A Drop Pinned by a Designed Patch on a Tilted Superhydrophobic Surface: Mimicking Desert Beetle

A desert beetle tilts its body forward into the fog-laden wind to collect water by the hydrophilic patches on its superhydrophobic back. In this study, the pinning and dewetting mechanism of a tilted drop pinned by a designed patch on a superhydrophobic surface with negligible contact angle hysteresis (CAH) is explored both experimentally and theoretically. The patch is designed in different shapes including square, rectangle, and triangle. For a square or rectangular patch, the uphill contact angle (CA) of the tilted drop vary with the inclined angle (alpha) of the plate. The drop remains pinned until the critical inclined angle (alpha(c)) is achieved. As alpha = alpha(c), the uphill CA of the drop reduces to the receding angle of the patch. The magnitude of alpha(c) grows approximately linearly with the pinning length (omega(p)), which is related to the patch size. It is found that cop equals the side length (w) of square or rectangular patch perpendicular to the sliding direction. While omega(p) on square patches remains essentially unchanged before sliding, omega(p) on the triangular patch grows with increasing alpha. However, the relation between sin(alpha) and omega(p) for the triangular patch is consistent with that between sin(alpha(c)) and w for square and rectangular patches. Surface evolver simulations based on free energy minimization are performed to reproduce the wetting and dewetting behavior. The simulation outcomes agree quite well with the experimental results.