Tracer Transport Probes Relaxation and Structure of Attractive and Repulsive Glassy Liquids

Dynamic coupling of small penetrants to slow, cooperative relaxations within crowded cells, supercooled liquids, and polymer matrices has broad consequences for applications ranging from drug delivery to nanocomposite processing. Interactions between the constituents of these and other disordered media alter the cooperative relaxations, but their effect on penetrant dynamics remains incompletely understood. We use molecular dynamics simulations to show that the motions of hard-sphere tracer particles probe differences in local structure and cooperative relaxation processes in attractive and repulsive supercooled liquid matrices with equal bulk packing fractions and long-time diffusivities. Coupling of the tracer dynamics to collective matrix relaxations affects the shape of tracer trajectories, which are string-like within the repulsive matrix and compact in the attractive. These results reveal that the structure of relaxations controls penetrant transport and dispersion in cooperatively relaxing systems. We further explore this connection by calculating the tracer and matrix dynamic susceptibilities, characterizing cooperative rearrangements in the matrices, and simulating tracer diffusion in arrested glasses.