Spontaneous aqueous foaming with fluorosurfactants from a hydrocarbon liquid at ambient conditions

Aqueous foams are ubiquitous in nature and have important applications in enhanced oil recovery, food industry, pharmaceuticals, and firefighting. Foams are intrinsically metastable and degrade quickly to form water when exposed to hydrocarbons. Contrary to the hydrocarbon-induced foam degradation, we describe spontaneous foam generation from a thin layer of immiscible, volatile, hydrocarbon liquid floating on an aqueous fluorosurfactant solution at quiescent, ambient, conditions in a 17-cm diameter vessel. Foaming occurs at undersaturation and is different from the spontaneous foam generation from boiling liquids or cavitation or effervescence of dissolved gases at over-saturated conditions. By varying parameters, we reveal the underlying processes and conditions and quantify their effects on foam volume. We also quantify the unique ability of fluorosurfactants to spontaneously form foams unlike fluorine-free surfactants. This is because the hydrocarbon evaporation drives foam generation and foam-degradation simultaneously, and the degradation dominates for fluorine-free surfactants. To understand the mechanisms of foaming, experiments with a single hydrocarbon droplet floating on the aqueous solution are performed. They show that the vapor formed by the evaporating drop diffuses directly into ambient air below its boiling point but some of it nucleates bubbles at the surfactant laden drop-aqueous-air interface. As the vapor diffuses and accumulates into bubbles, the bubbles grow, rupture, and re-nucleate at the interface. Increased hydrocarbon volatility increases the bubble growth rates. The bubbles trap the hydrocarbon vapors with potential benefits in applications including firefighting and oil spills. Also, the fluorine-free surfactants could be made to foam spontaneously by varying the process parameters identified here and could be important in designing environmentally friendly alternatives to per- and poly- fluoroalkyl substances (PFAS).

Automated summary

Aqueous foams have a wide range of applications and play an important role in various industries. In this study, we explore the spontaneous generation of foam from a layer of volatile hydrocarbon liquid floating on an aqueous fluorosurfactant solution. By investigating the underlying processes, we demonstrate the unique ability of fluorosurfactants to form foams. Additionally, we examine the mechanisms of foaming using experiments with a single hydrocarbon droplet floating on the aqueous solution.