A perspective on active glassy dynamics in biological systems

Abstract

Dynamics is central to living systems. Many experiments in the last two decades have revealed glassy dynamics in diverse biological systems, showing a transition between a solid-like and a fluid-like state. The biological systems have nontrivial characteristics: they are active with novel control parameters and immense complexity. Moreover, glassiness in these systems has many nontrivial features, such as the behavior of dynamical heterogeneity and readily found sub-Arrhenius relaxation dynamics. Theoretical treatments of these systems are generally challenging due to their nonequilibrium nature and large number of control parameters. We first discuss the primary characteristics of a glassy system and then review the experiments that started this field and simulations that have led to a deeper understanding. We also show that despite many challenges in these systems, it has been possible to develop theories that have played a significant role in unifying diverse phenomena and bringing insights. The field is at the interface of physics and biology, freely borrowing tools from both disciplines. We first discuss the known equilibrium scenario and then present the primary changes under activity.

Motility driven glassy dynamics in confluent epithelial monolayers

“…The plateau in Q(t) and sub-diffusive behavior in Dr 2 (t) at intermediate times are typical characteristics of glassy systems. 11,12,15 We define the relaxation time t as Q(t = t) = 0.3. Fig.…”

“…The strong correlation between cell shape, a static observable, and dynamics 71,84,105 makes the glassiness in these systems distinctive compared to that in particulate systems. 11,15,32,69,104 We have revealed a novel effect of activity, where the time scale of cell shape change leads to a modified rotational diffusivity, D eff r , of the motile forces. This modification is crucial for the relaxation dynamics of the system.…”

“…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. For example, complex non-exponential relaxation, 16,17 dynamical heterogeneity, 17,18 caging, 19 non-Gaussian particle displacements, 14,20,21 etc.…”

“…12,34,35 These systems comprise self-propelled particles (SPPs) with a self-propulsion force, f 0 , and a persistence time, t p . 15,28,29 t p leads to a separation of time scales between the thermal and active dynamics. When t p is not too large, an equilibrium-like description with an extended fluctuation-dissipation relation applies.…”

Emergence of bacterial glass

“…5 c), within a distance of , which is consistent with the typical size of the microdomains reported above. Note that the velocity correlation is generally absent in thermal systems, hence another athermal characteristic of our system, and is known to play a significant role in active glassy dynamics ( 13 , 15 , 33 ). In our system, the velocity correlation is formed as a result of the microdomain structure of the orientations, but it is noteworthy that the observed velocity correlation indicates the presence of polar order, despite the essentially nematic nature of the steric interaction.…”