  Catalytically active Janus particles suspended in solution create gradients
in the chemical composition of the solution along their surfaces, as well as
along any nearby container walls. The former leads to self-phoresis, while the
latter gives rise to chemi-osmosis, providing an additional contribution to
self-motility. Chemi-osmosis strongly depends on the molecular interactions
between the diffusing chemical species and the wall. We show analytically,
using an approximate "point-particle" approach, that by chemically patterning a
planar substrate one can direct the motion of Janus particles: the induced
chemi-osmotic flows can cause particles to either "dock" at the chemical step
between the two materials, or to follow a chemical stripe. These theoretical
predictions are confirmed by full numerical calculations. Generically, docking
occurs for particles which tend to move away from their catalytic caps, while
stripe-following occurs in the opposite case. Our analysis reveals the physical
mechanisms governing this behavior.
