# Active glass: ergodicity breaking dramatically affects response to self-propulsion

## TL;DR

This study experimentally investigates how self-propulsion affects dense sediment of particles, revealing nonmonotonic relaxation behavior in glassy states due to competing effects of activity, challenging existing theoretical predictions.

## Contribution

It demonstrates that activity can both slow down and speed up glass relaxation, highlighting the importance of active directionality in glass transition dynamics.

## Key findings

- Active particles enhance relaxation in supercooled liquids.
- In nonergodic glasses, activity causes a slowdown before fluidization.
- Results challenge existing ergodic-based theoretical models.

## Abstract

We study experimentally the response of a dense sediment of Brownian particles to self-propulsion. We observe that the ergodic supercooled liquid relaxation is monotonically enhanced by activity. By contrast the nonergodic glass shows an order of magnitude slowdown at low activities with respect to passive case, followed by fluidization at higher activities. Our results contrast with theoretical predictions of the ergodic approach to glass transition summing up to a shift of the glass line. We propose that nonmonotonicity is due to competing effects of activity: (i) extra energy that helps breaking cages (ii) directionality that hinders cage exploration. We call it "Deadlock from the Emergence of Active Directionality" (DEAD). It suggests further theoretical works should include thermal motion.