Active string fluids and gels formed by dipolar active Brownian particles in 3D
arxiv(2024)
摘要
Self-propelled particles possessing permanent magnetic dipole moments occur
naturally in magnetotactic bacteria and in man-made systems like active
colloids or micro-robots. Yet, the interplay between self-propulsion and
anisotropic dipole-dipole interactions on dynamic self-assembly in three
dimensions (3D) remains poorly understood. We conduct Brownian dynamics
simulations of active dipolar particles in 3D, focusing on the low-density
regime, where dipolar hard spheres tend to form chain-like aggregates and
percolated networks with increasing dipolar coupling strength. We find that
strong active forces override dipolar attractions, effectively inhibiting
chain-like aggregation and network formation. Conversely, activating particles
with low to moderate forces results in a fluid composed of active chains and
rings. At strong dipolar coupling strengths, this active fluid transitions into
an active gel, consisting of a percolated network of active chains. Although
the overall structure of the active gel remains interconnected, the network
experiences more frequent configurational rearrangements due to the reduced
bond lifetime of active dipolar particles. Consequently, particles exhibit
enhanced translational and rotational diffusion within the active fluid of
strings and active gels compared to their passive counterparts. We quantify the
influence of activity on aggregates topology, as they transition from branched
structures to unconnected chains and rings. Our findings are summarized in a
state diagram, delineating the impact of dipolar coupling strength and active
force magnitude on the system.
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