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Contact Angle Hysteresis and Dynamics on Real Surfaces
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 a contact angle hysteresis due to the pinning of the contact line. Over 30 years ago Joanny and de Gennes developed what is arguably still the best simple model of hysteresis and yet only a handful of experiments have attempted to test this model quantitatively on controlled surfaces. One can also ask how this same small-scale disorder affects the motion of the contact line. 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 to dispersed nanobeads and adsorbed polymer films. We find that the hysteresis scales as predicted with the shape and density of defects and deduce an effective contact-line spring constant showing that the line is stiffer in the presence of defects. We are able to identify a regime of thermally-activated dynamics at low velocities, but which depends surprisingly little on the details of the disorder, and in the case of an adsorbed polymer film, we find that the dynamics is dominated by the visco-elastic dissipation in the polymer, despite its nanometric thickness.