| Contact angle hysteresis and film formation are generic phenomena observed in wetting of rough surfaces. A large class of real surfaces can be described as random arrangements of topographic defects which act as pinning sites for the interface. To understand the fundamental mechanisms leading to contact angle hysteresis, we chose a regular array of cylindrical posts as a model system and compute the equilibrium shapes of the liquid interface by numerical energy minimizations [1]. Starting from an interface attached to a row of posts, a number of transitions between different interfacial morphologies is observed while the apparent contact angle is quasi-statically increased. As the static advancing contact angle, we take the largest apparent contact angle that permits a stable pinned interface. It is found that the coalescence of liquid menisci ahead of the posts drastically decreases pinning. For high material contact angles a soft deformation modes of the interface occurs leading to a pinch-off from the posts. Small material contact angles favor the formation of a wetting film which is spreading ahead of the front. A complete contact angle hysteresis plot can be constructed from a simple mapping between advancing and receding contact angles which is based on a symmetry argument. [1] Ciro Semprebon, Stephan Herminghaus, and Martin Brinkmann: "Advancing modes on regularly patterned substrates" Soft Matter 8 (2012) 6301 |
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