Coalescence of micrometer-scale droplets is impacted by several parameters, including droplet size, viscosities of the two phases, droplet velocity, angle of approach, as well as interfacial tension and surfactant coverage. The thinning dynamics of films between coalescing droplets can be particularly complex in the presence of surfactants, due to the generation of Marangoni stresses and reduced film mobility. Here, a microfluidic hydrodynamic "Stokes" trap is used to gently steer and trap surfactantladen micrometer-sized droplets at the center of a cross-slot. Water droplets are formed upstream of the cross-slot using a microfluidic T-junction, in heavy and light mineral oils and stabilized using SPAN 80, an oil-soluble surfactant. Incoming droplets are made to coalesce with the trapped droplet, yielding measurements of the film drainage time. Film drainage times are measured as a function of continuous phase viscosity, incoming droplet speed, trapped droplet size, and surfactant concentrations above and below the critical micelle concentration (CMC). As expected, systems with higher surfactant concentrations and slower incoming droplet speed exhibit longer film drainage times. At low surfactant concentrations, the drainage time is longer for the more viscous heavy mineral oil in the continuous phase, whereas at high surfactant concentrations, the dependence on continuous phase viscosity vanishes. Perhaps more surprisingly, larger droplets and high confinement also result in longer film drainage times, potentially due to deformation of the droplet interfaces. The results are used here to determine critical conditions for coalescence, including both an upper and a lower critical capillary number. Moreover, it is shown that induced surfactant concentration gradient effects enable coalescence events after the droplets had originally flocculated, at surfactant concentrations above the CMC. The microfluidic hydrodynamic trap provides new insights into the role of surfactants in film drainage and opens avenues for controlled coalescence studies at micrometer length scales and millisecond time scales.

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Well-mixed atmospheric aqueous aerosol droplets containing multiple chemical species can undergo processes such as liquid–liquid phase separation (LLPS) and crystallization depending on the ambient temperature and relative humidity (RH). So far, only a handful of single droplet studies have examined the effect of temperature in conjunction with the organic to inorganic ratio (OIR) on the separation RH for LLPS. In this work, we present a temperature-controlled microfluidic static trap approach to study the LLPS and efflorescence phenomenon in multiple ternary systems in a quasi-equilibrium manner. Ammonium sulfate or sodium chloride is used as the inorganic phase and 3-methylglutaric acid (3-MGA), poly­(ethylene glycol), poly­(propylene glycol), or poly­(ethylene glycol) diacrylate is used as the organic phase. Results show a clear trend in droplets containing 3-MGA with either salt of the initial LLPS and efflorescence events occurring at higher RH at lower temperatures, while this trend is less obvious for the other organics. The organic to inorganic ratio (OIR) of the system also affected the type of first phase transition, which can be either LLPS or efflorescence. Finally, the rate of RH change also had an impact on the temperature dependence of the formation of either anhydrous or dihydrous crystals of sodium chloride upon efflorescence. These results help inform the effects of temperature, OIR, and rate of RH change on the phase state of aqueous aerosol droplets containing multiple species.

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