(490c) Surface Viscoelasticity Hinders the Instabilities in Confined Multiphase Flows

Liquid-liquid interfaces are common in both nature and industry, arising commonly in foams, respiratory droplets, thin films, coatings, and flotation. Generally, stabilizing agents or 'surfactants' such as fatty acids, alcohols, proteins, and particles are used to stabilize the interface against rupture and coalescence. However, interfacial instabilities can still occur even in the presence of surfactants. These instabilities are often undesirable and present challenges in common industrial processes involving multiphase flows. This work focuses on the effect of interfacial rheology on Saffman-Taylor instability, which involves the formation of finger-like protrusions when a less viscous fluid displaces a more viscous fluid. Recent studies have shown that complex surfactant-laden interfaces with surface rheological stresses resist interfacial deformation and alter the fluid dynamics of the system. Despite current progress, the impact of surface rheological stresses on the Saffman-Taylor instability is yet unknown. In this work, we demonstrate, for the first time, the stabilizing effect of surface rheology in radial viscous fingering using linear stability analysis. We analyze the effect of surface rheology on the growth of interfacial perturbations in a radial Hele-Shaw cell and illustrate that surface viscous stresses have a stabilizing effect: the most unstable wavenumber as well as the corresponding growth rate decreases with increasing surface viscosity. Based on these, we postulate that surface viscous resistance slows the growth of the instability and results in thicker fingers. Finally, we highlight the quantitative changes that are predicted to occur when a typical surface viscous surfactant is present. Our work suggests that surface rheology should be considered as a potential factor in future models and experiments involving complex surfactant-laden interfaces. We use these insights to propose qualitative changes that might occur when surface-attached microparticles that behave as surfactants might be present at the fluid-fluid interfaces.