Universal hydrodynamic mechanisms for crystallization in active colloidal suspensions

TL;DR

This paper reveals that hydrodynamic interactions in active colloidal suspensions lead to crystallization through a spinodal-like instability, with stability influenced by bottom-heaviness and chiral activity, providing a universal mechanism.

Contribution

It introduces a universal hydrodynamic mechanism for crystallization in active colloids, highlighting the role of viscous flow and stability factors unlike equilibrium processes.

Findings

Crystallization driven by hydrodynamic forces near walls.

Crystals form via a spinodal-like instability, not nucleation.

Stability can be achieved with bottom-heaviness or chiral activity.

Abstract

The lack of detailed balance in active colloidal suspensions allows dissipation to determine stationary states. Here we show that slow viscous flow produced by polar or apolar active colloids near plane walls mediates attractive hydrodynamic forces that drive crystallization. Hydrodynamically mediated torques tend to destabilize the crystal but stability can be regained through critical amounts of bottom-heaviness or chiral activity. Numerical simulations show that crystallization is not nucleational, as in equilibrium, but is preceded by a spinodal-like instability. Harmonic excitations of the active crystal relax diffusively but the normal modes are distinct from an equilibrium colloidal crystal. The hydrodynamic mechanisms presented here are universal and rationalize recent experiments on the crystallization of active colloids.