78th Annual Meeting of the APS Division of Fluid Dynamics (Nov 23 — 25, 2025)

V043: Active Brownian Particles in Circular Confinement

Brownian motion provides a classical stochastic framework for the thermal fluctuations of microscopic particles suspended in a fluid. Active Brownian Particle (ABP) theory generalizes this framework to self-propelled microswimmers-such as motile bacteria, algae, or synthetic Janus colloids-whose persistent propulsion drives motion while random reorientation, e.g. rotational diffusion or tumbling, introduces stochasticity. However, how confinement and microscopic boundary physics reshape the balance between persistence and noise-and thereby control transport, accumulation, and residence times-remains poorly understood. Here, we investigate ABPs confined to a circular domain of radius R, moving at speed U with rotational diffusivity Dr acting solely on their heading. In open space, the dynamics exhibit a rotational decorrelation time τdiff ∼ 1/Dr: for t « τdiff, trajectories remain anisotropic due to memory of the initial heading direction, while for t » τdiff the system recovers diffusive behaviour. Confinement introduces an advective time τadv ∼ R/U, giving a natural Péclet number Pe = U/(RDr). We compare two wall interactions: (i) reflective boundaries that enforce mirror-like reflections, and (ii) sticky boundaries that transiently trap particles before reorienting. At low Pe, strong angular noise suppresses boundary effects, producing near-uniform steady-state densities insensitive to boundary physics. At high Pe, persistence amplifies boundary physics: reflective walls support rapid circulation with brief contact times, whereas sticky walls yield clustering and extended residence. These results, supported by high-resolution simulations and video visualizations, reveal how confinement, persistence, and noise conspire to regulate active-particle transport and steady-state organization, providing a minimal physical model for boundary-controlled phenomena in active matter.