Non-equilibrium phase separation in mixtures of catalytically active particles: size dispersity and screening effects

TL;DR

This paper analytically investigates non-equilibrium phase separation in mixtures of catalytically-active particles, revealing effects of size dispersity and screening, with implications for biological condensates and cellular organization.

Contribution

It introduces a model capturing non-reciprocal interactions and phase behaviors in active particle mixtures, emphasizing the role of activity dependence and size dispersity.

Findings

Screening of interactions depends on Michaelis-Menten kinetics.

Mixtures of differently-sized particles can show phase separation with oscillations.

A detailed phase diagram for stability of active particle mixtures.

Abstract

Biomolecular condensates in cells are often rich in catalytically-active enzymes. This is particularly true in the case of the large enzymatic complexes known as metabolons, which contain different enzymes that participate in the same catalytic pathway. One possible explanation for this self-organization is the combination of the catalytic activity of the enzymes and a chemotactic response to gradients of their substrate, which leads to a substrate-mediated effective interaction between enzymes. These interactions constitute a purely non-equilibrium effect and show exotic features such as non-reciprocity. Here, we analytically study a model describing the phase separation of a mixture of such catalytically-active particles. We show that a Michaelis-Menten-like dependence of the particles' activities manifests itself as a screening of the interactions, and that a mixture of two differently-sized active species can exhibit phase separation with transient oscillations. We also derive a rich stability phase diagram for a mixture of two species with both concentration-dependent activity and size dispersity. This work highlights the variety of possible phase separation behaviours in mixtures of chemically-active particles, which provides an alternative pathway to the passive interactions more commonly associated with phase separation in cells. Our results highlight non-equilibrium organizing principles that can be important for biologically relevant liquid-liquid phase separation.