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 (α) of the plate. The drop remains pinned until the critical inclined angle (α_c) is achieved. As α = α_c, the uphill CA of the drop reduces to the receding angle of the patch. The magnitude of α_c grows approximately linearly with the pinning length (ω_p), which is related to the patch size. It is found that ω_p equals the side length (w) of square or rectangular patch perpendicular to the sliding direction. While ω_p on square patches remains essentially unchanged before sliding, ω_p on the triangular patch grows with increasing α. However, the relation between sin(α) and ω_p for the triangular patch is consistent with that between sin(α_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.