Impact of soluble surfactants on the instabilities in liquid jets

Interfaces are common in both nature and industry, arising in foams, respiratory droplets, 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. This work focuses on the effect of soluble surfactants on the instabilities in liquid jets. Recent studies have shown that insoluble surfactants delay thread thinning and suppress instabilities in Newtonian jet fluids. However, the understanding of soluble surfactants in jet fluid stability is still lacking, specifically the adsorption and desorption kinetics of the soluble surfactants near the interface. In this work, 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.