Experimental observation of flow fields around active Janus spheres

Andrew I Campbell,Pierre Illien,Stephen J Ebbens,Ramin Golestanian

doi:10.1038/s41467-019-11842-1

Abstract

The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years. In particular, the importance of electrokinetic effects on the motility of Pt-PS Janus sphere was recently discussed. Here, we probe the hydrodynamic flow field around a catalytically active colloid using particle tracking velocimetry both in the freely swimming state and when kept stationary with an external force. Our measurements provide information about the fluid velocity in the vicinity of the surface of the colloid, and confirm a mechanism for propulsion that was proposed recently. In addition to offering a unified understanding of the nonequilibrium interfacial transport processes at stake, our results open the way to a thorough description of the hydrodynamic interactions between such active particles and understanding their collective dynamics.

Highlights

The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years
An optical cuvette filled with a mixture of tracer particles and Pt-coated Janus colloids was prepared, and the tracer particles at the top of the cuvette were brought into focus
We note that focussed at the top of the cuvette the bulk of the tracer particles are not in focus. This provided a background level of illumination that enabled us to image the non-fluorescent Janus spheres, where they appeared as a transparent disc with a shadow covering one-half of the disc indicating the position and rotation of the Pt cap. For both the moving and stuck colloids the Janus particle appeared as a “half-moon” shape consistent with the near surface alignment effect reported in ref

Summary

Introduction

The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years. We recently proposed a mechanism where the motility of the Pt-PS sphere is linked to closed current loops, which start at the equator of the particle and end at its pole (Fig. 1a) These conjectured closed proton loops are expected to yield a strongly asymmetric slip velocity across the surface of the colloid[13,16]. Control experiments conducted near a symmetrical catalytically active Pt colloid showed that the tracer motion is dominated by fluid flow rather than chemical gradients This method allowed us to arrive at average flow fields derived from observations of many different Pt-PS Janus particles

Methods

Results

Conclusion