# Dependence of Wenzel–Cassie Transition on Droplet Size: The Critical Water Droplet

## TL;DR

This study uses simulations to explore how droplet size affects the Wenzel–Cassie wetting transition on rough surfaces.

## Contribution

The paper identifies a critical droplet size and roughness parameter that determine the Wenzel–Cassie transition.

## Key findings

- The critical droplet size (RDroplet,c) depends on the wetting parameter of surface roughness (WRoughness,c).
- A denser surface roughness distribution lowers the critical droplet size.
- Superhydrophobicity is influenced by both surface roughness characteristics and droplet size.

## Abstract

In this work, molecular dynamics (MD) simulations are applied to investigate the dependence of the Wenzel–Cassie transition on water droplet size. During the Wenzel–Cassie transition, the critical water droplet and corresponding critical roughness may be expected, which are respectively described as the critical radius (RDroplet,c) and wetting parameter (WRoughness,c). From the work, RDroplet,c may be termed as the smallest droplet size at which the Cassie state is expected for the corresponding WRoughness,c. In combination with the structural study of water, it is due to the structural competition between interfacial and bulk water. Additionally, RDroplet,c may be dependent on the WRoughness,c. It is found that the RDroplet,c is influenced by the distribution and geometric characteristics of surface roughness. A denser distribution of roughness is expected to result in a lower RDroplet,c. Consequently, superhydrophobicity may be influenced by the characteristics of surface roughness and the size of the water droplet. The Cassie state is achieved when the wetting parameter of roughness is less than the WRoughness,c and the water droplet is larger than the RDroplet,c.