  A model to study the dynamics of colloidal particles in nonuniform electric
fields is proposed. For an isolated sphere, the conditions and threshold for
sustained (Quincke) rotation in a linear direct current (dc) field are
determined. Particle dynamics becomes more complex with increasing electric
field strength, changing from steady spinning around the particle center to
time-dependent orbiting motion around the minimum field location. Pairs of
particles exhibit intricate trajectories, which are combination of translation,
due to dielectrophoresis, and rotation, due to the Quincke effect. Our model
provides a basis to study the collective dynamics of many particles in a
general electric field.
