Magnetically Assisted Trapping of Passive Colloids by Active Dipolar Chains

Abstract: We investigate a trapping mechanism for passive Brownian particles based on mixtures with self-propelled dipolar colloids. Active dipoles, whose magnetic moment is oriented perpendicularly to their propulsion direction, spontaneously form dynamic chains that collapse into clusters through dipole-dipole interactions. These transient structures efficiently capture nearby passive particles, forming dense phases at relatively low global densities. Using Brownian dynamics simulations, we analyze how the capture efficiency depends on the P\'eclet number (Pe) and dipolar interaction strength ($\lambda$). We demonstrate that an external magnetic field, applied briefly to align the active dipoles, significantly enhances trapping efficiency, with capture rates exceeding 50% under optimal conditions. Our results reveal a nontrivial competition between activity and dipolar forces, governed by the ratio $\lambda$/Pe, and offer insights into designing self-organized trapping strategies for passive colloids.