Distribution of tracer particles around a catalytic Janus particle

Active Janus particles self-propel by catalyzing the decomposition of molecular “fuel” available in the surrounding solution. The resulting self-generated chemical gradients drive phoretic flow in an interfacial layer surrounding the particle, as well as chemi-osmotic interfacial flows along nearby container walls. Through experiments and theory, we consider the distribution of small tracer particles around a Janus particle in the vicinity of a planar wall. The Janus particle is either free to move or stuck to the wall, and its axis of symmetry is oriented in the plane of the wall. Experimentally, we observe that, under certain conditions, the catalytic cap of a particle is surrounded by a tracer-free exclusion zone. To understand this finding, we model the motion of tracer particles. In our model, the tracers are advected by fluid flows driven in the bulk solution by phoretic and/or chemi-osmotic interfacial flows. Additionally, the tracers can respond to chemical gradients through phoresis, providing a third contribution to tracer velocity. We find that these three ingredients can combine to create an exclusion zone. In particular, we highlight the essential role of chemi-osmotic flow, which is often neglected in modeling and interpretation of experiments.