Mesoscopic Turbulence and Local Order in Janus Particles Self-Propelling under an AC Electric Field

TL;DR

This study investigates mesoscopic turbulence in dense suspensions of Janus particles under an AC electric field, revealing local order, vortex formation, and energy cascades in active colloidal systems.

Contribution

It experimentally demonstrates the coexistence of polar and anti-parallel alignments leading to turbulence in Janus particles, highlighting mechanisms of energy transfer and local clustering.

Findings

Velocity fields exhibit turbulence with vortices and abrupt velocity changes.

Correlation functions show coexistence of polar and anti-parallel alignments.

Energy spectrum peaks at scales associated with local clusters and alignment.

Abstract

To elucidate mechanisms of mesoscopic turbulence exhibited by active particles, we experimentally study turbulent states of non-living self-propelled particles. We realize an experimental system with dense suspensions of asymmetrical colloidal particles (Janus particles) self-propelling on a two-dimensional surface under an AC electric field. Velocity fields of the Janus particles in the crowded situation can be regarded as a sort of turbulence because it contains many vortices and their velocities change abruptly. Correlation functions of their velocity field reveal the coexistence of polar alignment and anti-parallel alignment interactions, which is considered to trigger mesoscopic turbulence. Probability distributions of local order parameters for polar and nematic orders indicate the formation of local clusters with particles moving in the same direction. A broad peak in the energy spectrum of the velocity field appears at the spatial scales where the polar alignment and the cluster formation are observed. Energy is injected at the particle scale and such conserved quantity as energy could be cascading toward the larger clusters.