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Orientational ordering of colloidal dispersions by application of time dependent external forces

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.