact line with heat input is shorter.

Commentary by Dr. Valentin Fuster
2011;():495-499. doi:10.1115/ICNMM2011-58125.

We consider the two-dimensional problem of steady natural convection in a narrow (micro) horizontal cylindrical annulus filled with porous medium due with linear volumetric heat generation. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of two, and Rayleigh number. The series is extended by means of symbolic calculation up to 28 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is small, but pade approximation leads our results to be good even for much higher value of the similarity parameter.

Commentary by Dr. Valentin Fuster
2011;():501-503. doi:10.1115/ICNMM2011-58132.

A numerical procedure has successfully predicted accurate values of thermodynamic properties in seven cubic equations of state (EOS) in predicting thermodynamic properties of nine ozone-safe refrigerants both in super and sub-critical regions. Refrigerants include R22, R32, R123, R124, R125, R134a, R141b, R143, and R152a and equations of state, considered here, are Ihm-Song-Mason (ISM), Peng-Robinson (PR) [2], Redlich-Kwong (RK), Soave-Redlikh-Kwong (SRK), Modified Redlickh-Kwong (MRK), Nasrifar-Moshfeghian (NM), and TCC were shown in this paper. In general, the results are in favor of the preference of TCC and PR EOS over other remaining EOS’s in predicting gas densities of all aforementioned refrigerants in both super and sub critical regions. Typically, PR and SRK are in good agreement with those obtained from recent correlations and speed of sound measurements. Therefore, these two EOS stand over other EOS both in sub and super critical regions. All EOS follow two-parameter principle of corresponding states at T/Tc higher than 8 and lower than 1 except NM EOS. In the temperature range 1<T/Tc<8, PR and SRK still follow above mentioned principle. The same trend has been observed for other refrigerants.

Commentary by Dr. Valentin Fuster
2011;():505-510. doi:10.1115/ICNMM2011-58133.

From practical point of view, Refrigerants and refrigerant mixtures are widely used as working fluids in many industrial applications, such as refrigerators, heat pumps, and power plants the present work is devoted to evaluating seven cubic equations of state (EOS) in predicting gas and liquid phase volumetric properties of nine ozone-safe refrigerants both in super and sub-critical regions. The evaluations, in sub-critical region, show that TWU and PR EOS are capable of predicting PVT properties of refrigerants R32 within 2%, R22, R134a, R152a and R143a within 1% and R123, R124, R125, and TWU and PR EOS’s, from literature data are 0.5% for R22, R32, R152a, R143a, and R125, 1% for R123, R134a, and R141b, and 2% for R124. Moreover, SRK EOS predicts PVT properties of R22, R125, and R123 to within aforementioned errors. The remaining EOS’s predicts volumetric properties of this class of fluids with higher errors than those above mentioned which are at most 8%.In general, the results are in favor of the preference of TWU and PR EOS over other remaining EOS’s in predicting densities of all mentioned refrigerants in both super and sub critical regions. Typically, this refrigerant is known to offer advantages such as ozone depleting potential equal to zero, Global warming potential equal to 140, and no toxic.

Commentary by Dr. Valentin Fuster
2011;():511-514. doi:10.1115/ICNMM2011-58136.

A unique way for maximizing turbulent free convection from heated vertical plates to cold gases is studied in this paper. The central idea is to examine the attributes that binary gas mixtures having helium as the principal gas and xenon, nitrogen, oxygen, carbon dioxide, methane, tetrafluoromethane and sulfur hexafluoride as secondary gases may bring forward. From fluid physics, it is known that the thermo-physical properties affecting free convection with binary gas mixtures are viscosity ηmix , thermal conductivity λmix , density ρmix , and heat capacity at constant pressure. The quartet ηmix , λmix , ρmix , and Cp,mix is represented by triple-valued functions of the film temperature the pressure P, and the molar gas composition w. The viscosity is obtained from the Kinetic Theory of Gases conjoined with the Chapman-Enskog solution of the Boltzmann Transport Equation. The thermal conductivity is computed from the Kinetic Theory of Gases. The density is determined with a truncated virial equation of state. The heat capacity at constant pressure is calculated from Statistical Thermodynamics merged with the standard mixing rule. Using the similarity variable method, the descriptive Navier-Stokes and energy equations for turbulent Grashof numbers Grx > 109 are transformed into a system of two nonlinear ordinary differential equations, which is solved by the shooting method and the efficient fourth-order Runge-Kutta-Fehlberg algorithm. The numerical temperature fields T(x, y) for the five binary gas mixtures He-Xe, He-N2 , He-O2 , He-CO2 , He-CH4 , He-CF4 and He-SF6 are channeled through the allied mean convection coefficient hmix/B varying with the molar gas composition w in proper w-domain [0, 1]. For the seven binary gas mixtures utilized, the allied mean convection coefficient hmix /B versus the molar gas composition w is graphed in congruous diagrams. At a low film temperature Tf = 300 K, the global maximum allied mean convection coefficient hmix,max /B = 85 is furnished by the He-SF6 gas mixture at an optimal molar gas composition wopt = 0.93. The global maximum allied mean convection coefficient hmix,max /B = 57 is supplied by pure methane gas SF6 (w = 1) at a high film temperature Tf = 1000 K instead of the He-SF6 gas mixture.

Commentary by Dr. Valentin Fuster
2011;():515-521. doi:10.1115/ICNMM2011-58158.

This study presents numerical simulations of forced convection with parachute-shaped encapsulated phase-change material particles in water, flowing through a square cross-section duct with top and bottom iso-flux surfaces. The system is inspired by the gas exchange process in the alveolar capillaries between the red blood cells (RBC) and the lung tissue. The numerical model was developed for the motion of elongated encapsulated phase change particles along a channel in a particulate flow where particle diameters are comparable with the channel height. Results of the heat transfer enhancement for the parachute-shaped particles are compared with the circular particles. Results reveal that the key role in heat transfer enhancement is the snugness movement of the particles and the parachute-shaped geometry yields small changes in heat transfer coefficient when compared to the circular ones. The effects of various parameters including particle diameter and volume-fraction, as well as fluid speed, on the heat transfer coefficient is investigated and reported in this paper.

Commentary by Dr. Valentin Fuster
2011;():523-530. doi:10.1115/ICNMM2011-58175.

There is a growing interest for applications of heat and mass transfer in microchannels. Consequently, several numerical and experimental studies related to transport phenomena in microchannels have been carried-out. The flow problem in microchannels is different from the macro-scale problems due to rarefaction effects, surface roughness, viscous dissipation heating as well as other effects. As a result, a number of studies have been proposed for investigating the micro-flow problem and how each of these phenomena affect heat and mass transfer characteristics. Naturally, there is particular focus on how the observed micro-scale phenomena differ from the traditionally known macro-scale effects. In the realm of simulation studies for heat transfer in micro-sized channels, this paper proposes a comparison between hybrid solution strategies for solving steady heat transfer problems within microchannels. The Generalized Integral Transform Technique (GITT) is employed as the main solution methodology; however, different solution approaches are investigated in order to determine advantages and drawbacks of each alternative. The presented results can serve as guidance for choosing an optimum solution methodology for thermally developing heat transfer in microchannels using GITT implementations.

Commentary by Dr. Valentin Fuster
2011;():531-539. doi:10.1115/ICNMM2011-58187.

Understanding heat transfer through saturated porous media is of great importance to many engineering and geophysical systems such as cooling the electronic devices and solar power collectors, and post-accidental heat removal in nuclear reactors. Large numbers of research studies have been and are conducted on the expanding field of porous media due to the high rate of heat transfer in these systems. Despite the efforts made towards the study of the mechanics of fluid flow through porous media, little is studied the rate of exergy which is the only factor presenting the rate of reusable energy potentially produced by a heat generating body. The objective of this study is to develop a design of experiment to carry out a numerical analysis of heat transfer in a rectangular enclosure filled with a saturated porous medium. The optimum heat transfer rate will be obtained for various configuration-related parameters, namely different inlet to outlet ratios and different inlet width to cavity width ratios. These parameters will be optimized to achieve maximum rate of heat transfer and minimum rate of entropy generation. The results of this study will also help to determine relationships for predicting the heat transfer characteristics of the enclosure.

Commentary by Dr. Valentin Fuster
2011;():541-550. doi:10.1115/ICNMM2011-58190.

This paper deals with the experimental analysis of forced micro-convection features of heated gas flows through commercial stainless steel microtubes having an inner diameter of 172 μm and 500 μm. The experimental results, in terms of Nusselt numbers, are compared to the classical correlations validated for conventional pipes and to the correlations proposed for gas flows through microtubes under laminar and transitional conditions (Re ∊ [400–3500]). The cross sections of the tested microtubes enabled the analysis of the effects of wall axial heat conduction on the Nusselt number which determines a dependence of the convective heat transfer coefficients on the Reynolds number even in the laminar regime, especially for low inner diameters. It is highlighted in the paper that the effects due to overall heat losses, to viscous dissipation and the problems in the right determination of the axial gas bulk temperature distribution cannot be ignored in the thermal analysis of gas flows through microtubes.

Commentary by Dr. Valentin Fuster
2011;():551-555. doi:10.1115/ICNMM2011-58194.

In this paper, the characteristics of bubble dynamic behaviors and the impacts on the triple-phase contact line are studied by a visualization investigation. A high-speed digital camera with maximum speed of 30000 frames per second is adopted to record the period of bubble growth and the geometry of the splashed liquid drops. The information of the bubble dynamic behavior and the liquid drops volume can be analyzed through the software MATLAB. The statistics of the splashed liquid drops is adopted under different heat flux conditions. The experimental results show that the bubble dynamic behaviors lead to the fluctuation of the triple-phase contact line and the splashed liquid drops make the heat transfer capability of the film in microgrooves less than its theoretical maximum value. The investigation indicates that the bubble behaviors can influence the performance of heat transfer through the fluctuations of the triple-phase contact line in the thin liquid film in microgrooves. And the splashed liquid drops appearing in boiling process can also affect the heat transfer of the liquid film in open capillary microgrooves.

Topics: Bubbles , Liquid films
Commentary by Dr. Valentin Fuster
2011;():557-564. doi:10.1115/ICNMM2011-58221.

The presence of current flow in an electric and magnetic field results in electromagnetic force and joule heating. It is desirable to understand the roles of electromagnetic force and joule heating on gas microflow and heat transfer. In this study, a mathematical model is developed of the pressure-driven gas flow through a long isothermally heated horizontal planar microchannel in the presence of an external electric and magnetic field. The solutions for flow and thermal field and characteristics are derived analytically and presented in terms of dimensionless parameters. It is found that an electromagnetic driving force can be produced by a combined non-zero electric field and a negative magnetic field and results in an additional velocity slip and an additional flow drag. Also, a joule heating can be enhanced by an applied positive magnetic field and therefore results in an additional temperature jump and an additional heat transfer.

Commentary by Dr. Valentin Fuster
2011;():565-570. doi:10.1115/ICNMM2011-58223.

In this paper a general heat conduction law has been proposed based on the thermomass theory, which can be derived from the Boltzmann equation for phonons in dielectrics. The Boltzmann equation for phonons gives a balance between the drift and friction parts of the distribution function. When the normal scattering term is omitted in the friction part, the Fourier’s law and Cattaneo-Vernotte thermal wave equation can be obtained by the zeroth order and first order approximations of the drift term, respectively. A second order approximation of the drift part lead to the thermomass theory based general heat conduction law, which is a nonlocal damped heat wave equation and consists of driving, inertial and resistant forces for phonon gas motion. In nanosystems, the normal scattering term of the friction part reflecting the boundary effect is required and induces a Laplacian term in governing equations by solving the phonon Boltzmann equation. A general law containing the viscosity of the thermomass fluid is obtained in analogy with the Brinkman extension in porous hydrodynamics. The general law is then applied to investigate the effective thermal conductivity of a nanosystem, which covers the effect of the ultra-high heat flux and the boundary confinement and scattering. The present research not only presents a powerful heat conduction law for heat transport in nanosystems, but also bridges the microscopic Boltzmann transport equation and the macroscopic gas dynamics of phonons in terms of the thermomass theory.

Topics: Heat conduction
Commentary by Dr. Valentin Fuster
2011;():571-577. doi:10.1115/ICNMM2011-58245.

A numerical study was performed for the laminar forced convection of water over a bank of staggered micro fins with cross section of the elongated hexagon. A 3-dimensional mathematical model, for conjugate heat transfer in both solid and liquid is developed. For this aim the Navier-Stokes and energy equations for the liquid region and the energy equation for the solid region are solved simultaneously and the pressure drop as well as the heat transfer characteristics was investigated. The length and width of the studied heat sinks are one centimeter and different heights in the range of 200–500 micrometer were examined for the fluid media. The heat removal of the finned heat sink is compared with an optimum simple mirochannel heat sink. The comparison which is presented at equal pumping powers depicts the enhancement of the heat removal for some specific sizes of the finned heat sink.

Commentary by Dr. Valentin Fuster

Interfacial Phenomena at Micro and Nanoscale

2011;():579-586. doi:10.1115/ICNMM2011-58032.

We evaluate enhanced mass detection possibilities with electrostatic actuation of a mechanical microoscillator positioned at a fluid-liquid interface. An analytical model is used to simulate the rotational motion of such configuration inside a microchannel either completely filled with two immiscible liquids, or partially filled forming an air-liquid interface. Simulation results show that improvements in sensitivity can be obtained through the use of fluid-liquid interfaces. For point mass detection, sensitivities of at least 9 –15 Hz/ng can be achieved using liquid-liquid interfaces. For air-liquid interfaces, it is conceivable to detect point masses in the order of tens of picograms.

Commentary by Dr. Valentin Fuster
2011;():587-596. doi:10.1115/ICNMM2011-58088.

This paper presents numerical results of disperse liquid droplets forming in the dripping regime at the tip of a microtube into another co-flowing immiscible liquid in a coaxial microtube of larger diameter. Investigated are the effects of the interfacial surface tension, velocities and viscosities of the liquids and the diameters of the coaxial microtubes on the forming dynamics and the size of the droplet. The 2-D, transient Navier-Stockes equations, in conjunction with the momentum jump condition across the interface between the co-flowing liquids are solved using a finite element method. The solution tracks the interface and the growth of the droplet and predicts droplet size and forming frequency. The droplet’s dimensionless radius (rd*) is correlated within ± 10% in terms of the continuous liquid capillary number (Cac ) and ratios of Reynolds numbers (Red /Rec ) and microtube radii (Rc /Rd ) of the co-flowing liquids as:

rd* = 0.225 R*0.466/(Cac0.5)(Red/Rec).0.05

Commentary by Dr. Valentin Fuster
2011;():597-602. doi:10.1115/ICNMM2011-58188.

Freezing of drops on surfaces has many consequences in icing of various systems, e.g. micro-condensers. It is known that when a water drop is placed on a cold surface and the surface temperature is reduced, it will not necessarily freeze when the surface temperature has reached zero degrees Celsius. The delay in freezing of a drop on a cold surface is not well understood; especially the effect that micro- and nano-texture of a surface has this delay. In this study, freezing and melting points of water drops on various micro-textured surfaces, i.e. superhydrophilic, and superhydrophobic have been measured by differential scanning calorimetry (DSC). A comparison of the experimental results with smooth hydrophilic and hydrophobic surfaces allows us to understand the roles of surface chemistry and roughness in freezing of drops in contact with such surfaces. It is found that when the surface chemistry is hydrophobic, roughness will delay the freezing and a drop may not freeze until the surface temperature has been lower than −15 ° C. On the contrary, for hydrophilic surfaces, roughness will shorten the freezing delay and facilitate formation of ice on the surface. This can explain the benefit of the superhydrophobic surfaces (SHS) in preventing ice formation.

Commentary by Dr. Valentin Fuster
2011;():603-608. doi:10.1115/ICNMM2011-58189.

The field of microfluidics is developing with advances in MEMS, biotechnology and μ-TAS technologies. In various devices, interfacial energy is a dominant factor for liquid movement in a microchannel. The surface tension and interfacial tension values are necessary to analyze the liquid behavior in the microchannel. Evaluating the values of interfacial tension is especially important for multiphase flow. A pendant drop method is usually used to measure the interfacial tension, however, this method has some inconveniences. For example, the pendant drop method demands strict accuracy for measuring the droplet size when the droplet has a non-spherical shape. Moreover, it needs an accurate value of the density difference between the two liquids. In this work, a new measurement method named “Liquid-bridging Induced Micro Contact Method” has been developed to overcome the weaknesses of the existing methods. In a previous study, we obtained the interfacial tension from bridging of two liquid droplets on the tip of opposing round metal rods. In this study, we have examined the liquid-bridging of two extruded liquid droplets out of a micro glass tube. By measuring the radii of curvature of each liquid surface and interface, we calculate the Laplace pressure on the surface and interface, and derive the interfacial tension value using the Laplace equation. To prove these two methods are reliable, we have compared the results obtained in this experiment to that of the pendant drop method. As a liquid droplet comes into contact with an opposite liquid droplet the phenomenon is recorded using a CCD camera and high speed camera. The results show that the values of interfacial tension obtained from two methods are approximately the same. Therefore, the liquid-bridging induced micro contact method has been shown to be capable of interfacial tension measurements.

Commentary by Dr. Valentin Fuster
2011;():609-618. doi:10.1115/ICNMM2011-58197.

In this paper, experiments and numerical simulations are performed to study the effects of electric field on the contact angle of a sessile liquid drop resting on a substrate in electrowetting-on-dielectric (EWOD) application. In the experiments for studying the electrowetting, a mercury droplet of 20μL was dispensed manually on a soldermask-coated PCB (Printed Circuit Board) and different values of AC voltage were applied between the droplet and the insulator. High quality images were captured using a CCD camera in all experiments and a program was developed using the MATLAB software for image-processing purposes to obtain the contact angle and other geometrical parameters of the droplet. A numerical model was also used to simulate the drop deformation under an electric field. The continuity and momentum equations along with an equation for tracking the liquid free surface were solved. The free surface advection and reconstruction were performed based on the volume-of-fluid method using Youngs’ algorithm. To evaluate the effect of the electric field on the free surface, the electrostatic potential was first solved for the entire computational domain. Next, the electric field intensity and the surface density of the electric charge were calculated on the free surface after which the electric force could be determined. Calculated droplet shapes agreed well with those of the experiments.

Commentary by Dr. Valentin Fuster
2011;():619-625. doi:10.1115/ICNMM2011-58219.

Evaporation of liquid meniscus formed in microgrooves is associated with very high heat transfer rates, but the cross-section shape of air-liquid interface has a great influence to the heat transfer in microgrooves. But the real cross-section shape of interface in microgrooves is still unknown for us. In this work, the micro-PIV (Particle Image Velocimetry) method is used to test the cross-section shape of air-liquid interface in microgrooves. In the experiment, the camera is focus on different planes from top of the microgrooves to the bottom of the microgrooves. In each plane, we can see the boundary between the air and liquid through small particles added into the liquid. The positions of boundary in each plane for a given cross section are drawn in two-dimension coordinate. Then the cross-section shape of interface in microgrooves can be seen from the fitting curve. The results show that the cross-section shapes of the interface in microgrooves are not round, but polynomial curves. The curvature of interface in microgrooves changes along a single curve. Besides, the polynomial curves also vary along axial direction of the microgrooves. The variations are more obvious in vertical microgrooves than in horizontal microgrooves.

Topics: Shapes
Commentary by Dr. Valentin Fuster
2011;():627-637. doi:10.1115/ICNMM2011-58237.

The shear stress and heat transfer of moving liquid slugs between two parallel plates are studied numerically. The problem is solved initially as a steady state problem for the velocity field and shear stress, and then, the thermal problem is solved. The thermal boundary condition is constant wall temperature. The fluid properties are assumed to be constant. The finite volume method is applied using the ANSYS Fluent software package. The results show good agreement with the published literature. Effects of different interface shapes (contact angles) on wall shear stress and heat transfer is discussed. Dimensionless heat transfer plots are also presented.

Commentary by Dr. Valentin Fuster

Lab-On-Chip

2011;():639-646. doi:10.1115/ICNMM2011-58178.

Recently, EWOD (Electrowetting on dielectric) has attracted a great deal of interest with applications of digital lab-on-a-chip in which microfluids are manipulated in a discrete form of droplets using electrical inputs. In most EWOD applications, the commonly used powering method is wired transmission, which may not be suitable for implantable lab-on-a-chip applications. In this paper, we will investigate wireless power transmission for EWOD utilizing the inductive coupling. Unlike the conventional inductive coupling, wireless EWOD requires a high voltage (> 50 V) at the receiver side which is connected to the EWOD chip since EWOD naturally operates under high input voltages. To satisfy this condition, the resonant inductive coupling method at a high resonant frequency is introduced and investigated. To optimize the transmission efficiency, we study the effects of many parameters such as the frequency, the inductance and the capacitance at the transmitter as well as receiver, the gap between the transmitter coil and receiver coil, and so on, by measuring the voltage at the receiver and the contact angle of droplets placed on wirelessly operated EWOD chip. In addition, by introducing amplitude modulation (AM) to the resonant inductive coupling, wireless AC electrowetting which generates droplet oscillations and is one of the commonly used operational modes is also achieved.

Commentary by Dr. Valentin Fuster

Single-Phase Liquid Flow

2011;():647-655. doi:10.1115/ICNMM2011-58140.

In this study, the effect of two important parameters have been evaluated for pressure driven liquid flows in microchannel in laminar regime by analytical modeling, followed by experimental measurement. These parameters are wettability conditions of microchannel surfaces and aspect ratio of rectangular microchannels. For small values of aspect ratio, the channel was considered to have a rectangular cross-section, instead of being two parallel plates. Novel expressions for these kinds of channels were derived using eigenfunction expansion method. The obtained two-dimensional solutions based on dual finite series were then extended to the case of a constant slip velocity at the bottom wall. In addition, for large values of aspect ratio, a general equation was obtained which is capable of accounting for different values of slip lengths for both upper and lower channel walls. Firstly, it was found out that for low aspect ratio microchannels, the results obtained by analytical rectangular 2-D model agree well with the experimental measurements as compared to one dimensional solution. For high aspect ratio microchannels, both models predict the same trend. This finding indicates that using the conventional 1-D solution may not be accurate for the channels where the width is of the same order as the height. Secondly, experimental results showed that up to 2.5% and 16% drag reduction can be achieved for 1000 and 250 micron channel height, respectively. It can be concluded that increasing the surface wettability can reduce the pressure drop in laminar regime and the effect is more pronounced by decreasing the channel height.

Commentary by Dr. Valentin Fuster
2011;():657-665. doi:10.1115/ICNMM2011-58144.

Heat transfer in a spiral heat sink is examined experimentally and analytically. The spiral channel was fabricated on a base plate of copper. The cross section of the channel is square with 1 mm sides. A copper cap plate was bolted tight to seal the channel. Water and four low viscosity silicone oils (0.65 cSt, 1 cSt, 3 cSt and 10 cSt) were used as a medium; thus a Prandtl number from 5 to 100 was examined. Tests considered fluid entering from the side of the heat sink and exiting from the middle of heat sink and entering from the side and exiting from the middle. Heat transfer behavior over a wide range of flow rates from laminar to turbulent has been examined. Enhancement due to the spiral geometry was observed, and no significant difference was reported between the side and middle inlet condition. The dimensionless mean wall flux and the dimensionless thermal flow length were used to analyse the experimental data instead of Nusselt number and channel length. The spiral channel has been discretized, so that a single Dean number can be assumed in each cell, and two current models were applied to obtain the average Nusselt number. These are used to obtain the dimensionless mean wall flux and comparisons made with the experimental points.

Commentary by Dr. Valentin Fuster
2011;():667-677. doi:10.1115/ICNMM2011-58148.

Xurography is an inexpensive rapid prototyping technology for the development of microfluidic systems. Imprecision in the xurographic tape cutting process can result in undesired changes in channel dimensions near features that require a change in cutting direction, such as 90° miter bends. An experimental study of water flow in rectangular xurographic microchannels incorporating 90° miter bends with different channel widths in each leg is reported. A set of twelve microchannels, with channel depth approximately 105 micrometers and aspect ratio ranging from 0.071 to 0.435, were fabricated from double-sided adhesive Kapton® polyimide tape and two rectangular glass plates. The channels were reinforced with a mechanical clamping system, enabling high Reynolds number, Re, flows (up to Re = 3200) where Re was based upon hydraulic diameter and average velocity. Reported data include friction factor and critical Reynolds number for straight microchannels and loss coefficients for flow through 90° miter bends that contain either a contraction or expansion with cross-sectional area ratios of 0.5, 0.333 and 0.2. The critical Reynolds number, Recr , ranged from 1750 to 2300 and was found to be dependent on channel defects such as sidewall roughness, adhesive droplets, and corner imperfections. Loss coefficients through 90° miter bends with expansion decrease rapidly for Re < Recr . At the transition, the loss coefficient suddenly drops and approaches an asymptotic value for Re > Recr . For 90° miter bends with contractions, loss coefficients gradually decrease with increasing Re for 150 < Re < 1400. In addition, the loss coefficient decreases with decreasing area ratio through the contraction or expansion. The minor loss coefficient data were found to be dependent on Reynolds numbers and area ratio of contraction/expansion at the bend. The results suggest that the effect of the contraction/expansion was the dominant mechanism for minor losses in the 90° miter bend.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2011;():679-684. doi:10.1115/ICNMM2011-58248.

This paper presents a numerical study for the single phase heat transfer of water in the heat sinks with different types of the grooved microchannels. The cross section of the grooves is either rectangular or arced shape. The grooves are embedded vertically in the side walls of the microchannel but for the floor, different orientation angles of the grooves in the range of 0–60° are investigated. As well, for the grooves on the floor of the channel, the chevron-shape is another pattern which has bee studied. A 3-D computational model is developed for each of the studied cases and the conjugate heat transfer in both solid and liquid is investigated. The governing equations are solved numerically to determine the pressure drop and heat transfer through the heat sink. The results of the heat removal and coefficient of performance (COP) for different types of the grooved microchannel heat sinks are compared to each other as well with those for a simple microchannel heat sink with minimum fin thickness. The comparison shows that the case with minimum vertical fin thickness and arc grooves aligned in 60° on the floor has the maximum heat removal and COP among the studied cases.

Commentary by Dr. Valentin Fuster

Mass Transfer

2011;():685-695. doi:10.1115/ICNMM2011-58155.

The aim of this work is to investigate numerically the mass transfer characteristics in a Taylor flow microchannel reactor. Previous attempts to model gas-liquid mass transfer in microchannels have mainly been done by the unit cell based models. Limitations of this approach are its incapability to account for the mass transfer in the inlet mixing region and the dependence on empirical data to define the unit cell geometry. The present work attempts to overcome both these shortcomings by adopting a purely numerical approach to model the mass transfer in a Taylor flow microreactor. A finite-element implementation of the phase field method was used to predict the hydrodynamics of the two-phase flow The flow pattern obtained was used to define the computational domain to model the mass transfer. The reaction system of CO2 absorption into aqueous NaOH solution was considered for gas superficial velocities ranging from 0.09 to 0.25 m/s with the liquid phase superficial velocities ranging from 0.02 to 0.21 m/s. Channels with hydraulic diameters ranging from 100 μm to 500 μm were considered with flow focusing and cross flow types of inlet configuration. The effect of channel length was also studied by varying the residence time in the transient simulation. Results suggest that the conventional unit cell based approaches which do not model the inlet mixing region could over predict the mass transfer by up to 16%. Smaller diameter channels were found to have improved mass transfer characteristics. This was found to be further enhanced by higher concentration levels of the liquid reactant and higher temperatures. The channel wall wettability was found to negligibly affect the mass transfer characteristics. The predictions from the present model were compared with experimental data as well as with predictions of the unit cell based model and a good agreement was obtained with both models.

Commentary by Dr. Valentin Fuster
2011;():697-704. doi:10.1115/ICNMM2011-58157.

Multiphase flow is often found in chemical engineering, food processing, or analytics. First contacting and droplet generation as well as coalescence and re-dispersion have high importance for the flow characteristics. In all processes, the channel geometry, fluid properties, and flow velocity determine the flow regime, droplet size, and interfacial area. The hydrolysis of alkyl acetates in organic phase with sodium hydroxide NaOH in the aqueous phase is investigated as flexible test reaction for mass transfer and interfacial area. For right design of the characteristic time for mass transfer, the alkyl group is chosen from ethyl, isopropyl or n-butyl, which differ in water solubility, diffusivity and rate constant. The consumption of NaOH is used for calculation of specific area and related mass transfer coefficient. Different channel geometries are characterized and design considerations are conducted.

Commentary by Dr. Valentin Fuster

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