Active string fluids and gels formed by dipolar active Brownian particles in 3D

Area of Science:

• Soft Matter Physics
• Active Matter Systems
• Statistical Mechanics

Background:

• Self-propelled particles with magnetic dipoles are found in nature (magnetotactic bacteria) and engineered systems (active colloids, micro-robots).
• Understanding the combined effects of self-propulsion and magnetic dipole interactions on three-dimensional (3D) self-assembly is crucial but remains limited.

Purpose of the Study:

• To investigate the dynamic self-assembly of active dipolar particles in 3D.
• To elucidate the influence of self-propulsion (activity) and magnetic dipole interactions on particle aggregation and dynamics.

Main Methods:

• Brownian dynamics simulations were employed to model active dipolar particles in a 3D low-density regime.
• The study systematically varied dipolar coupling strength and active force magnitude.

Main Results:

• At high activity, self-propulsion overcomes dipolar attractions, preventing chain and network formation.
• Moderate activity leads to the formation of active fluids with chains and rings.
• Strong dipolar coupling combined with moderate activity results in an active gel, a percolated network of active chains with enhanced particle diffusion.

Conclusions:

• A state diagram was developed, mapping the system's behavior based on dipolar coupling and activity levels.
• Particle activity significantly influences aggregate topology, transitioning structures from branched to discrete chains and rings.
• Active dipolar systems exhibit enhanced translational and rotational diffusion compared to passive counterparts.