# Nonmonotonic behavior in the dense assemblies of active colloids

## TL;DR

This study reveals a nonmonotonic response in active colloid assemblies, showing slowdown then speedup in relaxation with increasing activity, due to the failure of passive mapping beyond the glass transition.

## Contribution

It demonstrates the breakdown of passive glass mapping in active systems beyond the glass transition, explaining the nonmonotonic relaxation behavior in active colloids and polycrystals.

## Key findings

- Nonmonotonic relaxation response to activity in active colloids.
- Failure of passive mapping beyond the glass transition.
- Domain size correlates with relaxation speed, showing nonmonotonic behavior.

## Abstract

We study experimentally a sediment of self-propelled Brownian particles with densities ranging from dilute to ergodic supercooled, to nonergodic glass, to nonergodic polycrystal. In a compagnon letter, we observe a nonmonotonic response to activity of relaxation of the nonergodic glass state: a dramatic slowdown when particles become weakly self-propelled, followed by a speedup at higher activities. Here we map ergodic supercooled states to standard passive glassy physics, provided a monotonic shift of the glass packing fraction and the replacement of the ambient temperature by the effective temperature. However we show that this mapping fails beyond glass transition. This failure is responsible for the nonmonotonic response. Furthermore, we generalize our finding by examining the dynamical response of an other class of nonergodic systems : polycrystals. We observe the same nonmonotinic response to activity. To explain this phenomenon, we measure the size of domains were particles move in the same direction. This size also shows a nonmonotonic response, with small lengths corresponding to slow relaxation. This suggests that the failure of the mapping of nonergodic active states to a passive situation is general and is linked to anisotropic relaxation mechanisms specific to active matter.