Dynamical heterogeneity in aging colloidal glasses of Laponite

TL;DR

This study provides experimental evidence of increasing dynamical heterogeneity length scales in aging Laponite colloidal glasses, revealing decoupled rotational and translational dynamics and their relation to macroscopic rheology.

Contribution

It demonstrates the growth of dynamical heterogeneity length scales during aging and links microscopic particle dynamics to macroscopic rheological properties.

Findings

Dynamical heterogeneity length scale increases with waiting time.

Rotational diffusion slows down faster than translational diffusion.

Shear viscosity increases more rapidly than particle relaxation times.

Abstract

Glasses behave as solids due to their long relaxation time; however the origin of this slow response remains a puzzle. Growing dynamic length scales due to cooperative motion of particles are believed to be central to the understanding of both the slow dynamics and the emergence of rigidity. Here, we provide experimental evidence of a growing dynamical heterogeneity length scale that increases with increasing waiting time in an aging colloidal glass of Laponite. The signature of heterogeneity in the dynamics follows from dynamic light scattering measurements in which we study both the rotational and translational diffusion of the disk-shaped particles of Laponite in suspension. These measurements are accompanied by simultaneous microrheology and macroscopic rheology experiments. We find that rotational diffusion of particles slows down at a faster rate than their translational motion. Such decoupling of translational and orientational degrees of freedom finds its origin in the dynamic heterogeneity since rotation and translation probe different length scales in the sample. The macroscopic rheology experiments show that the low frequency shear viscosity increases at a much faster rate than both rotational and translational diffusive relaxation times.