Spatial and temporal dynamical heterogeneities approaching the binary colloidal glass transition

TL;DR

This study investigates how particle motion varies in space and time near the glass transition in binary colloidal suspensions, revealing growing heterogeneity and the significant role of smaller particles in facilitating rearrangements.

Contribution

It provides direct experimental observations of spatial and temporal dynamical heterogeneities approaching the colloidal glass transition, highlighting the influence of particle size disparity.

Findings

Particle dynamics are heterogeneous in space and time near the glass transition.

Mobile particles form localized groups with increasing size scales as transition approaches.

Smaller particles significantly facilitate particle rearrangements.

Abstract

We study concentrated binary colloidal suspensions, a model system which has a glass transition as the volume fraction $\phi$ of particles is increased. We use confocal microscopy to directly observe particle motion within dense samples with $\phi$ ranging from 0.4 to 0.7. Our binary mixtures have a particle diameter ratio $d_S/d_L=1/1.3$ and particle number ratio $N_S/N_L=1.56$, which are chosen to inhibit crystallization and enable long-time observations. Near the glass transition we find that particle dynamics are heterogeneous in both space and time. The most mobile particles occur in spatially localized groups. The length scales characterizing these mobile regions grow slightly as the glass transition is approached, with the largest length scales seen being $\sim 4$ small particle diameters. We also study temporal fluctuations using the dynamic susceptibility $\chi_4$, and find that the fluctuations grow as the glass transition is approached. Analysis of both spatial and temporal dynamical heterogeneity show that the smaller species play an important role in facilitating particle rearrangements. The glass transition in our sample occurs at $\phi_g \approx 0.58$, with characteristic signs of aging observed for all samples with $\phi>\phi_g$.