Confinement and Interaction Effects on the Diffusion of Passive Particles

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We use Molecular Dynamics simulations to study the effect of interactions and confinement (walls) on particle diffusion. We extend previous studies by analyzing the mean squared displacement (MSD) of an interacting fluid constrained to a circular, square and triangular cavity of nanometric size. The interactions among particles and walls are modeled by means of three classic potentials namely, Lenard-Jones (CLJ), soft Lenard-Jones (SLJ) and hard Lenard-Jones (HLJ) potentials. For hard spheres, for all cavities, and for very diluted densities, diffusion is shown to be less favorable in comparison with particles interacting with a CLJ. It is also observed that HLJ particles do not show difference in their MSD with SLJ particles at these densities. Confinement effects also appear at these densities and it is shown that diffusion decreases in the following cavity shape order: triangular, square and circular. For moderated densities, the combination of confinement and interactions shows a non-trivial effect. It is observed that particles inside a triangular cavity interacting by means of HLJ, reduce their MSD in comparison with CLJ or SLJ particles, since for this cavity shape, hard collisions reduce the particles’ speed. For higher densities, another non-trivial effect appears. Once again, the combination of interactions and confinement gives rise to order in the system that clearly reduces the system MSD. It is also shown that order appears for SLJ particles but it is absent for CLJ or HLJ particles.