Shedding new light on the mystery of wetting on soft solids

TL;DR

This paper uncovers a universal wetting principle on soft solids by visualizing wetting ridge tips, revealing their asymmetric and bent geometry, and linking macroscopic and microscopic contact angles through a dual-scale model.

Contribution

It introduces a new dual-scale model explaining wetting ridge geometry on soft solids, bridging macroscopic and microscopic contact angles.

Findings

Wetting ridge tips are asymmetric and bent.

The geometry remains invariant during ridge growth.

A dual-scale model links macroscopic and microscopic contact angles.

Abstract

One of the most questionable issues in wetting is the vertical force balance that is excluded in Young's law. On soft deformable solids, such as biotic materials and synthetic polymers, the vertical force of liquid leads to a microscopic protrusion of the contact line, i.e. a "wetting ridge". The wetting principle that determines the geometry of the ridge-tip is at the heart of the issues over the past half century. Here, we reveal a universal wetting principle by directly visualizing ridge-tips with high spatio-temporal resolution using x-ray microscopy. We find that the tip-geometry is asymmetric and bent, and invariant during ridge growth or by surface softness. This singular geometry is explained by linking the macroscopic and microscopic contact angles to Young's and Neumann's laws, respectively. Our dual-scale model would be applicable to a general framework in wetting and give new clues to various issues in cell-substrate interaction and elasto-capillary problems.