Gravitaxis of asymmetric self-propelled colloidal particles

TL;DR

This study demonstrates that shape asymmetry alone can induce gravitactic motion in self-propelled colloidal particles, leading to upward or downward swimming and complex trajectories, without the need for inhomogeneous mass distribution.

Contribution

It provides experimental and theoretical evidence that geometric asymmetry can cause gravitaxis in self-propelled particles, a phenomenon previously attributed mainly to mass distribution.

Findings

Shape anisotropy induces gravitactic motion.

Particles exhibit upward or downward swimming.

Trochoid-like trajectories observed.

Abstract

Many motile microorganisms adjust their swimming motion relative to the gravitational field and thus counteract sedimentation to the ground. This gravitactic behavior is often the result of an inhomogeneous mass distribution which aligns the microorganism similar to a buoy. However, it has been suggested that gravitaxis can also result from a geometric fore-rear asymmetry, typical for many self-propelling organisms. Despite several attempts, no conclusive evidence for such an asymmetry-induced gravitactic motion exists. Here, we study the motion of asymmetric self-propelled colloidal particles which have a homogeneous mass density and a well-defined shape. In experiments and by theoretical modeling we demonstrate that a shape anisotropy alone is sufficient to induce gravitactic motion with either preferential upward or downward swimming. In addition, also trochoid-like trajectories transversal to the direction of gravity are observed.