Diffusive behavior of a thin particle layer in fluid by hydrodynamic interaction.

The hydrodynamic effect on a thin particle layer, which moves relative to fluid by an external force, is investigated theoretically and numerically. Because of the presence of layer ends, the arrangement of particles in the layer is anisotropic and the drag force acting on them varies according to the position. The resulting relative motion of particles brings about the spreading of the layer. We have studied such a diffusive behavior of particle layers, which have various internal arrangements. We have assumed a non-Brownian system in which the particles move relatively owing to only the variance of hydrodynamic force. The hydrodynamic force on each particle was calculated by Stokesian dynamics approach. The results show that the relative motion of particles is greatly influenced by the internal arrangement of the particle layer. In consequence, the overall diffusive motion of particle layer varies with the arrangement even if the particle concentration is similar. It is in contrast to the gradient diffusion of Brownian particles.