Passage of surfactant-laden and particle-laden drops through a contraction

Abstract : We report on the experimental behavior of oil drops suspended in water and passing in a constricted capillary tube under an imposed pressure gradient. The surfaces of droplets are covered either by colloidal solid particles or soluble surfactants. We investigate the coupling between the flow behavior and the concentration gradient in adsorbed species that are induced by surface expansion, when a water lubrication film persists between the drop and the capillary walls. For both particle-laden and surfactantladen drops, we evidence the formation of strong concentrations gradients resulting in surface tension gradients. We show how local values of surface tension can be monitored using flowrate measurements. In the case of particle-laden drops, we demonstrate the surface tension gradient is balanced by viscous friction in the lubrication film. In the case of surfactant-laden drops, we suggest a Marangoni flow opposes the decrease of surfactant concentration at the front of the drop, up to a threshold value of the surface expansion rate. Finally, we discuss how these effects increase the passage time of surfactant-laden drops in the constriction.