Hot Particles Attract in a Cold Bath

TL;DR

This paper reveals a novel long-range attraction mechanism between active particles in a passive bath, driven by differences in diffusivity, which could be experimentally realized despite not aligning with typical biological swimmer parameters.

Contribution

The study introduces a new active interaction mechanism based on diffusivity differences, extending understanding of non-equilibrium self-organization in particle systems.

Findings

Long-range attraction exceeds particle size by over ten times.

Mechanism relies on hot particles creating depletion-like holes.

Potential for laboratory realization despite biological limitations.

Abstract

Controlling interactions out of thermodynamic equilibrium is crucial for designing addressable and functional self-organizing structures. These active interactions also underpin collective behavior in biological systems. Here we study a general setting of active particles in a bath of passive particles, and demonstrate a novel mechanism for long range attraction between active particles. The mechanism operates when the translational persistence length of the active particle motion is smaller than the particle diameter. In this limit, the system reduces to particles of higher diffusivity ("hot" particles) in a bath of particles with lower diffusivity ("cold" particles). This attractive interaction arises as a hot particle pushes cold particles away to create a large hole around itself, and the holes interact via a depletion-like attraction. Strikingly, the interaction range is more than an order of magnitude larger than the particle radius, well beyond the range of conventional depletion force. Although the mechanism occurs outside the parameter regime of typical biological swimmers, the mechanism could be realized in the laboratory.