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

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 the 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 surfactant-laden drops, we evidence the formation of strong concentration gradients resulting in surface tension gradients. We show how local values of surface tension can be monitored using flow rate measurements. In the case of particle-laden drops, we demonstrate that 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.

Automated summary

The experimental behavior of oil drops suspended in water and passing through a constricted capillary tube under imposed pressure gradient is investigated, revealing strong concentration gradients and surface tension gradients. Local values of surface tension can be monitored using flow rate measurements. Different mechanisms are proposed to balance the surface tension gradient in particle-laden drops and surfactant-laden drops, affecting the passage time of the drops in the constriction.