Tuning the motility and directionality of self-propelled colloids

Juan Ruben Gomez-Solano,Pablo Ruedas-Batuecas,René Van Roij,Clemens Bechinger,Sela Samin,Celia Lozano

doi:10.1038/s41598-017-14126-0

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.

Highlights

Locomotion at the microscale is an intriguing non-equilibrium phenomenon that has recently attracted a great deal of attention in soft matter physics[1,2,3] and applied science[4,5]
Catalytic colloids made of inert polystyrene or SiO2 and with a partial active Pt-coating can self-propel in aqueous H2O2 solutions by diffusiophoresis either toward or away from the active site[21,22,23], where the swimming direction can be strongly affected by the detailed particle
The particles are suspended in a binary mixture of water and propylene glycol n-propyl ether (PnP), whose lower critical point is Tc = 31.9 °C and 0.4 PnP mass fraction[34], and its viscosity at 25° is ηf = 0.004 Pa s

Summary

Introduction

Locomotion at the microscale is an intriguing non-equilibrium phenomenon that has recently attracted a great deal of attention in soft matter physics[1,2,3] and applied science[4,5]. Microorganisms, such as bacteria and algal cells, are capable to propel themselves through viscous liquids despite the inertialess flows they create and the random thermal collisions with the surrounding fluid molecules Their motility is achieved by means of internal biochemical processes that allow them to break the time-reversibility at extremely low Reynolds numbers, e.g. by performing non-reciprocal flagellar beating and rotation, converting energy into directed motion with a well-defined polarity[6]. In agreement with numerical calculations, we demonstrate that such an intensity threashold linearly depends on the inverse of the particle radius and on the environmental temperature This non-monotonic dependence of the propulsion speed enables a change in the swimming directionality that can be reversibly varied by means of laser illumination. These unique features allows us to experimentally realize in a straightforward manner both positive and negative phototaxis, i.e. the ability of these synthetic microswimmers to sense a light gradient and to move toward or away from it, respectively

Methods

Results

Conclusion