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Wetting Hysteresis, Metastability, and Droplet Impact on Superhydrophobic Surfaces

[+] Author Affiliations
Kripa K. Varanasi

Massachusetts Institute of Technology, Cambridge, MA

Tao Deng, Ming F. Hsu, Nitin Bhate

GE Global Research Center, Niskayuna, NY

Paper No. InterPACK2009-89350, pp. 623-630; 8 pages
doi:10.1115/InterPACK2009-89350
From:
  • ASME 2009 InterPACK Conference collocated with the ASME 2009 Summer Heat Transfer Conference and the ASME 2009 3rd International Conference on Energy Sustainability
  • ASME 2009 InterPACK Conference, Volume 1
  • San Francisco, California, USA, July 19–23, 2009
  • Conference Sponsors: Electronic and Photonic Packaging Division
  • ISBN: 978-0-7918-4359-8 | eISBN: 978-0-7918-3851-8
  • Copyright © 2009 by ASME

abstract

We study the wetting energetics and wetting hysteresis of sessile and impacting water droplets on superhydrophobic surfaces as a function of surface texture and surface energy. For sessile drops, we find three wetting regimes on these surfaces: equilibrium Cassie at small feature spacing, equilibrium Wenzel at large feature spacing, and an intermediate state at medium feature spacing. We observe minimum wetting hysteresis not on surfaces that exhibit Cassie wetting but rather on surfaces in the intermediate regime. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the surface. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop to the capillary pressure associated with the wetting-energy barrier of the textured surface. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets show very good agreement with this simple pressure-balance model.

Copyright © 2009 by ASME
Topics: Wetting

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