Glass transition in active matter
ABSTRACT :
Glassy state is ubiquitous in nature. Examples range from window glass to complex fluids such as suspensions, emulsions, gels, polymers and granular materials. One often wonders what is glassy state. The answer lies at a very fundamental level. All systems which have relaxation times much larger than their typical molecular relaxation time are generally called as glassy systems. In this context, it is very important to know why the relaxation time is very high and what are the actual governing mechanisms behind the glassy state. By observation from outside a glassy material bears the resemblances of both a solid & a liquid. Purely from a mechanical point of view they are solid-like but from a structural point of view they are liquid-like i.e. they lack true long range order. In general, high viscous liquids show glassy state when they undergo rapid cooling. Once the system reaches glassy state, some concomitant properties arise spontaneously such as dynamic heterogeneity, cage effect and aging. While the dynamical properties of the system change drastically, the structural properties remain more or less the same as one approaches to Tg. Several theories are developed to explain the glass transition but none of them give satisfactory explanation for all the properties simultaneously. The phenomenon does not fall under the purview of conventional equilibrium & non-equilibrium statistical physics. It is not a thermodynamic phase transition in any sense but appears to be a transition from ergodic (liquid) to non-ergodic (solid) phase in space-time. While a lot of work has been done in the area of glass transition in equilibrium systems, very little is known for non-equilibrium (active) systems undergoing glass transition. Well known examples of such active systems include bacterial suspensions and self propelled granual media. Due to emergence of research in active matter system (living & non-living) in recent years, it will be very interesting to study how activity influences the glass transition.​