Hydrodynamic assembly of active colloids: chiral spinners and dynamic crystals

TL;DR

This study uses large-scale simulations to explore how hydrodynamic interactions and external fields influence the self-assembly of active colloids into chiral spinners and dynamic crystals, revealing the role of fluid flow in chiral transfer and structural topology.

Contribution

It demonstrates how hydrodynamic interactions and a gravity-like field enable tunable self-assembly of active colloids into chiral and crystalline structures, highlighting fluid flow's role in chiral transfer.

Findings

Puller-driven particles form small chiral spinners.

Pushers assemble into large dynamic aggregates.

Chirality controls the topology of self-assembled structures.

Abstract

Active colloids self-organise to a variety of collective states, ranging from highly motile 'molecules' to complex 3D structures. Using large-scale simulations, we show that hydrodynamic interactions, together with a gravity-like aligning field, lead to tunable self-assembly of active colloidal spheres near a surface. The observed structures depend on the hydrodynamic characteristics: particles driven at the front, pullers, form small chiral spinners consisting of two or three particles, whereas those driven at the rear, pushers, assemble to large dynamic aggregates. The rotational motion of the puller spinners, arises from spontaneous breaking of the internal chirality. Our results show that the fluid flow mediates chiral transfer between neighboring spinners. Finally we show that the chirality of the individual spinners controls the topology of the self-assembly in solution: homochiral samples assemble into a hexagonally symmetric 2D crystal lattice while racemic mixtures show reduced hexatic order with diffusion-like dynamics.