Wetting, Spreading, and Adsorption on Randomly Rough Surfaces

TL;DR

This paper develops analytical models to understand how microscopic roughness affects wetting, spreading, and adsorption on solid surfaces, revealing the importance of roughness characteristics and identifying a new first order transition.

Contribution

It introduces analytic expressions for wetting behavior on rough surfaces, highlighting the role of roughness parameters and discovering the Wenzel prewetting transition.

Findings

Wetting properties depend mainly on key roughness parameters.

A first order transition, 'Wenzel prewetting', occurs on typical roughness.

Gaussian roughness lacks the prewetting transition.

Abstract

The wetting properties of solid substrates with customary (i.e., macroscopic) random roughness are considered as a function of the microscopic contact angle of the wetting liquid and its partial pressure in the surrounding gas phase. Analytic expressions are derived which allow for any given lateral correlation function and height distribution of the roughness to calculate the wetting phase diagram, the adsorption isotherms, and to locate the percolation transition in the adsorbed liquid film. Most features turn out to depend only on a few key parameters of the roughness, which can be clearly identified. It is shown that a first order transition in the adsorbed film thickness, which we term 'Wenzel prewetting', occurs generically on typical roughness topographies, but is absent on purely Gaussian roughness. It is thereby shown that even subtle deviations from Gaussian roughness characteristics may be essential for correctly predicting even qualitative aspects of wetting.