Evaporation-triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures

TL;DR

This paper investigates how evaporation causes water droplets on hydrophobic microstructures to transition from a non-wetting to a fully wetting state, revealing the underlying dynamics and energy considerations.

Contribution

It introduces a detailed microscopic analysis and a global energy model explaining the evaporation-triggered wetting transition on hydrophobic microstructured surfaces.

Findings

Wetting transition occurs at droplet radii of a few hundreds of micrometers.

Evaporation can induce a transition from Cassie-Baxter to Wenzel state.

A global energy argument estimates the critical radius for the transition.

Abstract

When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.