Spreading on viscoelastic solids: Are contact angles selected by Neumann's law?

TL;DR

This study visualizes the wetting ridge during droplet spreading on soft solids, confirming Neumann's law governs the ridge shape and introducing a new theory that accounts for surface strain effects and the Shuttleworth effect in soft wetting dynamics.

Contribution

It provides direct experimental evidence that Neumann's law applies during dynamic wetting on viscoelastic solids and develops a novel theoretical framework incorporating surface strain and the Shuttleworth effect.

Findings

Wetting ridge rotation matches the dynamic contact angle.

Neumann's law governs the wetting ridge shape during spreading.

A new theory includes surface strain and Shuttleworth effect in soft wetting.

Abstract

The spreading of liquid drops on soft substrates is extremely slow, owing to strong viscoelastic dissipation inside the solid. A detailed understanding of the spreading dynamics has remained elusive, partly owing to the difficulty in quantifying the strong viscoelastic deformations below the contact line that determine the shape of moving wetting ridges. Here we present direct experimental visualisations of the dynamic wetting ridge, complemented with measurements of the liquid contact angle. It is observed that the wetting ridge exhibits a rotation that follows exactly the dynamic liquid contact angle -- as was previously hypothesized [Karpitschka \emph{et al.} Nature Communications \textbf{6}, 7891 (2015)]. This experimentally proves that, despite the contact line motion, the wetting ridge is still governed by Neumann's law. Furthermore, our experiments suggest that moving contact lines lead to a variable surface tension of the substrate. We therefore set up a new theory that incorporates the influence of surface strain, for the first time including the so-called Shuttleworth effect into the dynamical theory for soft wetting. It includes a detailed analysis of the boundary conditions at the contact line, complemented by a dissipation analysis, which shows, again, the validity of Neumann's balance.