ANOMALOUS DYNAMICS IN GLASSY COLLOIDAL LIQUIDS

A liquid that is slowly cooled below the melting temperature usually undergoes a first-order transition to a crystal with long-range periodic order. Rapid cooling, by contrast, may suppress nucleation and result in a glass transition, in which the liquid viscosity and relaxation time appear to diverge despite the structure remaining liquid-like. With decreasing temperature, liquid dynamics become spatially and temporally heterogeneous as the system segments into ephemeral regions of relatively mobile and immobile particles. In this work, we use molecular dynamics simulations of model, colloidal glass-formers to probe anomalous dynamics of glassy liquids. In one investigation, we probe anomalous dynamics of dilute, hard sphere tracers within supercooled liquid and glass matrices with varying interparticle attraction strength. We find that tracers with diameters that are 35% of the matrix diameter exhibit anomalous dynamics indicative of competing relaxation mechanisms. This competition is associated with particular time and length scales that increase as the dominant interactions within the matrix are modulated from repulsive to attractive. As a result, tracer trajectories in attractive systems are more compact than trajectories in repulsive systems, reflecting the underlying rearrangements of the matrices. Further, we simulate tracers of this characteristic size in glasses with no long-time matrix rearrangement and find that vitrification does not prohibit tracer rearrangement. In fact, tracers are delocalized in repulsive glasses but localized in attractive glasses. This localization in attractive systems is heterogeneous in space and time because of the smaller matrix fluctuations in attractive systems, allowing the matrix structure to more strongly determine tracer dynamics. In another investigation, we examine the dynamics of polydisperse hard spheres confined between parallel hard walls separated by varying length $H$. We find a minimum in relaxation time at an intermediate $H$ that is incommensurate with average particle size, coinciding with a change from square to hexatic crystalline ordering in confined monodisperse hard spheres. Glassy slowdown in systems with commensurate $H$ is correlated with growing hexatically ordered domains, whereas no strong correlation between dynamics and crystalline ordering is observed for incommensurate $H$. Thus, dynamics can be driven by local ordering, but this relation is sensitive to confinement.