  Recently there is strong experimental and theoretical interest in studying
the self-assembly and the phase behavior of patchy and of Janus particles,
which form colloidal suspensions. Although in this quest a variety of effective
interactions have been proposed and used in order to achieve directed assembly,
the critical Casimir effect stands out as being particularly suitable in this
respect because it provides both attractive and repulsive interactions as well
as the potential of a sensitive temperature control of their strength.
Specifically, we have calculated the critical Casimir force between a single
Janus particle and a laterally homogeneous substrate as well as a substrate
with a chemical step. We have used the Derjaguin approximation and compared it
with results from full mean field theory. A modification of the Derjaguin
approximation turns out to be generally reliable. Based on this approach we
have derived the effective force and the effective potential between two Janus
cylinders as well as between two Janus spheres.
