  Surface bound catalytic chemical reactions self-propel chemically active
Janus particles. In the vicinity of boundaries, these particles exhibit rich
behavior, such as the occurrence of wall-bound steady states of "sliding". Most
active particles tend to sediment as they are density mismatched with the
solution. Moreover Janus spheres, which consist of an inert core material
decorated with a cap-like, thin layer of a catalyst, are gyrotactic
("bottom-heavy"). Occurrence of sliding states near the horizontal walls
depends on the interplay between the active motion and the gravity-driven
sedimentation and alignment. It is thus important to understand and quantify
the influence of these gravity-induced effects on the behavior of model
chemically active particles moving near walls. For model gyrotactic,
self-phoretic Janus particles, here we study theoretically the occurrence of
sliding states at horizontal planar walls that are either below ("floor") or
above ("ceiling") the particle. We construct "state diagrams" characterizing
the occurrence of such states as a function of the sedimentation velocity and
of the gyrotactic response of the particle, as well as of the phoretic mobility
of the particle. We show that in certain cases sliding states may emerge
simultaneously at both the ceiling and the floor, while the larger part of the
experimentally relevant parameter space corresponds to particles that would
exhibit sliding states only either at the floor or at the ceiling or there are
no sliding states at all. These predictions are critically compared with the
results of previous experimental studies and our experiments conducted on
Pt-coated polystyrene and silica-core particles suspended in aqueous hydrogen
peroxide solutions.
