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, with a directional persistence controlled by the driving force, <i>F</i>_D, and a persistence time scale, <i>τ</i>_m. Coarse-grained molecular dynamics simulations of these reversal-capable SPPs confined within a deformable two-dimensional enclosure reveal that the collective dynamics arise from a three-way feedback between active stresses, boundary elasticity, and particle-level memory. With increasing <i>F</i>_D, this stress-boundary-memory feedback generates a sequence of collective dynamical regimes. At low <i>F</i>_D, SPP motion is dominated by thermal fluctuations and activity plays a negligible role. At intermediate <i>F</i>_D, coherent vortical motion emerges with intermittent, noise-driven reversals. The frequency of reversals is modulated by boundary elasticity and <i>τ</i>_m, and their occurrence coincides with transient coherent polar motion. With further increase in <i>F</i>_D, reversals are suppressed, yielding sustained unidirectional vortical motion in which the enclosure exhibits diffusive propulsion with a diffusivity that varies non-monotonically with <i>F</i>_D. At sufficiently high <i>F</i>_D, 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, deforming it into an anisotropic shape and driving sustained ballistic propulsion of the enclosure with a slowly drifting direction. These results demonstrate how environmental sensitivity and soft confinement jointly regulate emergent collective states of confined active matter and identify boundary elasticity as a control parameter governing the balance between vortical and ballistic dynamics.