Superhydrophobic frictions

Timothée Mouterde, Pascal S. Raux, Christophe Clanet, and David Quéré
PNAS April 23, 2019 116 (17) 8220-8223; first published April 5, 2019 https://doi.org/10.1073/pnas.1819979116

  1. Edited by Laurent Limat, CNRS, and accepted by Editorial Board Member John D. Weeks March 5, 2019 (received for review November 26, 2018)

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Significance

The matchless mobility of water on superhydrophobic materials is often considered as the hallmark of water repellency. The friction of drops is indeed found to be unusual: instead of observing classical friction such as due to contact line and liquid viscosity, we show that the main source of resistance to the water motion arises from the air around it. This explains why the drop velocity becomes quasi-independent of its viscosity at low viscosity and more generally why the mobility is so high. We also discuss the case of more viscous liquids whose rolling motion induces a bulk viscous dissipation that caps the mobility.

Abstract

Contrasting with its sluggish behavior on standard solids, water is extremely mobile on superhydrophobic materials, as shown, for instance, by the continuous acceleration of drops on tilted water-repellent leaves. For much longer substrates, however, drops reach a terminal velocity that results from a balance between weight and friction, allowing us to question the nature of this friction. We report that the relationship between force and terminal velocity is nonlinear. This is interpreted by showing that classical sources of friction are minimized, so that the aerodynamical resistance to motion becomes dominant, which eventually explains the matchless mobility of water. Our results are finally extended to viscous liquids, also known to be unusually quick on these materials.

Footnotes

  • ↵1T.M. and P.S.R. contributed equally to this work.

  • ↵2To whom correspondence should be addressed. Email: david.quere{at}espci.fr.

  • Author contributions: T.M., P.S.R., and D.Q. designed research; T.M. and P.S.R. performed the experiments; T.M., P.S.R., C.C., and D.Q. analyzed data; and T.M., P.S.R., C.C., and D.Q. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission. L.L. is a guest editor invited by the Editorial Board.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1819979116/-/DCSupplemental.

Published under the PNAS license.

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Superhydrophobic frictions
Timothée Mouterde, Pascal S. Raux, Christophe Clanet, David Quéré
Proceedings of the National Academy of Sciences Apr 2019, 116 (17) 8220-8223; DOI: 10.1073/pnas.1819979116
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