High-bandwidth viscoelastic properties of aging colloidal glasses and gels

TL;DR

This study measures the frequency-dependent shear moduli of aging colloidal glasses and gels using microrheology, revealing a universal aging behavior characterized by a crossover in viscoelastic response and differences in homogeneity.

Contribution

It introduces a wide-range microrheology approach to distinguish and analyze the aging dynamics of colloidal glasses and gels, highlighting a universal scaling behavior.

Findings

Both systems show a crossover from single to dual power-law frequency dependence.

The aging data collapse onto a master curve when scaled by a characteristic time.

Gels exhibit heterogeneity, whereas glasses are homogeneous on micron scales.

Abstract

We report measurements of the frequency-dependent shear moduli of aging colloidal systems that evolve from a purely low-viscosity liquid to a predominantly elastic glass or gel. Using microrheology, we measure the local complex shear modulus $G^{*}(\omega)$ over a very wide range of frequencies (1 Hz- 100 kHz). The combined use of one- and two-particle microrheology allows us to differentiate between colloidal glasses and gels - the glass is homogenous, whereas the colloidal gel shows a considerable degree of heterogeneity on length scales larger than 0.5 micrometer. Despite this characteristic difference, both systems exhibit similar rheological behavior which evolve in time with aging, showing a crossover from a single power-law frequency dependence of the viscoelastic modulus to a sum of two power laws. The crossover occurs at a time $t_{0}$, which defines a mechanical transition point. We found that the data acquired during the aging of different samples can be collapsed onto a single master curve by scaling the aging time with $t_{0}$. This raises questions about the prior interpretation of two power laws in terms of a superposition of an elastic network embedded in a viscoelastic background. Keywords: Aging, colloidal glass, passive microrheology