Wind- and Gravity-Forced Drop Depinning

TL;DR

This study presents a mechanistic model for liquid drop depinning caused by combined wind and gravity forces, validated through experiments on roughened surfaces, revealing critical Weber numbers and different regimes of depinning behavior.

Contribution

It introduces a simple model for drop depinning under combined wind and gravity forcing and experimentally verifies the critical conditions for depinning on inclined and non-inclined surfaces.

Findings

Drops depin at a constant Weber number on flat surfaces.

Critical Weber number decreases linearly with Bond number and aspect ratio on inclined surfaces.

Different depinning regimes are identified based on wind and gravity dominance.

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.