  Gold-capped Janus particles immersed in a near-critical binary mixture can be
propelled using illumination. We employ a non-isothermal diffuse interface
approach to investigate the self-propulsion mechanism of a single colloid. We
attribute the motion to body forces at the edges of a micronsized droplet that
nucleates around the particle. Thus, the often-used concept of a surface
velocity cannot account for the self-propulsion. The particle's swimming
velocity is related to the droplet shape and size, which is determined by a
so-called critical isotherm. Two distinct swimming regimes exist, depending on
whether the droplet partially or completely covers the particle. Interestingly,
the dependence of the swimming velocity on temperature is non-monotonic in both
regimes.
