Flow induced by bistable dynamics of soft magnetic pillars near a magnetoelastic instability.

Abstract

Soft magnetic filaments driven by external fields provide a versatile model system for nonequilibrium dynamics in active soft matter. We investigate the magnetoelastic response of elastomeric micropillars containing superparamagnetic inclusions when actuated by a rotating magnetic field. Despite nominally identical geometries and forcing conditions, individual filaments display qualitatively distinct beating patterns, ranging from smooth periodic motion to abrupt snapping and bistable oscillations. Agreement between finite-element beam modeling and analytical theory, consistent with high-speed imaging data, demonstrates that these behaviors can arise from a magnetoelastic instability governed by a dimensionless control parameter comparing magnetic and elastic torques. This model indicates that near the instability threshold, small variations in micropillar composition, geometry, or magnetic field lead to large qualitative changes in dynamics, reflecting strong imperfection sensitivity characteristic of soft-matter bifurcations. Notably, the snapping transition produces brief episodes of high filament velocity, during which local Reynolds numbers approach unity. These transient inertial excursions contribute to the breaking of time-reversal symmetry and enhance net fluid transport, alongside other geometric and dynamic asymmetries in the filament motion. These results identify magnetoelastic instability-induced inertial bursts as a route to symmetry breaking in driven soft filaments, with implications for active matter, artificial cilia, and soft robotic systems.