  In this Account, we describe how synthetic motors that operate by
self-diffusiophoresis make use of a self-generated concentration gradient to
drive motor motion. A description of propulsion by self-diffusiophoresis is
presented for Janus particle motors comprising catalytic and noncatalytic
faces. The properties of the dynamics of chemically powered motors are
illustrated by presenting the results of particle-based simulations of
sphere-dimer motors constructed from linked catalytic and noncatalytic spheres.
The geometries of both Janus and sphere-dimer motors with asymmetric catalytic
activity support the formation of concentration gradients around the motors.
Because directed motion can occur only when the system is not in equilibrium,
the nature of the environment and the role it plays in motor dynamics are
described. Rotational Brownian motion also acts to limit directed motion, and
it has especially strong effects for very small motors. We address the
following question: how small can motors be and still exhibit effects due to
propulsion, even if only to enhance diffusion? Synthetic motors have the
potential to transform the manner in which chemical dynamical processes are
carried out for a wide range of applications.
