Evolution of particle-scale dynamics in an aging clay suspension

Abstract

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a suspension of highly charged, nanometer-sized disks. At wave vectors q corresponding to interparticle length scales, the dynamic structure factor follows a form f(q,t) ~exp[−(t/τ)^ß], where ß ≈1.5. The relaxation time τ increases with the sample age ta approximately as τ ~t^1.8_a and decreases with q as τ ~q⁻¹. Such behavior is consistent with models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The measured amplitude of f(q,t) varies with q in a manner that implies caged particle motion. The decrease in the range of this motion and an increase in suspension conductivity with increasing ta indicate a growth in interparticle repulsion as the mechanism for internal stress development implied by these models.