Segregation by size difference in binary suspensions of fluid droplets in channel flow

In channel flow of multicomponent suspensions, segregation behavior of suspended components perpendicular to the flow direction is often observed, which is considered to be caused by the differential properties of the lateral migration depending on their shape, size, flexibility, and other characteristics. In the present study, we investigate the effect of size differences between suspended components on the segregation behavior, by a two-dimensional numerical simulation for binary dispersed suspensions of fluid droplets of two different sizes subjected to a plane Poiseuille channel flow. The small and large droplets are assumed to have equal surface tensions and equal viscosity ratios of internal to external fluids. The time evolutions of the lateral positions of large and small droplets relative to the channel centerline were computed by changing the area fraction of the small droplets in a mixture with a constant total area fraction. The large droplets are found to migrate closer to the channel centerline and the small droplets are found to migrate closer to the channel wall compared to the corresponding lateral positions in mono-dispersed suspensions at the same area fractions, although the mean lateral positions of the large and small droplets in mono-dispersed suspension are comparable. This segregation behavior as well as the margination of small droplets are enhanced when the size difference between large and small droplets is increased and the area fraction of large droplets is increased. These results may arise from higher tendencies for the large droplets to approach the channel centerline compared to the small droplets, which consequently expel small droplets from the central region toward the channel walls.