Droplet microfluidics for studying surfactant-rich interfaces found in aerosols, emulsions and foams

Abstract

<h2>Abstract</h2> The presence of droplets suspended in a liquid or gas is known to play an essential role in fields ranging from materials to atmospheric science. Often, the dispersed phase is stabilized by surface active compounds and surfactants, resulting in complex chemical composition and material properties at the fluid-fluid interface. In this talk, recent advancements in use of microscale flow fields will be highlighted for measuring properties of aqueous multiphase, interface-rich systems. Systems studied will include atmospheric aerosol droplets and liquid-liquid emulsions. Microfluidic contractions, traps, and wells are used to measure surface-bulk partitioning and temperature-dependent liquid-liquid phase separation of aqueous droplets, towards better understanding of suspensions of aerosol droplets in our atmosphere. Advanced measurements of droplet shape deformations and coalescence will also be presented for both water-in-fuel and oil-in-water emulsions, for improved treatment and separation of emulsions. Dynamic interfacial tensions measurements were performed using a microfluidic tensiometer, demonstrating a dependence on if the surfactant approaches the interface from inside (dispersed) versus outside (continuous), implying phase dependent surfactant transport to curved interfaces at the microscale. Droplet coalescence and film drainage experiments are also performed in a microfluidic Stokes trap across a range of viscosity ratios and surfactant concentrations. Results are used to explore the influence of interfacial mobility and Marangoni stresses on film stability with soluble surfactants.