Pattern formation in colloidal mixtures under external driving fields

The influence of an external field acting differently on the two constituents of a binary colloidal mixture performing Brownian dynamics is investigated by computer simulations and a simple theory. In our model, one half of the particles ($A$-particles) are pulled by an external force ${vec F}^{(A)}$ while the other half of them ($B$-particles) are pulled by an external force ${vec F}^{(B)}$. If ${vec F}^{(A)}$ and ${vec F}^{(B)}$ are parallel and the field-free state is a mixed fluid, previous simulations (J. Dzubiella et al, Phys. Rev. E {bf 65} 021402 (2002)) have shown a nonequilibrium pattern formation involving lanes of $A$ or $B$ particles only which are sliding against each other in the direction of the external forces. In this paper, we generalize the situation both to non-parallel external forces and to field-free crystalline states. For non-parallel forces, lane formation is also observed but with an orientation {it tilted} with respect to the external forces. If the field-free state is crystalline, a continuous increase of the parallel external forces yields a novel {it reentrant freezing} behavior: the crystal first melts mechanically via the external force and then recrystallizes into demixed crystalline lanes sliding against each other.