Stress-Boundary-Memory Feedback Drives Vortical-Polar Transitions in Softly Confined Active Matter

Abstract

We computationally investigate how environmental sensitivity of active matter interacts with soft confinement to shape collective dynamics. In our model, the active constituents are represented as self-propelled particles (SPPs), implemented as nematic, disjoint ring polymers whose direction of motion can reverse without tumbling. Coarse-grained molecular dynamics simulations reveal that collective dynamics arise from a three-way feedback between active stresses, boundary elasticity, and particle-level memory. With increasing driving force, FD, this feedback generates a sequence of collective dynamical regimes. At low FD, SPP motion is dominated by thermal fluctuations. At intermediate FD, coherent vortical motion emerges with intermittent, noise-driven reversals. With further increase in FD, reversals are suppressed, yielding sustained unidirectional vortical motion. At sufficiently high FD, the system transitions to a polar state characterized by strong nematic ordering of the SPPs, symmetry breaking of the enclosure shape, and persistent polar collective motion. In this regime, the SPPs accumulate at the leading edge of the enclosure, driving sustained ballistic propulsion. These results demonstrate how environmental sensitivity and soft confinement jointly regulate emergent collective states and identify boundary elasticity as a control parameter governing the balance between vortical and ballistic dynamics.