Interfacial tension gradients may arise upon spatially dependent adsorption and desorption rates of surfactant. These interfacial tension gradients give rise to interfacial stresses and concomitant Marangoni flows. These flows can potentially be utilized for particulate transport along a liquid-liquid interface and are relevant in, for example, personal care products, topical medications, surfactant replacement therapy for respiratory distress syndrome and enhanced oil recovery.
The varying adsorption and desorption rates are governed by convection and diffusion, and may depend on adsorption and desorption kinetics of the surfactant. Furthermore, the geometry of the interface will affect the rate of change of the surface excess, in particular in the presence of interface curvature. In addition, the presence of any obstacle in the water phase may affect the convection and diffusion of surfactant species to and from a water/oil interface.
We discuss the effect of an obstacle on interfacial particulate transport as a result of surfactant-induced Marangoni flows. Specifically, we will address the influence of meniscus curvature on interfacial transport close to an obstacle in surfactant application and rinsing experiments. Meniscus curvature reflects whether the obstacle is wetted by the oil or not and may influence the directionality of the Marangoni transport upon surfactant adsorption or desorption from flowing solutions. For targeted delivery towards an object, the directionality of the induced flows is crucial and a transport hysteresis upon surfactant application and rinsing is required to create a net transport towards the object of interest. Factors that will allow a net transport towards an obstacle will be discussed.
The Marangoni flows are induced in a radial flow cell between a ca 350 µm water phase and a ca 30 µm thin oil phase. The transport of 5 µm tracer particles are monitored by optical microscopy. Obstacles, partially wetted by the oil, that penetrate the oil/water interface, are fabricated by pouring PDMS with cross-linker into a negative template to create features that are ca 150 µm tall with a diameter of 200 µm after curing. In order to vary the meniscus shape of the water/oil interface around the cylindrical objects, the PDMS molds are plasma treated to modify their wettability. The experimental results are compared to a numerical interfacial mass transfer model.