Morphology of cooperatively rearranging regions in active glass formers

Abstract

Super cooled liquids display increasingly heterogeneous dynamics as temperature is lowered towards the glass transition ($T_{g}$). A hallmark of this dynamical heterogeneity is the spontaneous emergence of cooperative rearranging regions (CRRs) composed of fast moving particles. While these CRRs in passive glass formers have been explored in great detail, thus understanding is severely limited in active glass formers. The existing consensus on the morphology of CRRs in a passive glass former prioritizes its fast subsets, composed of fast moving particles. In the present study, we focus on a synthetic athermal active glass former and show an equal contribution for the morphology of CRRs from slow subsets as well. Both these subsets exhibit an exponential distribution in their structure which strongly correlates with the existence of CRRs. Interestingly, we also observe that the fractal dimensions ($d_{\text{f}}$) of these subsets share both string and compact like morphology that tends to vary in opposite fashion with the control parameters, namely the persistent time ($\tau_{p}$) and the effective temperature ($T_{\text{eff}}$). The fractal dimension $d_{\text{f}}$ measures the roughness or put simply the compactness of fractal objects at their boundaries. More precisely, molecules are loosely bound in a structure for which boundary is rough and thereby this condition facilitates its structural change in terms of size and shape. It is also a fact that any structural change is a signature of relaxation dynamics in the context of glass forming liquids. Thus, in the present study, we observe a change in $d_{\text{f}}$ with $T_{\text{eff}}$ and $\tau_{p}$ from the insights of morphology variation that causes structural change, both in the BD limit and non-equilibrium limit.