Full alignment of colloidal objects by programmed forcing

By analysis and simulation we demonstrate two methods for achieving complete orientational alignment of a set of identical, asymmetric colloidal objects dispersed randomly in a fluid. Sedimentation or electrophoresis in a constant field can lead to partial alignment, in which the objects rotate about a common body axis, but the phases of rotation for these objects are random. We show that this phase disorder can be removed by two forms of programmed forcing. First, simply alternating the forcing between two directions reduces the statistical entropy of the orientation arbitrarily. Second, addition of a small rotating component to the applied field in analogy to magnetic resonance can lead to phase locking of the objects' orientation. We identify conditions for alignment of a broad class of generic objects and discuss practical limitations.