Microscopic structure of electrowetting-driven transitions on superhydrophobic surfaces

TL;DR

This study uses microscopy to reveal how electrowetting causes water droplets on superhydrophobic surfaces to transition from a state with trapped air to a fully wetted state, highlighting the microscopic mechanisms involved.

Contribution

It provides direct microscopic evidence of the transition mechanism between Cassie-Baxter and Wenzel states under electrowetting, focusing on the role of the contact line region.

Findings

Transition begins in a narrow annular region near the contact line.

High voltages cause microscopic air pockets to collapse, leading to partial wetting.

Local contact angles near the transition are close to advancing contact angles on smooth surfaces.

Abstract

We investigate directly at the microscale the morphology of the electrowetting induced transition between the Cassie-Baxter and Wenzel states for a water droplet on a superhydrophobic surface. Our experiments demonstrate that the transition originates in a very narrow annular region near the macroscopic contact line, which is first invaded by water and causes a thin film of air to be entrapped below. At high applied voltages, a growing fraction of microscopic air-pockets collapse, resulting in a partialWenzel state. Modulations in the intensity of the light reflected from individual micro-menisci clarify that the local contact angles near the filling transition are close to the usual advancing values for contact lines on smooth surfaces.