Directional wetting properties of topographically micro-patterned surfaces

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Surfaces equipped with microscopic structures often display unusual wetting properties. Liquids droplets brought into contact with the microstructures typically exhibit a directional depedence of their mobility. In many cases, one can observe an enhanced mobility of the liquid into certain directions of the structures. Goal of the present project is a comprehensive study of directional wetting phenomena on surfaces that are decorated with a regular pattern of micro-posts with uniform cross-section. First, we will consider the case of partially wetting liquids, i.e. liquids which form droplets on the micropatterned surface that are large compared to the typical dimensions of the structures. The apparent contact angle at which the liquid-air interface just starts to move is determined by the relative orientation of the interface with respect to the micro-pattern. These "advancing" and "receding" contact angles shall be computed with numerical models of the interface in contact to the micro-patterned surface. The typically unsteady advancement of the interface can be described by a spectrum of characteristic interfacial instabilities that occur in the vicintiy of the three-phase contact line between the liquid, air, and the substrate material. In case of highly wetting liquids, one may observe the formation of a liquid film that is wicking the micro-structures. Also in the latter case, a strong directional dependence of the liquid motion is observed which can lead to a facetting of the propagating film edge. Similar to the advancing modes of an inteface for liquid that do not form a film, we will investigate the mechanisms underlying the facetting in numerical models. In the limit of large constrasts of the advancing contact angle into different directions, we propose the possibilty to observe distinct characteristic shapes of growing droplets. These shapes shall be computed for a arbitrary dependence of the advancing contact angle on the orientation of the interface. For lage contrasts, we expect to observe a unidirectional, i.e. filamenteous droplet growth, while for small constrats, the growing liquid will form deformed spherical caps. To reach further conclusions about the directional dependence of the propagation velocity of a liquid film in the microstructures, we will compute the viscous flow resistance of the film for various structures and filling degress. This flow resistance will be further employed to fomulate a coarse grained model for the liquid film dynamics.