Traveling strings of active dipolar colloids

TL;DR

This paper investigates the self-assembly and dynamics of active dipolar colloids forming string-like structures in 3D, combining experiments, simulations, and analytical modeling to understand their collective behavior.

Contribution

It introduces a new active colloidal system with dipolar interactions, providing experimental, simulation, and theoretical insights into its collective dynamics and phase transitions.

Findings

Dipolar colloids form self-propelled 3D columns at low density.

An activity-dependent transition to string phases occurs with increasing dipole strength.

Long-range correlations in string fluctuations grow with active persistence time.

Abstract

We study an intriguing new type of self-assembled active colloidal polymer system in 3D. It is obtained from a suspension of Janus particles in an electric field that induces parallel dipoles in the particles as well as self-propulsion in the plane perpendicular to the field. At low packing fractions, in experiment, the particles self-assemble into 3D columns that are self-propelled in 2D. Explicit numerical simulations combining dipolar interactions and active self-propulsion find an activity dependent transition to a string phase by increasing dipole strength. We classify the collective dynamics of strings as a function of rotational and translational diffusion. Using an anisotropic version of the Rouse model of polymers with active driving, we analytically compute the strings' collective dynamics and centre of mass motion, which matches simulations and is consistent with experimental data. We also discover long range correlations of the fluctuations along the string contour that grow with the active persistence time, a purely active effect that disappears in the thermal limit.