Colloidal Gels Tuned by Oscillatory Shear

TL;DR

This study investigates how oscillatory shear flow influences the microstructure and mechanical properties of colloidal gels, revealing three regimes of structural change and demonstrating oscillatory shear's effectiveness in tuning gel properties.

Contribution

It introduces a detailed analysis of the microstructural and mechanical effects of oscillatory shear on colloidal gels, highlighting regimes not observed with steady shear.

Findings

Large shear fully breaks the gel, leading to stronger, more homogeneous gels.

Intermediate shear densifies clusters, resulting in heterogeneous, weaker gels.

Oscillatory shear creates more structural heterogeneity than steady shear.

Abstract

We examine microstructural and mechanical changes which occur during oscillatory shear flow and reformation after flow cessation of an intermediate volume fraction colloidal gel using rheometry and Brownian Dynamics (BD) simulations. A model depletion colloid-polymer mixture is used, comprising of a hard sphere colloidal suspension with the addition of non-adsorbing linear polymer chains. Results reveal three distinct regimes depending on the strain amplitude of oscillatory shear. Large shear strain amplitudes fully break the structure which results into a more homogenous and stronger gel after flow cessation. Intermediate strain amplitudes densify the clusters and lead to highly heterogeneous and weak gels. Shearing the gel to even lower strain amplitudes creates a less heterogonous stronger solid. These three regimes of shearing are connected to the microscopic shear-induced structural heterogeneity. A comparison with steady shear flow reveals that the latter does not produce structural heterogeneities as large as oscillatory shear. Therefore oscillatory shear is a much more efficient way of tuning the mechanical properties of colloidal gels. Moreover, colloidal gels presheared at large strain amplitudes exhibit a distinct nonlinear response characterized largely by a single yielding process while in those presheared at lower rates a two step yield process is promoted due to the creation of highly heterogeneous structures.