(414d) Dynamic Simulation of Aging in a Hard-Sphere Colloidal Glass after Volume-Fraction Jumps

We study the colloidal glass transition from a micromechanical perspective via large-scale dynamic simulation. Many molecular liquids transition from a liquid to a crystalline state upon cooling. If the cooling is sufficiently rapid, however, the material can undergo a phase transition whereby it solidifies without crystallization; the amorphous structure is retained but the dynamics become extremely slow (Angell, 1995). The new “state” is path dependent, i.e. is not an equilibrium phase: material properties such as enthalpy continue to evolve in time. The material relaxes over time toward an “intransient” state, as measured by the Kovacs signature experiments, i.e. the intrinsic isotherms, the asymmetry of approach and the memory effect (Kovacs, 1963). The reasons for this path- and time-dependence are not fully understood in molecular glass-formers, owing partly to molecular relaxation that is so fast that the transition is conflated with the subsequent aging process, making it difficult to capture structural changes during the transition. Colloidal glass-formers provide a natural way to model such behavior, owing to the disparity in colloidal versus solvent time scales that allows tracking of particle dynamics and structure, and how these evolve with liquid-to-glassy macroscopic behavior. To shed light on the ambiguity of the glass transition and the glassy state, we study via large-scale dynamic simulation the structural relaxation, dynamics and rheology of a hard-sphere colloidal glass after various volume-fraction jumps. We execute such volume-fraction jumps with a range of quench rates, where particle size increases at constant system volume. Here we focus on post-jump particle dynamics, where we implement the protocols of the McKenna-Kovacs signature experiments (intrinsic iso-volume fraction) to study the approach to the stationary state. During and following each jump, the positions, velocities, and particle-phase stress are tracked and utilized to characterize relaxation time scales. The impact of both quench depth and quench rate on arrested dynamics and “state” variables is explored. A comparison to the experiments (Peng and McKenna, 2016; Li, Peng and McKenna, 2017) shows qualitative agreement. In addition, we expand our view to various structural signatures, and rearrangement mechanism is proposed. The results provide insight into not only the existence of an “ideal” glass transition, but also the role of structure in such a dense amorphous system.