Tuning the motility and directionality of self-propelled colloids

TL;DR

This study demonstrates how laser-induced forces and wetting properties can be used to precisely control the speed and direction of self-propelled colloids, enabling reversible switching and phototaxis in microswimmers.

Contribution

It introduces a method to tune both the motility and directionality of colloidal microswimmers using local body forces and droplet formation, with experimental and numerical validation.

Findings

Direction of motion can be reversibly switched.

Colloid speed and directionality depend on droplet size and shape.

Enables realization of positive and negative phototaxis.

Abstract

Microorganisms are able to overcome the thermal randomness of their surroundings by harvesting energy to navigate in viscous fluid environments. In a similar manner, synthetic colloidal microswimmers are capable of mimicking complex biolocomotion by means of simple self-propulsion mechanisms. Although experimentally the speed of active particles can be controlled by e.g. self-generated chemical and thermal gradients, an in-situ change of swimming direction remains a challenge. In this work, we study self-propulsion of half-coated spherical colloids in critical binary mixtures and show that the coupling of local body forces, induced by laser illumination, and the wetting properties of the colloid, can be used to finely tune both the colloid's swimming speed and its directionality. We experimentally and numerically demonstrate that the direction of motion can be reversibly switched by means of the size and shape of the droplet(s) nucleated around the colloid, depending on the particle radius and the fluid's ambient temperature. Moreover, the aforementioned features enable the possibility to realize both negative and positive phototaxis in light intensity gradients. Our results can be extended to other types of half-coated microswimmers, provided that both of their hemispheres are selectively made active but with distinct physical properties.