The role of crosslinking density in surface stress and surface energy of soft solids

TL;DR

This study investigates how crosslinking density affects surface stress and energy in soft silicone gels, revealing a critical transition point where surface stress increases while surface energy stays constant, linked to network percolation.

Contribution

It provides new experimental insights into the relationship between crosslinking density, surface stress, and surface energy in soft gels, highlighting a critical transition point.

Findings

Surface stress increases with crosslinking density above a critical point.

Surface energy remains unchanged across varying crosslinking densities.

Surface properties are linked to the percolation transition of polymer networks.

Abstract

Surface stress and surface energy are two fundamental parameters that determine the surface properties of any materials. While it is commonly believed that the surface stress and surface energy of liquids are identical, the relationship between the two parameters in soft polymeric gels remains debatable. In this work, we measured the surface stress and surface energy of soft silicone gels with varying weight ratios of crosslinkers in soft wetting experiments. Above a critical density, $k_0$, the surface stress was found to increase significantly with crosslinking density while the surface energy remained unchanged. In this regime, we can estimate a non-zero surface elastic modulus that also increases with the ratio of crosslinkers. By comparing the surface mechanics of the soft gels with their bulk rheology, the surface properties near the critical density $k_0$ were found to be closely related to the underlying percolation transition of the polymer networks.