Interphase diffusion in two-phase fluids: local dynamics and finite-size effects

Diffusion across the fluid-fluid interface during phase separation is ubiquitous in industry separation processes and biological systems. This process is strongly dependent on the local nature of the interface separating the two phases. Hence, it is expected that a strong finite-size effect will be observed in the process of simulating such system.

In this study, molecular dynamics were performed to determine the global and local dynamics across the fluid-fluid interface of a binary mixture of Lennard-Jones fluids with various degrees of immiscibility and various sizes.

Strong local variations were observed for both the normal and lateral local diffusion with the lateral diffusion showing a maximum at the interface while the normal diffusion is highest in the unfavourable phase. We identified two different contributions to the finite-size effects to the normal and lateral diffusion of the fluid-fluid systems, which bear resemblance to nano-confined fluids. Interestingly, the lateral diffusion is not only affected by the changes in the distribution of species across the interface, but also by the hydrodynamic effects, which are an artefact of the relative size of the periodic cell.