Gulliver Seminar : Mathieu Leocmach, ILM Lyon

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4 janvier 2021 11:30 » 12:30

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

We study experimentally a sediment of self-propelled Brownian particles with densities ranging from dilute to ergodic supercooled to nonergodic glass, to nonergodic polycrystal. We observe a dramatic slowdown of relaxation of nonergodic states when particles become weakly self-propelled. By contrast, ergodic supercooled states always relax faster with self-propulsion. Our system is a monolayer of micron-size gold-platinum Janus particles, which become active upon adding a solution of hydrogen peroxide due to self-phoretic propulsion mechanisms. We characterise the activity level in our system with an effective temperature defined from the density profile. Standard glassy physics describes well the ergodic regime provided the replacement of the ambient temperature by this effective temperature : higher temperature implies faster relaxation. However beyond the glass transition, the relaxation of the nonergodic system abruptly slows down at low but nonzero activity. As we increase further activity, the relaxation speeds up until it exceeds the passive situation. This nonmonotonic behavior cannot be described by a simple increase in temperature. The same nonmonotonic response is observed in polycrystal. To explain this phenomenon, we correlated particle displacement orientation and calculated the average length of correlated domains. This length is inversely correlated with relaxation times, with small lengths corresponding to slow relaxation. This suggests that relaxation in sufficiently active nonergodic phase follows collective motion mechanisms, while cooperative motion dominates at zero and low activities. We propose that directed motion makes cage exploration less efficient and thus slows down cooperative relaxation with respect to a passive glass. We finally weight this model agaist recent simulation results.

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