(150e) Structure, Elasticity, and Non-Equilibrium State Diagram of Depletion Gels

Depletion gels, soft solids that form in a mixture of colloidal particles and non-adsorbing polymer, are ubiquitous in industries where fine (colloidal) solids are dispersed in polymers, and have often served as model systems to understand the fundamental origins and rheology of particulate gels, including their elasticity and yielding. Using a recent model system that enables the rheology, structure, and particle interactions to be measured in concert [1], we show that the elastic modulus of a depletion gel is governed by the spatial organization and interactions of particle clusters. Each cluster acts as a mechanical unit that propagates the elastic deformation. As the attractive strength between particles increases, the gel elastic modulus increases, but this change cannot be accounted for by the immediate increase in bond stiffness between particles and clusters alone. The modulus is, however, consistent with an increasing number of cluster-cluster contacts. Two principal length scales thus emerge: at the particle level, the internal cluster structure becomes less dense with increasing attractive strength, and extends from the attractive glass line of colloids with short range attraction into the lower density gel region. However, the overall size of the clusters, the length scale over which stress is transmitted during elastic deformation, does not depend on the magnitude of the attractive strength.

1. Hsiao, L. C., Solomon, M. J., Whitaker, K. A. & Furst, E. M. A model colloidal gel for coordinated measurements of force, structure, and rheology. J. Rheol. 58, 1485–1504 (2014).