Far-from-equilibrium Sheared Colloidal Liquids: Disentangling Relaxation, Advection, and Shear-induced Diffusion

TL;DR

This study investigates the behavior of colloidal liquids under large amplitude oscillatory shear, revealing distinct nonlinear and linear responses, and demonstrates a universal scaling law linking equilibrium and far-from-equilibrium states.

Contribution

It uncovers the microscopic mechanisms behind shear-induced responses and introduces a colloidal analog of the Cox-Merz rule with a unifying master curve.

Findings

Identification of nonlinear amplitude saturation due to shear advection

Observation of linear frequency saturation from relaxation-shear competition

Scaling of all data onto a master curve linking different shear regimes

Abstract

Using high-speed confocal microscopy, we measure the particle positions in a colloidal suspension under large amplitude oscillatory shear. Using the particle positions we quantify the in situ anisotropy of the pair-correlation function -- a measure of the Brownian stress. From these data, we find two distinct types of responses as the system crosses over from equilibrium to far-from-equilibrium states. The first is a nonlinear amplitude saturation that arises from shear-induced advection, while the second is a linear frequency saturation due to competition between suspension relaxation and shear rate. In spite of their different underlying mechanisms, we show that all the data can be scaled onto a master curve that spans the equilibrium and far-from-equilibrium regimes, linking small amplitude oscillatory to continuous shear. This observation illustrates a colloidal analog of the Cox-Merz rule and its microscopic underpinning. Brownian Dynamics simulations show that interparticle interactions are sufficient for generating both experimentally observed saturations.