Active colloidal chains with cilia- and flagella-like motion

TL;DR

This paper demonstrates that simple active colloidal chains can exhibit cilia- and flagella-like beating and propulsion through non-reciprocal interactions and symmetry breaking, highlighting a general mechanism in active matter.

Contribution

It introduces a minimal model of active colloidal chains that self-organize into beating and propelling motions due to non-reciprocal forces and asymmetry, revealing fundamental principles of active matter behavior.

Findings

Chains exhibit sustained oscillations and traveling wave patterns.

Net force on the anchor vanishes before bifurcation and increases afterward.

Chains can self-propel like flagella when asymmetry mimics cargo loading.

Abstract

It has been shown that self-assembled chains of active colloidal particles can present sustained oscillations. These oscillations are possible because of the effective diffusiophoretic forces that mediate the interactions of colloids do not respect the action--reaction principle and hence, a Hopf bifurcation is possible even for overdamped dynamics. Anchoring the particles in one extreme breaks the head-tail symmetry and the oscillation is transformed into a traveling wave pattern and thus the chain behaves like a beating cilium. The net force on the anchor, estimated using the resistive force theory, vanishes before the bifurcation and thereafter grows linearly with the bifurcation parameter. If the mobilities of the particles on one extreme are reduced to mimic an elongated cargo, the traveling wave generates a net velocity on the chain that now behaves like a moving flagellum. The average velocity again grows linearly with the bifurcation parameter. Our results demonstrate that simplified systems, consisting only of a few particles with non-reciprocal interaction and head-tail asymmetry show beating motion and self-propulsion. Both properties are present in many non-equilibrium models thus making our results a general feature of active matter.