Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX), key isoprene oxidation products, with inorganic sulfate aerosol yields substantial amounts of secondary organic aerosol (SOA) through the formation of organosulfur compounds. The extent and implications of inorganic-to-organic sulfate conversion, however, are unknown. In this article, we demonstrate that extensive consumption of inorganic sulfate occurs, which increases with the IEPOX-to-inorganic sulfate concentration ratio (IEPOX/Sulfinorg), as determined by laboratory measurements. Characterization of the total sulfur aerosol observed at Look Rock, Tennessee, from 2007 to 2016 shows that organosulfur mass fractions will likely continue to increase with ongoing declines in anthropogenic Sulfinorg, consistent with our laboratory findings. We further demonstrate that organosulfur compounds greatly modify critical aerosol properties, such as acidity, morphology, viscosity, and phase state. These new mechanistic insights demonstrate that changes in SO2 emissions, especially in isoprene-dominated environments, will significantly alter biogenic SOA physicochemical properties. Consequently, IEPOX/Sulfinorg will play an important role in understanding the historical climate and determining future impacts of biogenic SOA on the global climate and air quality.

Small amounts of water can enter diesel fuel during usage, causing major damage and failure of engine parts. Water is dispersed in fuel as droplets stabilized by the presence of surface-active compounds in the original fuel mixture as well as in fuel additives, including lubricity improvers and deposit control agents. Additives partition to the fuel–water interface and lower the interfacial tension (IFT), decreasing the ability to coalesce and separate water from fuel. The ability of standard coalescing filters to capture and coalesce emulsion droplets depends on dynamic IFT, conventionally measured for large millimeter-sized drops or planar interfaces. In this work, a microfluidic platform is employed to generate a monodisperse stream of small micrometer-sized water droplets in model fuel and ultralow sulfur diesel, mimicking the size of droplets in actual fuel–water emulsions. The deformation of hundreds of droplets is tracked at high speed through twenty-six geometric contractions to find time-dependent apparent IFT. It is found that the time scale associated with the decrease of IFT is orders of magnitude smaller in micrometer-sized droplets compared to millimeter-sized drops from pendant drop experiments. This finding suggests that, in real emulsion processing conditions such as fuel filtration, the residence time of droplets from the point of formation to filtration is such that IFT has already decreased to the equilibrium value. This work results in clear implications that standardized tests used by industry for qualifying diesel fuels must be reconsidered to account for droplet size, to enable design of efficient fuel filtration systems.

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.

Microfluidics is used in a broad range of applications, from biology and medicine to chemistry and polymer science, because this versatile platform enables rapid and precise repeatability of measurements and experiments on a relatively low-cost laboratory platform. Despite wide-ranging uses, this powerful research platform remains under-utilized by the atmospheric aerosol science community. This review will summarize selected microfluidic concepts and tools with potential applications to aerosol science. Where appropriate, the basic operating conditions and tunable parameters in microfluidics will be compared to typical aerosol experimental methods. Microfluidics offers a number of advantages over larger-scale experiments; for example, the small volumes of sample required for experiments open a number of avenues for sample collection that are accessible to the aerosol community. Filter extraction, spot sampling, and particle-into-liquid sampling techniques could all be used to capture aerosol samples to supply microfluidic measurements and experiments. Microfluidic concepts, such as device geometries for creating emulsions and developments in particle and droplet manipulation techniques will be reviewed, and current and potential microfluidic applications to aerosol science will be discussed.

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