Cross-stream migration of a surfactant-laden deformable droplet in a Poiseuille flow

Abstract

The motion of a viscous deformable droplet suspended in an unbounded Poiseuille flow in the presence of bulk-insoluble surfactants is studied analytically. Assuming the convective transport of fluid to be negligible, we perform a small-deformation perturbation analysis to obtain the droplet migration velocity. The droplet dynamics strongly depends on the distribution of surfactants along the droplet interface, which is governed by the relative strength of convective transport of surfactants as compared with the diffusive transport of surfactants. The present study is focused on the following two limits: (i) when the surfactant transport is dominated by surface diffusion and (ii) when the surfactant transport is dominated by surface convection. In the first limiting case, it is seen that the axial velocity of the droplet decreases with an increase in the advection of the surfactants along the surface. The variation of cross-stream migration velocity, on the other hand, is analyzed over three different regimes based on the ratio of the viscosity of the droplet phase to that of the carrier phase (λ). In the first regime (∼λ < 0.75), the migration velocity decreases with an increase in surface advection of the surfactants, although there is no change in the direction of droplet migration. For the second regime (∼0.75 < λ < 11), the direction of the cross-stream migration of the droplet changes (which means the droplet moves either towards the flow centerline or away from it) depending on different parameters. In the third regime (∼λ > 11), the migration velocity is merely affected by any change in the surfactant distribution. For the other limit of higher surface advection in comparison with surface diffusion of the surfactants, the droplet always moves towards the flow centerline and the axial velocity of the droplet is found to be independent of the surfactant distribution. However, the cross-stream velocity is found to decrease with an increase in nonuniformity in surfactant distribution.