Generic long-range interactions between passive bodies in an active fluid

TL;DR

This paper demonstrates that passive bodies in an active fluid experience long-range, anisotropic interactions due to induced currents, leading to novel self-assembly behaviors and dynamic phenomena.

Contribution

It introduces a multipole expansion framework to characterize long-range interactions between passive bodies in active fluids, revealing their decay, anisotropy, and non-reciprocity.

Findings

Passive bodies induce currents that cause long-range interactions.

Interactions decay as a power law with distance.

Bodies can spontaneously synchronize or form bound pairs.

Abstract

Because active particles break time-reversal symmetry, a single non-spherical body placed in an active fluid generates currents. We show that when two or more passive bodies are placed in an active fluid these currents lead to long-range interactions. Using a multipole expansion we characterize their leading-order behaviors in terms of single-body properties and show that they decay as a power law with the distance between the bodies, are anisotropic, and do not obey an action--reaction principle. The interactions lead to rich dynamics of the bodies, illustrated by the spontaneous synchronized rotation of pinned non-chiral bodies and the formation of traveling bound pairs. The occurrence of these phenomena depends on tunable properties of the bodies, thus opening new possibilities for self-assembly mediated by active fluids.