  The force and torque on a Janus sphere moving in a rarefied gas with a
thermal gradient are calculated. The regime of large Knudsen number is
considered, with the momenta of impinging gas molecules either obtained from a
Chapman-Enskog distribution or from a binary Maxwellian distribution between
two opposing parallel plates at different temperature. The reflection
properties at the surface of the Janus particle are characterized by
accommodation coefficients having constant but dissimilar values on each
hemisphere. It is shown that the Janus particle preferentially orients such
that the hemisphere with a larger accommodation coefficient points towards the
lower temperature. The thermophoretic velocity of the particle is computed, and
the influence of the thermophoretic motion on the magnitude of the torque
responsible for the particle orientation is studied. The analytical
calculations are supported by Direct Simulation Monte Carlo results, extending
the scope of the study towards smaller Knudsen numbers. The results shed light
on the efficiency of oriented deposition of nanoparticles from the gas phase
onto a cold surface.
