UC Davis

Liquid-liquid interfaces are common in both nature and industry, arising in foams, respiratory droplets, thin films, coatings, and inkjet printing. Generally, surfactants are used to stabilize the interface against rupture and coalescence. However, interfacial instabilities can still occur even in the presence of surfactants. As surfaces deform, surfactants alter the surface flows by causing gradients in surface tension and inducing additional surface rheological effects. Quantitatively characterizing these effects has been a key research interest in the past decades. Our work examines the effects of surface rheology on the interfacial instabilities in two areas. First, we demonstrate the stabilizing effect of surface rheology in radial viscous fingering using linear stability analysis. We quantify the growth rates of perturbations to show that surface viscosity slows the growth of the instability and results in thicker fingers. In addition, we highlight the quantitative changes that are predicted to occur when a typical surface viscous surfactant is present. Our second area focuses on the effect of soluble surfactants on the instabilities in liquid jets. We use linear stability analysis to quantitatively show the stabilizing effects of increasing Marangoni stresses, surfactant adsorption and desorption time, and surface viscous stresses of soluble surfactants in jet fluid. In addition, we identify the surface viscous-like force contributed by the Marangoni flow with a finite adsorption and desorption time interval. Our work suggests that surface rheology should be considered as a potential factor in future models and experiments involving complex surfactant-laden interfaces.