Interactions in Active Colloids

TL;DR

This paper reviews the complex interactions among active colloids, including hydrodynamic, phoretic, and osmotic effects, highlighting their long-range, non-reciprocal nature and importance for designing nonequilibrium materials.

Contribution

It provides a comprehensive overview of the current understanding of active colloid interactions and emphasizes open questions and challenges in the field.

Findings

Active colloids exhibit long-range, non-instantaneous interactions.

Interactions include hydrodynamic, phoretic, and osmotic effects.

Understanding these interactions is key for designing nonequilibrium colloidal materials.

Abstract

The past two decades have seen a remarkable progress in the development of synthetic colloidal agents which are capable of creating directed motion in an unbiased environment at the microscale. These self-propelling particles are often praised for their enormous potential to self-organize into dynamic nonequilibrium structures such as living clusters, synchronized super-rotor structures or self-propelling molecules featuring a complexity which is rarely found outside of the living world. However, the precise mechanisms underlying the formation and dynamics of many of these structures are still barely understood, which is likely to hinge on the gaps in our understanding of how active colloids interact. In particular, besides showing comparatively short-ranged interactions which are well known from passive colloids (Van der Waals, electrostatic etc.), active colloids show novel hydrodynamic interactions as well as phoretic and substrate-mediated "osmotic" cross-interactions which hinge on the action of the phoretic field gradients which are induced by the colloids on other colloids in the system. The present article discusses the complexity and the intriguing properties of these interactions which in general are long-ranged, non-instantaneous, non-pairwise and non-reciprocal and which may serve as key ingredients for the design of future nonequilibrium colloidal materials. Besides providing a brief overview on the state of the art of our understanding of these interactions a key aim of this review is to emphasize open key questions and corresponding open challenges.