  We discuss the non-equilibrium statistical mechanics of a thermally driven
micromachine consisting of three spheres and two harmonic springs [Y. Hosaka et
al., J. Phys. Soc. Jpn. 86, 113801 (2017)]. We obtain the non-equilibrium
steady state probability distribution function of such a micromachine and
calculate its probability flux in the corresponding configuration space. The
resulting probability flux can be expressed in terms of a frequency matrix that
is used to distinguish between a non-equilibrium steady state and a thermal
equilibrium state satisfying detailed balance. The frequency matrix is shown to
be proportional to the temperature difference between the spheres. We obtain a
linear relation between the eigenvalue of the frequency matrix and the average
velocity of a thermally driven micromachine that can undergo a directed motion
in a viscous fluid. This relation is consistent with the scallop theorem for a
deterministic three-sphere microswimmer.
