Does surface roughness amplify wetting?

TL;DR

This paper challenges the conventional belief that surface roughness always amplifies wetting, demonstrating through density functional theory that microscopic roughness can hinder wetting and even induce hydrophobicity.

Contribution

The study provides a theoretical demonstration that microscopic surface corrugation can hinder wetting, contrasting with macroscopic predictions like Wenzel's law.

Findings

Microscopic roughness can increase wetting temperature.

Surface corrugation can induce hydrophobicity.

A crossover length scale determines wetting response.

Abstract

Any solid surface is intrinsically rough on the microscopic scale. In this paper, we study the effect of this roughness on the wetting properties of hydrophilic substrates. Macroscopic arguments, such as those leading to the well-known Wenzel's law, predict that surface roughness should \emph{amplify} the wetting properties of such adsorbents. We use a fundamental measure density functional theory (DFT) to demonstrate the opposite effect from roughness for microscopically corrugated surfaces, i.e., wetting is \emph{hindered}. Based on three independent analyses we show that microscopic surface corrugation increases the wetting temperature or even makes the surface hydrophobic. Since for macroscopically corrugated surfaces the solid texture does indeed amplify wetting there must exist a crossover between two length-scale regimes that are distinguished by opposite response on surface roughening. This demonstrates how deceptive can be affords to extend the thermodynamical laws beyond their macroscopic territory.