Nonreciprocal collective dynamics in a mixture of phoretic Janus colloids

TL;DR

This paper investigates the collective behaviors of a binary mixture of Janus colloids with nonreciprocal interactions, revealing instabilities and deriving a microscopic model that captures their complex dynamics.

Contribution

It introduces a continuum theory linking microscopics to collective behavior in Janus colloids, and derives a nonreciprocal Cahn-Hilliard model for active mixtures.

Findings

Unstable unpatterned states due to various instabilities including oscillatory ones.

Derivation of a microscopic Nonreciprocal Cahn-Hilliard model.

Janus colloids serve as a versatile platform for active mixture studies.

Abstract

A multicomponent mixture of Janus colloids with distinct catalytic coats and phoretic mobilities is a promising theoretical system to explore the collective behavior arising from nonreciprocal interactions. An active colloid produces (or consumes) chemicals, self-propels, drifts along chemical gradients, and rotates its intrinsic polarity to align with a gradient. As a result the connection from microscopics to continuum theories through coarse-graining couples densities and polarization fields in unique ways. Focusing on a binary mixture, we show that these couplings render the unpatterned reference state unstable to small perturbations through a variety of instabilities including oscillatory ones which arise on crossing an exceptional point or through a Hopf bifurcation. For fast relaxation of the polar fields, they can be eliminated in favor of the density fields to obtain a microscopic realization of the Nonreciprocal Cahn-Hilliard model for two conserved species with two distinct sources of non-reciprocity, one in the interaction coefficient and the other in the interfacial tension. Our work establishes Janus colloids as a versatile model for a bottom-up approach to both scalar and polar active mixtures.