NUMERICAL SIMULATION OF DROP SPREADING OVER A PILLARED SURFACE

Abstract

Understanding drop-level interactions with micron-size pillars over flat textured surfaces is required in applications such as condensation of water vapor from a humid environment. Accordingly, the spreading of water drops with diameters of ~ 45 μm over micro-pillars has been studied. The studied cylindrical pillars had a diameter of 3.2 μm, whereas the height and pitch were varied from 15 to 20 μm and 6 to 9 μm, respectively. The impact velocity was varied from 0.02 to 1.89 m/s. The stability of the equilibrium and the transitions in the Cassie-Wenzel wetting states were examined. Three-dimensional simulations showed that drops rebound in closely spaced pillars. In contrast, for a relatively large pitch, drops may rebound and partially or entirely wet the pillars. These details depended on the impact velocity and pillar height. The structure and mechanism of moving contact lines over a pillared surface during impact was also examined. In the simulations, the spreading details were correctly reproduced when a time-dependent contact angle model was adopted, which took into account the nonlinear contribution of friction as well as hysteresis owing to finite pinning. The presence of pinning sites at the edges of the pillars was found to be a major factor affecting the possibility of rebounding and the resulting spreading rate. The simulations of drop shapes using this approach matched the experimental results reported in the literature.