Dynamics of inert spheres in active suspensions of micro-rotors

TL;DR

This study numerically explores how inert passive particles influence the collective behavior and pattern formation in suspensions of self-rotating active particles, revealing complex state transitions and dynamics.

Contribution

It introduces a detailed numerical analysis of passive particle effects on active rotor suspensions, highlighting new state behaviors and interaction dynamics.

Findings

Passive particles alter the density thresholds for different states.

Inert particles influence the formation of lanes, vortices, and crystals.

System composition critically affects inert particle dynamics.

Abstract

Inert particles suspended in active fluids of self-propelled particles are known to often exhibit enhanced diffusion and novel coherent structures. Here we numerically investigate the dynamical behavior and self-organization in a system consisting of passive and actively rotating spheres. The particles interact through direct collisions and the fluid flows generated as they move. In the absence of passive particles, three states emerge in a binary mixture of spinning spheres depending on particle fraction: a dilute gas-like state where the rotors move chaotically, a phase-separated state where like-rotors move in lanes or vortices, and a jammed state where crystals continuously assemble, melt and move (K. Yeo, E. Lushi, and P. M. Vlahovska, Phys. Rev. Lett. 114, 188301 (2015)). Passive particles added to the rotor suspension modify the system dynamics and pattern formation: while states identified in the pure active suspension still emerge, they occur at different densities and mixture proportions. The dynamical behavior of the inert particles is also non-trivially dependent on the system composition.