Activity induced delocalization and freezing in self-propelled systems

TL;DR

This paper investigates how activity time influences particle distribution and phase behavior in active particle systems confined by anharmonic potentials, revealing delocalization, reentrant freezing, and the effects of interactions.

Contribution

It introduces a detailed study of activity-induced phenomena in active particles with repulsive interactions, including phase diagrams and analytical density profile predictions.

Findings

Increasing activity time pushes particles away from the potential minimum.

Interaction induces liquid- or solid-like structures without suppressing delocalization.

Reentrant behavior: activity initially fluidizes, then induces freezing at higher values.

Abstract

We study a system of interacting active particles, propelled by colored noises, characterized by an activity time {\tau}, and confined by a single-well anharmonic potential. We assume pair-wise repulsive forces among particles, modelling the steric interactions among microswimmers. This system has been experimentally studied in the case of a dilute suspension of Janus particles confined through acoustic traps. We observe that already in the dilute regime - when inter-particle interactions are negligible - increasing the persistent time pushes the particles away from the potential minimum, until a saturation distance is reached. We compute the phase diagram (activity versus interaction length), showing that the interaction does not suppress this delocalization phenomenon but induces a liquid- or solid-like structure in the densest regions. Interestingly a reentrant behavior is observed: a first increase of {\tau} from small values acts as an effective warming, favouring fluidization; at higher values, when the delocalization occurs, a further increase of {\tau} induces freezing inside the densest regions. An approximate analytical scheme gives fair predictions for the density profiles in the weakly interacting case. The analysis of non-equilibrium heat fluxes reveals that in the region of largest particle concentration equilibrium is restored in several aspects.