  Very small synthetic motors that make use of chemical reactions to propel
themselves in solution hold promise for new applications in the development of
new materials, science and medicine. The prospect of such potential
applications, along with the fact that systems with many motors or active
elements display interesting cooperative phenomena of fundamental interest, has
made the study of synthetic motors an active research area. Janus motors,
comprising catalytic and noncatalytic hemispheres, figure prominently in
experimental and theoretical studies of these systems. While continuum models
of Janus particle systems are often used to describe motor dynamics,
microscopic models that are able to account for intermolecular interactions,
many-body concentration gradients, fluid flows and thermal fluctuations provide
a way to explore the dynamical behavior of these complex out-of-equilibrium
systems that does not rely on approximations that are often made in continuum
theories. The analysis of microscopic models from first principles provides a
foundation from which the range of validity and limitations of approximate
theories of the dynamics may be assessed. In this paper, a microscopic model
for the diffusiophoretic propulsion of Janus motors, where motor interactions
with the environment occur only through hard collisions, is constructed,
analyzed and compared to theoretical predictions. Microscopic simulations of
both single-motor and many-motor systems are carried out to illustrate the
results.
