Dynamical instability in driven colloids

– Brownian-dynamics computer simulations show a dynamical crossover in a strongly interacting colloidal suspension consisting of oppositely driven particles, wherein a uniform state transforms, with increasing driving force, into a locally demixed state characterized by strongly anisotropic stripe-like domains which are homogeneous in the direction parallel to the drive but have finite spatial extent of a double-correlation length in the transverse direction. A phenomenological dynamic density-functional theory has been proposed which accounts for such a strongly anisotropic state as arising from an instability of a homogeneous state. Introduction. – Nonequilibrium phase transitions are ubiquitous in nature, since they occur in simple and complex fluids [1–3], in granular matter [4], in enormously complicated biological systems [5, 6] as well as in pedestrian dynamics [7]. In order to understand the principles of such phase transformations, simple lattice models of driven diffusive systems have been studied [4, 8, 9] extensively in the recent past. However, most of the realizations in nature are off-lattice, implying a much richer scenario of nonequilibrium transitions. Here, phenomenological treatments have had considerable success [1], but full microscopic theories involving the interparticle interactions as the only input are still missing. Mesoscopic colloidal suspensions [10] represent excellent realizations of driven diffusive systems with a number of advantages: The samples are well characterized and the effective particle interactions can be tailored. Moreover, external driving fields can be applied in a controlled way [11]. The mesoscopic length scales and the slow relaxation times of the order of milliseconds open the way for a direct experimental investigation in real space and real time which is impossible for molecular systems. For instance, colloidal samples can be confined between glass plates to two dimensions [12] such that their individual trajectories can be followed by video microscopy [13]. For equilibrium phase transformations, colloids have played an explicit role as model systems for condensed matter in general, and it can be expected that they will play a similarly dominant role for nonequilibrium phase transitions. In this paper we examine a particular kind of nonequilibrium segregation in a driven two-component colloidal mixture by theory and simulation. In our two-dimensional model, the two components are driven in opposite directions by constant external forces. Using Brownian-dynamics simulations, we observe a dynamical crossover upon increasing the driving force. Above a critical drive, the system proceeds from a uniform mixed fluid into a state