(550d) Spontaneous Droplet Formation in a 2D Porous Structured Device

Using a 2-dimensional (2D) transparent porous structured device, we studied oil mobilization and emulsion droplet formation mechanisms due to surfactant fluid flooding. By controlling the interfacial tension between the immiscible liquids, we show that monodispersed emulsion droplets can be produced at a higher rate than the usual single channel droplet-maker devices. The oil transport is visualized via automated flooding experiments in a porous chip (PC). It is found that the theoretical critical capillary number needed to mobilize an oil ganglion is less relevant when the liquid-liquid interfacial tension is low enough to yield a highly deformable interface. The edges of the oil ganglia, in contact with the surfactant flooding fluid, erode into pore-size free drops in a cascade of various droplet-formation fluid dynamics. Due to the spontaneous formation of these droplets, oil saturation in the device is almost completely depleted at capillary numbers an order of magnitude lower than that needed for simple mobilization. With direct observation of oil transportation in porous medium, we see that a cascade mechanism is responsible for the erosion of large size initial ganglia where free droplets are formed with size and velocities essentially that of the pore constriction and the average flow. This work highlights the independent role interfacial tension plays in mobilization of the oil ganglia and sets the stage for new methods for emulsion droplet production.