  The interactions of autonomous microswimmers play an important role for the
formation of collective states of motile active matter. We study them in detail
for the common microswimmer-design of two-faced Janus spheres with hemispheres
made from different materials. Their chemical and physical surface properties
may be tailored to fine-tune their mutual attractive, repulsive or aligning
behavior. To investigate these effects systematically, we monitor the dynamics
of a single gold-capped Janus particle in the external temperature field
created by an optically heated metal nanoparticle. We quantify the
orientation-dependent repulsion and alignment of the Janus particle and explain
it in terms of a simple theoretical model for the induced thermoosmotic surface
fluxes. The model reveals that the particle's angular velocity is solely
determined by the temperature profile on the equator between the Janus
particle's hemispehres and their phoretic mobility contrast. The distortion of
the external temperature field by their heterogeneous heat conductivity is
moreover shown to break the apparent symmetry of the problem.
