Elastowetting of Soft Hydrogel Spheres

TL;DR

This study investigates how soft hydrogel spheres spread on rigid surfaces, revealing that elasticity influences contact angles and foot length, with experimental results supported by energy-based scaling analysis.

Contribution

It provides a systematic experimental analysis of elastowetting in hydrogel spheres and introduces a minimal energy-based model to explain the observed behaviors.

Findings

Contact angle increases as work of adhesion decreases.

Elastic modulus affects contact angle and foot length.

Contact foot length depends on adhesion, size, and elasticity.

Abstract

When a soft hydrogel sphere is placed on a rigid hydrophilic substrate, it undergoes arrested spreading by forming an axisymmetric foot near the contact line, while conserving its global spherical shape. In contrast, liquid water (that constitutes greater than 90% of the hydrogel's volume) spreads into a thin film on the same surface. We study systematically this elastowetting of gel spheres on substrates of different surface energies, and find that their contact angle increases as the work of adhesion between the gel and the substrate decreases, as one would observe for drops of pure water - albeit being larger than in the latter case. This difference in the contact angles of gel and water appears to be due to the elastic shear stresses that develop in the gel and oppose its spreading. Indeed, by increasing the elastic modulus of the gel spheres, we find that their contact angle also increases. In addition, the length of the contact foot increases with the work of adhesion and sphere size, while it decreases when the elastic modulus of the gel is increased. We discuss those experimental results in light of a minimal analysis based on energy minimization, volume conservation, and scaling arguments.