Different glassy characteristics are related to either caging or dynamical heterogeneity

Abstract

Despite the enormous theoretical and application interests, a fundamental understanding of the glassy dynamics remains elusive. The static properties of glassy and ordinary liquids are similar, but their dynamics are dramatically different. What leads to this difference is the central puzzle of the field. Even the primary defining glassy characteristics, their implications, and if they are related to a single mechanism remain unclear. This lack of clarity is a severe hindrance to theoretical progress. Here, we combine analytical arguments and simulations of various systems in different dimensions and address these questions. Our results suggest that the myriad of glassy features are manifestations of two distinct mechanisms. Particle caging controls the mean, and coexisting slow- and fast-moving regions govern the distribution of particle displacements. All the other glassy characteristics are manifestations of these two mechanisms; thus, the Fickian yet non-Gaussian nature of glassy liquids is not surprising. We discover a crossover, from stretched exponential to a power law, in the behavior of the overlap function. This crossover is prominent in simulation data and forms the basis of our analyses. Our results have crucial implications on how the glassy dynamics data are analyzed, challenge some recent suggestions on the mechanisms governing glassy dynamics, and impose strict constraints that a correct theory of glasses must have.

How motility drives the glassy dynamics in confluent epithelial monolayers?

“…Since the Stokes-Einstein relation for a suitably chosen τ gives D eff τ = const. [12], we can equivalently define the glass transition via τ . We therefore define the glass transition when τ /τ 0 = 10 6 .…”

“…The glassy nature of collective cellular dynamics in epithelial monolayers is crucial for many biological processes, such as embryogenesis [1][2][3][4], wound healing [5][6][7][8], and cancer progression [9,10]. Glass transition refers to the drastic change of dynamics, from a solid-like jammed to a fluid-like flowing state, without much apparent change in the static properties [11][12][13][14][15]. However, there are several dynamical signatures of glassiness, and they also appear in these biological systems.…”

“…The glassy nature of collective cellular dynamics in epithelial monolayers is crucial for many biological processes, such as embryogenesis [1][2][3][4], wound healing [5][6][7][8], cancer progression [9,10]. One defining feature of glassiness is the rapid growth of relaxation time by several orders of magnitude, with a modest change in the control parameter [11,12]. This enormous dynamical slow-down does not seem to accompany any apparent structural change in the static properties.…”

A perspective on active glassy dynamics in biological systems

“…Another way to determine the properties of particle displacements is to look at the van Hove function, G s (r, t). It gives the probability distribution of particle displacement r at time t [5,105,116]:…”

“…2 a The self overlap function, Q(t), as a function of time for different values of temperatures (Adapted with permission from Ref. [105]). b The self intermediate scattering function, Fs(k, t), at wave vector k = 7.25 as a function of time t for various T (Adapted with permission from Ref.…”