Evolution of particle-scale dynamics in an aging clay suspension

TL;DR

This study uses multispeckle x-ray photon correlation spectroscopy to analyze how the slow dynamics of an aging, charged colloidal suspension evolve over time and space, revealing strain-driven behavior and increasing interparticle repulsion.

Contribution

It provides new insights into the aging dynamics of charged colloids, demonstrating a specific aging-dependent relaxation behavior and supporting models of strain in disordered elastic media.

Findings

Relaxation time increases with sample age as τ ∼ t_a^{1.8}

Decay follows a compressed exponential form with β ≈ 1.5

Interparticle repulsion grows as suspension ages

Abstract

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a colloidal suspension formed by highly-charged, nanometer-sized disks. At scattering wave vectors $q$ corresponding to interparticle length scales, the dynamic structure factor follows a form $f(q,t) \sim \exp[-(t/\tau)^{\beta}$], where $\beta \approx$ 1.5. The characteristic relaxation time $\tau$ increases with the sample age $t_a$ approximately as $\tau \sim t_a^{1.8}$ and decreases with $q$ approximately as $\tau \sim q^{-1}$. Such a compressed exponential decay with relaxation time that varies inversely with $q$ is consistent with recent models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The amplitude of the measured decay in $f(q,t)$ varies with $q$ in a manner that implies caged particle motion at short times. The decrease in the range of this motion and an increase in suspension conductivity with increasing $t_a$ indicate a growth in the interparticle repulsion as the mechanism for internal stress development implied by the models.