Floor- or ceiling-sliding for chemically active, gyrotactic, sedimenting Janus particles

Chemically active particles achieve force- and torque-free motility via catalytic chemical reactions promoted on parts of their surface. These lead to inhomogeneity in the chemical composition of the solution ("chemical field") and hydrodynamic flow of the solution. By means of coupling distortions of these fields back to its motion, a chemically active particle experiences effective interactions with boundaries; this can lead to the occurrence of, e.g., states of steady "sliding" along a wall.

The often employed Janus spherical particles are density mismatched with the solution and, additionally, gyrotactic ("bottom-heavy"); the latter promotes alignment of the axis orthogonal to a horizontal wall. It is thus unclear under which conditions sliding states for such particles may occur. Here we study this issue theoretically for model gyrotactic, self-phoretic Janus spheres near horizontal planar walls which are either below ("floor") or above ("ceiling") the particle. We construct "state diagrams" as a function of the sedimentation velocity and gyrotatic response of the particle. These show that in certain cases sliding states may emerge simultaneously at both the ceiling and the floor. The predictions are critically compared with experimental results.