Deviation of viscous drops at chemical steps

TL;DR

This study combines experiments and simulations to understand how viscous drops slide across chemical steps, revealing that the optimal crossing angle depends on material properties and is influenced by contact angle hysteresis.

Contribution

It introduces a contact line mobility model to explain the influence of hysteresis on drop crossing behavior at chemical steps, supported by experimental phase diagrams.

Findings

Crossing angle depends on material parameters.

Contact angle hysteresis governs drop dynamics at the step.

Experimental phase diagram confirms the model's predictions.

Abstract

We present systematic wetting experiments and numerical simulations of gravity driven liquid drops sliding on a plane substrate decorated with a linear chemical step. Surprisingly, the optimal direction to observe crossing is not the one perpendicular to the step, but a finite angle that depends on the material parameters. We computed the landscapes of the force acting on the drop by means of a contact line mobility model showing that contact angle hysteresis dominates the dynamics at the step and determines whether the drop passes onto the lower substrate. This analysis is very well supported by the experimental dynamic phase diagram in terms of pinning, crossing, sliding and sliding followed by pinning.