Different scenarios of dynamic coupling in glassy colloidal mixtures

TL;DR

This study investigates how size ratio, volume fraction, and composition influence the coupled dynamics of small and large particles in dense colloidal mixtures, revealing mechanisms of dynamic coupling and cage formation through simulations and experiments.

Contribution

It provides new insights into the mechanisms of dynamic coupling and cage formation in colloidal mixtures with varying size ratios and compositions, combining simulations and microscopy data.

Findings

Dynamics become increasingly coupled at high small particle fractions for large size ratios.

Mixed cages form at moderate size ratios and high small particle fractions, leading to strong coupling.

Logarithmic decay in the intermediate scattering function indicates broad relaxation times.

Abstract

Colloidal mixtures represent a versatile model system to study transport in complex environments. They allow for a systematic variation of the control parameters, namely size ratio, total volume fraction and composition. We study the effects of these parameters on the dynamics of dense suspensions using molecular dynamics simulations and differential dynamic microscopy experiments. We investigate the motion of the small particles through the matrix of large particles as well as the motion of the large particles. A particular focus is on the coupling of the collective dynamics of the small and large particles and on the different mechanisms leading to this coupling. For large size ratios, about 1:5, and an increasing fraction of small particles, the dynamics of the two species become increasingly coupled and reflect the structure of the large particles. This is attributed to the dominant effect of the large particles on the motion of the small particles which is mediated by the increasing crowding of the small particles. Furthermore, for moderate size ratios, about 1:3, and sufficiently high fractions of small particles, mixed cages are formed and hence the dynamics are also strongly coupled. Again, the coupling becomes weaker as the fraction of small particles is decreased. In this case, however, the collective intermediate scattering function of the small particles shows a logarithmic decay corresponding to a broad range of relaxation times.