Wind- and Gravity-Forced Drop Depinning

Abstract: Liquid drops adhere to solid surfaces due to surface tension but can depin and run back along the surface due to wind or gravity forcing. This work develops a simple mechanistic model for depinning by combined gravity and high-Reynolds-number wind forcing and tests that model using water drops on a roughened aluminum surface. On non-inclined surfaces, drops depin at a constant critical Weber number, $W!e_{\mathrm{crit}}=7.9$, for the present wettability conditions. On inclined surfaces, $W!e_{\mathrm{crit}}$ decreases linearly with the product of the Bond number and the width-to-height aspect ratio of the unforced drop. The linear slope is different in distinct wind- and gravity-dominated forcing regimes above and below $W!e_{\mathrm{crit}}=4$. Contact line shapes and drop profile shapes are measured at depinning conditions but do not adequately explain the differences between the two forcing regimes.