  Recent experiments have demonstrated a fluctuation-induced lateral trapping
of spherical colloidal particles immersed in a binary liquid mixture near its
critical demixing point and exposed to chemically patterned substrates.
Inspired by these experiments, we study this kind of effective interaction,
known as the critical Casimir effect, for elongated colloids of cylindrical
shape. This adds orientational degrees of freedom. When the colloidal particles
are close to a chemically structured substrate, a critical Casimir torque
acting on the colloids emerges. We calculate this torque on the basis of the
Derjaguin approximation. The range of validity of the latter is assessed via
mean-field theory. This assessment shows that the Derjaguin approximation is
reliable in experimentally relevant regimes, so that we extend it to Janus
particles endowed with opposing adsorption preferences. Our analysis indicates
that critical Casimir interactions are capable of achieving well-defined,
reversible alignments both of chemically homogeneous and of Janus cylinders.
