Diffusion of Interacting Particles in a Concentration Gradient: Scaling, Critical Slowing Down and Phase Separation

A lattice gas of diffusing interacting particles in a concentration gradient is investigated by Monte Carlo simulations. The time evolution of the concentration profile is analysed in terms of scaling and the diffusion front is found to be fractal. Two qualitatively different regimes can be distinguished: for weak interactions, both attractive and repulsive, the system behaves similarly to diffusion of noninteracting particles, whereas for strong attraction one observes the equivalent of phase separation. At the critical point which separates the two regions large flsctuations slow down the diffusion process.