(398d) Effect of Surfactant Solubility on the Stability of Thin Liquid Jets

Interfaces are common in both nature and industry, arising in foams, respiratory droplets, coatings, and inkjet printing. In general, surfactants 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. Quantifying these effects has been a significant challenge in chemical physics and a topic of key research interest. Recent studies have shown that insoluble surfactants delay thread thinning and suppress instabilities in Newtonian jets. However, the role of surfactant solubility in liquid jet stability is still unknown. Quantitatively characterizing these effects has been a key research interest in the past decades. We use theory and simulations to illustrate the role of surfactant solubility on 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 Marangoni stresses, surfactant adsorption and desorption time, and intermolecular forces upon adsorption. We highlight the seemingly indistinguishable way in which various surfactant properties result in the same outcome. We also identify a surface dissipative contribution that arises from the interplay of Marangoni flows with finite adsorption and desorption, which acts as an apparent surface viscosity. We verify predictions of our linear stability results against numerical simulations and conclude by noting that tuning surface activity and kinetics of adsorbed surfactants or particles can potentially suppress droplet formation, which is of significant impact in the printing industry and in the control of the spread of aerosols.