Surfactant-induced instability of foam in a radial Hele-Shaw cell*

Foam, i.e., dispersion of gas bubbles in a liquid, is at the center of our daily activities, from medical applications to cleaning purposes, and enhanced oil production. In contrast to single-phase gas, foam demonstrates high viscosity which makes it a suitable agent to displace oil from an underground oil reservoir. In this study, we investigate the influence of surfactant dilution on the dynamical behavior of foam movement in a radial Hele-Shaw cell. To generate foam, the co-injection of nitrogen and Sodium Olefin Sulfonate (17 times above critical micelle concentration) into a micro-model is considered. The generated foam with the constant gas fraction (foam quality) is injected directly from the micro-model to the Hele-Shaw cell. The displacement is monitored when the Hele-Shaw cell is filled with (i) water and (ii) a surfactant solution that has the same surfactant concentration as the injected foam. Outcomes show that the displacement process remains stable in surfactant-saturated Hele-Shaw cell. The displacement of DI-water by foam, however, becomes unstable with the advancing fingers of foam into the DI-water. Measurement of the size of bubbles during the injection illustrates that the foam destabilization mechanisms, comprising coarsening, liquid drainage, and coalescence are not active. As the size of bubbles does not change over time, the branching pattern of foam in water-filled Hele-Shaw is not attributed to film rupturing. For one set of experiments, we add dye in the DI-water from which we can monitor the growth of the dilution zone, i.e., water mixing with the liquid fraction of foam. It is hypothesized that the water diffusion into foam reduces the surfactant concentration which enhances the capillary forces and consequently promotes the side branching pattern. Consistently, an increase in the liquid fraction of foam suppresses the fingers as the mixing zone becomes less diluted due to the larger volume fraction of surfactant solution in foam.