  We present a method of organizing incoherent motion of a colloidal suspension
to produce synchronized, coherent motion. This method exploits general features
of rotational response to time-dependent forcing, and it does not require
interaction between the particles. We report two methods of achieving
orientational alignment of an ensemble of identical colloids by means of a
time-dependent, but spatially uniform forcing: a) a piecewise constant force
alternating between two directions and b) a force uniformly rotating about an
axis. The physical origin of the forcing may be e.g., sedimentation or
electrophoresis. We will demonstrate that these forcing methods achieve
alignment both by analyzing the equations of motion and by simulation. We find
the conditions guaranteeing alignment, discuss the limitations of these
methods, and suggest possible applications. Examples of such forcing include
electrophoresis and sedimentation.
