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Wetting on real surfaces

Kristina Davitt Laboratoire de Physique Statistique de l’ENS

Real surfaces are rough, chemically heterogeneous, coated with molecular layers, or even all of the above. It is well known that liquid drops on such surfaces exhibit contact angle hysteresis due to the pinning of the contact line. Contact angle hysteresis is expected to depend on the details of the disorder, for example, the size, shape and density of defects. However, it was over 30 years ago that Joanny and de Gennes developed what is arguably still the best simple model of hysteresis on rough or chemically heterogeneous surfaces [1] and yet only a handful of experiments have attempted to test this model quantitatively on controlled surfaces [2-4]. One can also ask how this same small-scale disorder affects the motion of the contact line5. We use well-controlled systems in order to identify the essential elements that drive contact angle hysteresis and contact-line dynamics on real surfaces. I will talk about what we have learned from recent experiments performed on surfaces ranging from evaporated metallic films or dispersed nanobeads to adsorbed polymer films. Some things are as expected : the hysteresis appears to scale as predicted [1] and we are able to identify a thermally-activated dynamics at low velocities. There are also some surprises, such as the fact that defects appear to act independently up to astonishingly high densities, and the dynamics appear to depend very little on the details of the disorder.

[1] J. F. Joanny and P. G. de Gennes, J. Chem. Phys. 81, 552 (1984)
[2] V. de Jonghe and D. Chatain, Acta Metal. Mater. 43, 1505 (1995)
[3] S. M. M. Ramos, E. Charlaix, A. Benyagoub, M. Toulemonde, Phys. Rev. E 67, 031604 (2003)
[4] M. Delmas, M. Monthioux, T. Ondarçuhu, Phys. Rev. Lett. 106, 136102 (2011)
[5] K. Davitt, M. Pettersen, E. Rolley, Langmuir 29, 6884 (2013)