Entropy Calculations in Active Glass Formers
ABSTRACT :
Glass is ubiquitous in nature. The glass transition in general characterised by the slowing down of the relaxation time, caging effect and the dynamics heterogeneity. It is interesting to observe in the experi ments that many of the non-equilibrium living (active) systems such as the human crowd, embryos of zebrafish, cell motion in dense tissues etc., exhibit the similar phenomena as glass forming systems. Therefore, it is natural to study the glass transition in model active glass formers such as the Kob-Anderson system whose dynamics is governed by the Ornstein-Uhlenbeck process. We probe the thermodynamic aspects of the glass transition and study the relations between thermodynamic quantities and the dynamical quantities. We focus mainly on the entropic scaling relations with the dynamical quantities such as relaxation time and the diffusion constant in the case of active glass forming liquids. We show that increasing the duration of self-propulsion makes the pair excess entropy negatively larger. The associated reduction in the number of accessible configurations per particle leads to a reduction in self diffusivity. At moderate supercooling, the self-diffusivity is Arrhenius and in a reduced form obeys a Dzugutov like scaling law, directly yielding us a pair excess entropy that is inversely proportional to the effective temperature. In the strongly supercooled regime, Dzugutov law does not apply and we observe that the pair excess entropy shows a non-Arrhenius (power law) dependence on the effective temperature with an exponent that depends on the self propulsion time of the active particles. To demonstrate the generality of our scaling laws in moderately high temperatures, we set the particle interactions to be purely repulsive in one case and Lennard-Jones in the other, and find that in both the cases, the reported high temperature scaling laws are robust over variations in the duration of self propulsion. The pair excess entropy provides only the two particle contribution but to describe the supercooled liquids near glass transition the configurational entropy is an indispensable tool. Its calculation requires the enumeration of the basins in the poten tial energy landscape and when available, it reveals a direct connection with the relaxation time of the liquid. While there are several reports on the measurement of configurational entropy in passive liquids, very little is understood about its role in active liquids which have a propensity to undergo a glass transition at low temperatures. We report for the first time, a careful calculation of the configurational entropy in a model glass former where the constituent units are self propelled. We show that unlike passive liquids, the anharmonic contribution to the glass entropy in these self-propelled liquids can be of the same order as the harmonic contribution, and therefore must be included in the calculation of the configurational entropy. Our extracted configurational entropy is in good agreement with the prediction of the random first order transition theory that connects it to a static length scale obtained by random pinning.
ABOUT THE SPEAKER:
Biography of the Speaker :
PhD Scholar, Department of Physics, IIT Madras
Affiliation of the Speaker :
Guide: Dr. ASHWIN JOY?