Non-reciprocal interactions drive emergent chiral crystallites

TL;DR

This paper demonstrates that non-reciprocal interactions among achiral active colloids can lead to the spontaneous emergence of chiral phases and collective rotation, revealing new mechanisms for non-equilibrium self-organization.

Contribution

It introduces a field theory capturing non-reciprocal coupling of broken symmetries and experimentally shows emergent chiral phases in active colloids with controlled rotation.

Findings

Achiral active colloids form rotating hexatic and polar clusters.

Non-reciprocal interactions induce chiral phases with counter-rotating vortices.

External electric fields control cluster size and dynamics.

Abstract

We study a new type of 2D active material that exhibits macroscopic phases with two emergent broken symmetries: self-propelled achiral particles that form dense hexatic clusters, which spontaneously rotate. We experimentally realise active colloids that self-organise into both polar and hexatic crystallites, exhibiting exotic emergent phenomena. This is accompanied by a field theory of coupled order parameters formulated on symmetry principles, including non-reciprocity, to capture the non-equilibrium dynamics. We find that the presence of two interacting broken symmetry fields leads to the emergence of novel chiral phases built from (2D) achiral active colloids (here Quincke rollers). These phases are characterised by the presence of both clockwise and counterclockwise rotating clusters. We thus show that spontaneous rotation can emerge in non-equilibrium systems, even when the building blocks are achiral, due to non-reciprocally coupled broken symmetries. This interplay leads to self-organized stirring through counter-rotating vortices in confined colloidal systems, with cluster size controlled by external electric fields.