  We consider mixtures of two species of spherical colloidal particles that
differ in their hydrodynamic radii, but are otherwise identical, in the
presence of an external field. Since the particle-particle and particle-field
interactions are the same for both species, they are completely mixed in the
thermodynamic limit in the presence of any static field. Here, we combine
Brownian Dynamics and Dynamic Density Functional theory of fluids to show that
for sufficiently large differences in the hydrodynamic radius of the particles
(and corresponding differences in their electrophoretic mobilities) dynamical
demixing is observed. These demixed states are transient but, under certain
conditions, packing effects compromise the relaxation towards the thermodynamic
states and the lifetime of the demixed phases increases significantly.
