Universal behaviour in the mechanical properties of weakly aggregated colloidal particles

Abstract

We study the evolution of the elastic shear modulus of weakly aggregated colloidal particles during the onset of a delayed collapse under gravity. The early lifetime is characterised by an elastic shear modulus that increases logarithmically in time, following which the gels experience a catastrophic failure and the elastic modulus drops dramatically. As the gel collapses, various complex behaviours are seen, including a temporary stabilisation against collapse, and reformation of a new gel with its own elastic modulus that follows its own trajectory to collapse. Time-lapsed images acquired of identical samples in a transparent cell used to calibrate the measured shear modulus values allow observation of the sample-spanning collective rearrangement involved in the collapse. The loss of propagation of elastic stress in the gel is observed to precede the bulk collapse in all samples, with the two events always well-separated in time. The evolution of the viscoelastic response across a range of colloid volume fractions and polymer concentrations is significantly simplified by scaling the rheology curves for each sample together. From the scaling, we show a critical onset of the elastic modulus as a function of these system parameters. Moreover, our analyses of the time scale for collapse of the elastic shear modulus and of the gel itself over a range of colloid volume fractions and the polymer concentrations support a simple phenomenological model based on the dependence of the microscopic dynamics on the strength and number of sticky interparticle bonds.