Glasses of dynamically asymmetric binary colloidal mixtures: Quiescent properties and dynamics under shear

TL;DR

This study explores how mixing large and small particles in binary colloidal glasses affects their structure, dynamics, and response to shear, revealing a transition from caging by large particles to independent cage formation by small particles.

Contribution

It provides new insights into the effects of size asymmetry and mixing ratio on the glassy behavior and shear response of colloidal mixtures.

Findings

Mixing alters the cage structure and dynamics significantly.

Shear induces melting through facilitated diffusion and structural anisotropy.

Large particles' caging is strongly affected by small particle packing.

Abstract

We investigate mixing effects on the glass state of binary colloidal hard-sphere-like mixtures with large size asymmetry, at a constant volume fraction phi = 0.61. The structure, dynamics and viscoelastic response as a function of mixing ratio reflect a transition between caging by one or the other component. The strongest effect of mixing is observed in systems dominated by caging of the large component. The possibility to pack a large number of small spheres in the free volume left by the large ones induces a pronounced deformation of the cage of the large spheres, which become increasingly delocalised. This results in faster dynamics and a strong reduction of the elastic modulus. When the relative volume fraction of small spheres exceeds that of large spheres, the small particles start to form their own cages, slowing down the dynamics and increasing the elastic modulus of the system. The large spheres become the minority and act as an impurity in the ordering beyond the first neighbour shell, i.e. the cage, and do not directly affect the particle organisation on the cage level. In such a system, when shear at constant rate is applied, melting of the glass is observed due to facilitated out-of-cage diffusion which is associated with structural anisotropy induced by shear.