  A binary colloidal mixture of unequal sizes, subjected to an external
potential barrier, has been investigated using canonical ensemble molecular
dynamics simulations. The attractive depletion interaction between the external
barrier and larger species in the binary mixture causes the mixture to phase
separate. At higher volume fractions, a pure phase of larger particles forms
near the potential barrier, and the local density of this pure phase is high
enough that a face centered cubic crystalline domain is formed at this region.
This crystalline phase diffuses perpendicular to the external potential
barrier. The temperature dependence of diffusivity of larger particles is
non-Arrhenius and changes from sub-Arrhenius to super-Arrhenius as the volume
fraction increases. This crossover from sub-Arrhenius to super-Arrhenius
diffusion coincides with the crystalline formation near the potential barrier.
