Detergency and dissolution under flow in low interfacial tension systems

Adjusting the formulations of aqueous solutions to maximize their power to clean oil soils from solid substrates, often called detergency, is mostly based on empirical knowledge. Here, we quantitatively describe the removal of a single oil droplet attached to a solid wall under the action of a flow of surfactant solutions. To do so, we design a microfluidic set-up allowing control of the flow and monitoring of the droplet volume, shape, contact area and its detachment. We work with surfactant solutions whose varied composition allows us to span a wide range of both oil solubility and interfacial tension: the system forms microemulsions. Two detergency regimes are identified and described. (i) At large flow rates, the viscous shear forces the droplet to detach which is well accounted for by a single-valued capillary number. As a result, the detachment threshold depends on the antagonistic effects of the detergent composition on the wetting of oil on the solid and the interfacial tension that we describe. (ii) Below the threshold for detachment, the oil from the droplet is solubilized into the detergent according to the advection-diffusion model for transport. This allows us to predict the optimum composition. In both cases we release quantitative predictions, provided that independent characterization studies, using conventional methods, of the oil solubility, micelle size and concentration, interfacial tension and wetting angles are available.