Tracer Transport in Attractive and Repulsive Supercooled Liquids and Glasses*

The transport of small penetrants through disordered materials with glassy dynamics is encountered in drug delivery and chemical separations. Understanding the influence of matrix structure and fluctuations on penetrant motions remains a challenge. We use event-driven molecular dynamics to investigate the transport of small, hard-sphere tracers in matrices of square-well particles. Short-range attractions between matrix particles give rise to reentrant dynamics in the supercooled regime, in which the liquid's relaxation time increases dramatically upon heating or cooling. Heating results in a repulsive supercooled liquid where relaxations are frustrated by steric interactions between particles, whereas cooling produces an attractive liquid in which relaxations are hindered by long-lived interparticle bonds. Further cooling/heating, or compression, of the supercooled liquids results in the formation of distinct glasses. Our study reveals that tracer transport in these liquids and glasses is influenced by matrix structure and dynamics. The relative importance of each factor varies between matrices and is examined by analyzing mean-square displacements, caging behavior, and trajectories sampled from the isoconfigurational ensemble.