The collapse transition on superhydrophobic surfaces

TL;DR

This paper analyzes the transition mechanisms of a droplet on superhydrophobic surfaces during evaporation, identifying key factors influencing the Cassie-Baxter to Wenzel state change through analytical and simulation methods.

Contribution

It provides a detailed analysis of collapse mechanisms and highlights the role of contact line dynamics in droplet transitions on superhydrophobic surfaces.

Findings

Two collapse mechanisms identified: overcoming energy barrier and increased curvature touch-down.

Contact line retreat often triggers the collapse before the droplet reaches the surface.

Simulation shows droplet de-pinning and inverted bowl shape formation.

Abstract

We investigate the transition between the Cassie-Baxter and Wenzel states of a slowly evaporating, micron-scale drop on a superhydrophobic surface. In two dimensions analytical results show that there are two collapse mechanisms. For long posts the drop collapses when it is able to overcome the free energy barrier presented by the hydrophobic posts. For short posts, as the drop loses volume, its curvature increases allowing it to touch the surface below the posts. We emphasise the importance of the contact line retreating across the surface as the drop becomes smaller: this often preempts the collapse. In a quasi-three dimensional simulation we find similar behaviour, with the additional feature that the drop can de-pin from all but the peripheral posts, so that its base resembles an inverted bowl.