  Colloids immersed in a critical or near-critical binary liquid mixture and
close to a chemically patterned substrate are subject to normal and lateral
critical Casimir forces of dominating strength. For a single colloid we
calculate these attractive or repulsive forces and the corresponding critical
Casimir potentials within mean-field theory. Within this approach we also
discuss the quality of the Derjaguin approximation and apply it to Monte Carlo
simulation data available for the system under study. We find that the range of
validity of the Derjaguin approximation is rather large and that it fails only
for surface structures which are very small compared to the geometric mean of
the size of the colloid and its distance from the substrate. For certain
chemical structures of the substrate the critical Casimir force acting on the
colloid can change sign as a function of the distance between the particle and
the substrate; this provides a mechanism for stable levitation at a certain
distance which can be strongly tuned by temperature, i.e., with a sensitivity
of more than 200nm/K.
