Receding Contact Line Motion on Nanopatterned and Micropatterned Polymer Surfaces

Surface properties such as topography and chemistry affect the motion of the three-phase contact line (solid/liquid/air), which in turn affects the contact angle of a liquid moving on a solid surface. In this work, the motion of the receding water contact line was studied on chemically and topographically patterned surfaces obtained from the dewetting of thin polymer films. The patterned surfaces consisted of hydrophilic poly(4-vinylpyridine) (P4VP) bumps, which were either microsized and sparse or nanosized and dense, on top of a hydrophobic polystyrene (PS) background layer. These patterns are designed for atmospheric water capture, for which the easy roll off of water droplets is crucial to their efficient performance. The dynamic receding water contact angle and contact line height of the patterned surfaces were measured by vertically withdrawing the surfaces from a water bath and compared to those of a flat P4VP substrate. For both the micropatterned and nanopatterned surfaces, the height of the dynamic contact lines normalized by the capillary length was characterized by the equilibrium limit that was predicted from static states. The nanopatterned surface had a faster increase in the normalized height as the capillary number increased. The dynamic receding contact angles on all surfaces studied decreased with increasing withdrawing velocity. Surprisingly, even for these patterned surfaces with high hysteresis, the dynamic receding contact angle followed the Cox-Voinov relation at capillary numbers of between 1 × 10^-5 and 5 × 10^-5.