Active Janus colloids at chemically structured surfaces.

If catalytically active Janus particles are dispersed in certain liquid solutions, they can create a gradient in the chemical composition of this solution along their surfaces, as well as along any nearby confining surfaces. This gradient drives self-propulsion via a self-phoretic mechanism, while the compositional gradient along a wall gives rise to chemiosmosis, which additionally contributes to self-motility. In this study, we analyze theoretically the dynamics of an active colloid near chemically patterned walls. We use a point-particle approximation combined with a multipole expansion in order to discuss the effects of pattern geometry and chemical contrast on the particle trajectories. In particular, we consider planar walls patterned with chemical steps and stripes. We investigate in detail the changes in the topology of the corresponding phase portraits upon varying the chemical contrast and the stripe width.