Diffusive Motion in Confined Fluids: Mode-Coupling Results and Molecular-Dynamics Calculations

We present a mode-coupling calculation of the self-diffusion coefficient in a liquid confined between two parallel solid walls. The effect of confinement is to lower the diffusion constant in the direction parallel to the walls, compared to the bulk case. The relative decrease behaves as (σ/ h ) log ( h /σ), where h is the thickness of the fluid slab, and σ the atomic size. This behaviour is interpreted in terms of the suppression of long-wavelength modes due to confinement. It is also explained by a simple hydrodynamic argument. Molecular-dynamics simulations confirm the theoretically predicted behaviour, even at rather small values of h .