What hydrodynamic correlations tell us about the colloidal glass transition

Colloidal suspensions have been extensively used over the years to study the glass transition. The motions of the suspended particles slow down tremendously as the suspension approaches the glass transition, making an accurate study of the transition a decades-long challenge. We use an indirect approach to bypass this difficulty. Instead of examining the dynamics of the glass-forming particles directly, we learn about it from the dynamics of small tracers that freely move in the host liquid. Correlations between the thermal motions of the tracers report on the flow response of the host liquid, which in turn reports on the state of the larger particles obstructing the flow. A model-independent phenomenological theory predicts distinctive signatures of the glass transition in these hydrodynamic correlations: (a) the power of their decay with relative distance changes; (b) a length scale related to this decay steeply increases with the increasing viscosity; and (c) certain correlations switch sign. These predictions are confirmed by experiment.