Start-up Shear of Concentrated Colloidal Hard Spheres: Stresses, Dynamics and Structure

TL;DR

This study investigates the transient and steady-state responses of concentrated colloidal hard spheres under start-up shear, revealing stress behaviors, structural anisotropy, and cage dynamics through simulations and experiments.

Contribution

It combines Brownian Dynamics simulations, rheology, and microscopy to analyze stress, structure, and particle dynamics during start-up shear in colloidal glasses, highlighting the role of shear rate and volume fraction.

Findings

Stress exhibits a peak at yield and reaches a steady state.

Structural anisotropy increases under shear and persists in steady state.

High shear rates induce cage distortion and superdiffusive particle motion.

Abstract

The transient response of model hard sphere glasses is examined during the application of steady rate start-up shear using Brownian Dynamics (BD) simulations, experimental rheology and confocal microscopy. With increasing strain the glass initially exhibits an almost linear elastic stress increase, a stress peak at the yield point and then reaches a constant steady state. The stress overshoot has a non-monotonic dependence with Peclet number, Pe, and volume fraction, {\phi}, determined by the available free volume and a competition between structural relaxation and shear advection. Examination of the structural properties under shear revealed an increasing anisotropic radial distribution function, g(r), mostly in the velocity - gradient (xy) plane, which decreases after the stress peak with considerable anisotropy remaining in the steady-state. Low rates minimally distort the structure, while high rates show distortion with signatures of transient elongation. As a mechanism of storing energy, particles are trapped within a cage distorted more than Brownian relaxation allows, while at larger strains, stresses are relaxed as particles are forced out of the cage due to advection. Even in the steady state, intermediate super diffusion is observed at high rates and is a signature of the continuous breaking and reformation of cages under shear.