Can the self-propulsion of anisotropic microswimmers be described by using forces and torques?

TL;DR

This paper demonstrates that the motion of anisotropic microswimmers can be effectively described using forces and torques, despite their force-free nature, by mapping their dynamics onto passive particle models with shape-dependent resistance matrices.

Contribution

It shows that microswimmer equations of motion can be mapped onto passive particle models with effective forces and torques, clarifying their use in modeling active particle dynamics.

Findings

Effective forces and torques can describe microswimmer motion.

The mapping aligns with experimental observations of circular motion.

Limitations arise when interactions disturb the self-propulsion mechanism.

Abstract

The self-propulsion of artificial and biological microswimmers (i.e., active colloidal particles) has often been modelled by using a force and a torque entering into the overdamped equations for the Brownian motion of passive particles. This seemingly contradicts the fact that a swimmer is force-free and torque-free, i.e., that the net force and torque on the particle vanish. Using different models for mechanical and diffusiophoretic self-propulsion, we demonstrate here that the equations of motion of microswimmers can be mapped onto those of passive particles with the shape-dependent grand resistance matrix and formally external effective forces and torques. This is consistent with experimental findings on the circular motion of artificial asymmetric microswimmers driven by self-diffusiophoresis. The concept of effective self-propulsion forces and torques significantly facilitates the understanding of the swimming paths, e.g., for a microswimmer under gravity. However, this concept has its limitations when the self-propulsion mechanism of a swimmer is disturbed either by another particle in its close vicinity or by interactions with obstacles, such as a wall.