Confinement geometry regulates fluctuations, densities, and collective escape in interacting active matter

Abstract

Active matter, characterized by nonequilibrium signatures such as the violation of the fluctuation–dissipation theorem, giant number fluctuations, and non-Gaussian velocity distributions, is typically studied in bulk systems or trapped single-particle systems. Here, we investigate how the steady-state behavior of multiple sterically interacting active Ornstein–Uhlenbeck particles (AOUPs) confined within a double-well potential is influenced by the particle number and the convexity and concavity of the trapping potential. We find that increasing the particle number in our confined system leads to suppression of nonequilibrium fluctuations due to crowding unlike bulk active systems where fluctuations are enhanced at large particle numbers (Giant Number Fluctuations). This results in the steady-state density profiles of the AOUP system increasingly resembling those of passive systems for large particle numbers. However, the instantaneous density even at large particle numbers exhibits both unimodal and bimodal distributions, giving rise to large density fluctuations that are absent in passive systems, where the density remains bimodal. In the convex regions of the potential, both the velocity and single-particle position distributions are unimodal and Gaussian, whereas they become bimodal in the concave regions. Confinement suppresses long-range correlations while generating localized fluctuations that depend sensitively on the potential geometry: velocity fluctuations are suppressed in the high-density convex regions and are enhanced near the concave barrier, where they exhibit pronounced nonequilibrium characteristics. At large persistence times, collective effects emerge in the form of correlated barrier crossings and strong velocity-alignment correlations, features that are absent in passive systems. These findings demonstrate how confinement geometry, particle number, and activity cooperatively regulate the spatial organization of nonequilibrium fluctuations in active matter.