| 要旨： |
Experimental observations reveal that the response to an excitation in a complex condensed matter system may depend on the entire system’s history, and not just on the instantaneous value of the macroscopic state variables. These memory effects signal the breakdown of the macroscopic description. Some typical memory effects include shape memory in polymers, aging and rejuvenation in spin glasses, active matter, and granular gases. In this talk I will focus on the last class of systems and consider two prototypical memory phenomena: the Mpemba and Kovacs effects. The Mpemba effect is a counterintuitive phenomenon according to which, given two samples of fluid, the initially hotter one may cool more rapidly than the initially cooler one. In the Kovacs effect, a system relaxing to a low temperature is suddenly put in contact with a reservoir at the same temperature as the instantaneous value the system has after a given waiting time; however, the system’s temperature does not remain constant but exhibits a nonmonotonic evolution before reaching its asymptotic steady value. Both the Mpemba and Kovacs effects in granular gases will be addressed by minimal descriptions based on kinetic theory, the theoretical predictions being numerically confirmed by the direct simulation Monte Carlo method and by event-driven molecular dynamics. |
