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1. Predicting secondary organic aerosol phase state and viscosity and its effect on multiphase chemistry in a regional-scale air quality model

   https://doi.org/10.5194/acp-20-8201-2020  

   Schmedding, Ryan; Rasool, Quazi Z.; Zhang, Yue; Pye, Havala O.; Zhang, Haofei; Chen, Yuzhi; Surratt, Jason D.; Lopez-Hilfiker, Felipe D.; Thornton, Joel A.; Goldstein, Allen H.; et al (January 2020, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. Atmospheric aerosols are a significant public health hazard and havesubstantial impacts on the climate. Secondary organic aerosols (SOAs) havebeen shown to phase separate into a highly viscous organic outer layersurrounding an aqueous core. This phase separation can decrease thepartitioning of semi-volatile and low-volatile species to the organic phaseand alter the extent of acid-catalyzed reactions in the aqueous core. A newalgorithm that can determine SOA phase separation based on their glasstransition temperature (Tg), oxygen to carbon (O:C) ratio and organic massto sulfate ratio, and meteorological conditions was implemented into theCommunity Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 andwas used to simulate the conditions in the continental United States for thesummer of 2013. SOA formed at the ground/surface level was predicted to bephase separated with core–shell morphology, i.e., aqueous inorganic coresurrounded by organic coating 65.4 % of the time during the 2013 SouthernOxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeasternUnited States. Our estimate is in proximity to the previously reported∼70 % in literature. The phase states of organic coatingsswitched between semi-solid and liquid states, depending on theenvironmental conditions. The semi-solid shell occurring with lower aerosolliquid water content (western United States and at higher altitudes) has aviscosity that was predicted to be 102–1012 Pa s, whichresulted in organic mass being decreased due to diffusion limitation.Organic aerosol was primarily liquid where aerosol liquid water was dominant(eastern United States and at the surface), with a viscosity <102 Pa s.Phase separation while in a liquid phase state, i.e.,liquid–liquid phase separation (LLPS), also reduces reactive uptake ratesrelative to homogeneous internally mixed liquid morphology but was lowerthan aerosols with a thick viscous organic shell. The sensitivity casesperformed with different phase-separation parameterization and dissolutionrate of isoprene epoxydiol (IEPOX) into the particle phase in CMAQ can havevarying impact on fine particulate matter (PM2.5) organic mass, interms of bias and error compared to field data collected during the 2013 SOAS.This highlights the need to better constrain the parameters thatgovern phase state and morphology of SOA, as well as expand mechanisticrepresentation of multiphase chemistry for non-IEPOX SOA formation in modelsaided by novel experimental insights. 

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2. Core–shell droplets and microcapsules formed through liquid–liquid phase separation of a colloid–polymer mixture

   https://doi.org/10.1039/D1SM01091C  

   Dang, Steven; Brady, John; Rel, Ryle; Surineni, Sreenidhi; O’Shaughnessy, Conor; McGorty, Ryan (September 2021, Soft Matter)

   Microcapsules allow for the controlled containment, transport, and release of cargoes ranging from pharmaceuticals to fragrances. Given the interest from a variety of industries in microcapsules and other core–shell structures, a multitude of fabrication strategies exist. Here, we report on a method relying on a mixture of temperature-responsive microgel particles, poly( N -isopropylacrylamide) (pNIPAM), and a polymer which undergo fluid–fluid phase separation. At room temperature this mixture separates into colloid-rich (liquid) and colloid-poor (gas) fluids. By heating the sample above a critical temperature where the microgel particles shrink dramatically and develop a more deeply attractive interparticle potential, the droplets of the colloid-rich phase become gel-like. As the temperature is lowered back to room temperature, these droplets of gelled colloidal particles reliquefy and phase separation within the droplet occurs. This phase separation leads to colloid-poor droplets within the colloid-rich droplets surrounded by a continuous colloid-poor phase. The gas/liquid/gas all-aqueous double emulsion lasts only a few minutes before a majority of the inner droplets escape. However, the colloid-rich shell of the core–shell droplets can solidify with the addition of salt. That this method creates core–shell structures with a shell composed of stimuli-sensitive microgel colloidal particles using only aqueous components makes it attractive for encapsulating biological materials and making capsules that respond to changes in, for example, temperature, salt concentration, or pH. 

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3. Decrease in sulfate aerosol light backscattering by reactive uptake of isoprene epoxydiols

   https://doi.org/10.1039/d0cp05468b  

   Dubois, C.; Cholleton, D.; Gemayel, R.; Chen, Y.; Surratt, J. D.; George, C.; Rairoux, P.; Miffre, A.; Riva, M. (March 2021, Physical Chemistry Chemical Physics)

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   Sulfate aerosol is responsible for a net cooling of the Earth's atmosphere due to its ability to backscatter light. Through atmospheric multiphase chemistry, it reacts with isoprene epoxydiols leading to the formation of aerosol and organic compounds, including organosulfates and high-molecular weight compounds. In this study, we evaluate how sulfate aerosol light backscattering is modified in the presence of such organic compounds. Our laboratory experiments show that reactive uptake of isoprene epoxydiols on sulfate aerosol is responsible for a decrease in light backscattering compared to pure inorganic sulfate particles of up to – 12% at 355 nm wavelength and – 21% at 532 nm wavelength. Moreover, while such chemistry is known to yield a core–shell structure, the observed reduction in the backscattered light intensity is discussed with Mie core–shell light backscattering numerical simulations. We showed that the observed decrease can only be explained by considering effects from the complex optical refractive index. Since isoprene is the most abundant hydrocarbon emitted into the atmosphere, and isoprene epoxydiols are the most important isoprene secondary organic aerosol precursors, our laboratory findings can aid in quantifying the direct radiative forcing of sulfates in the presence of organic compounds, thus more clearly resolving the impact of such aerosol particles on the Earth's climate. 

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4. Nanoparticle-laden droplets of liquid crystals: Interactive morphogenesis and dynamic assembly

   https://doi.org/10.1126/sciadv.aav1035  

   Li, Yunfeng; Khuu, Nancy; Prince, Elisabeth; Alizadehgiashi, Moien; Galati, Elizabeth; Lavrentovich, Oleg D.; Kumacheva, Eugenia (July 2019, Science Advances)

   Defects in liquid crystals serve as templates for nanoparticle (NP) organization; however, NP assembly in cholesteric (Ch) liquid crystals is only beginning to emerge. We show interactive morphogenesis of NP assemblies and a Ch liquid crystalline host formed by cellulose nanocrystals (CNCs), in which both the host and the guest experience marked changes in shape and structure as a function of concentration. At low NP loading, Ch-CNC droplets exhibit flat-ellipsoidal packing of Ch pseudolayers, while the NPs form a toroidal ring- or two cone–shaped assemblies at droplet poles. Increase in NP loading triggers reversible droplet transformation to gain a core-shell morphology with an isotropic core and a Ch shell, with NPs partitioning in the core and in disclinations. We show programmable assembly of droplets carrying magnetic NPs. This work offers a strategy for NP organization in Ch liquid crystals, thus broadening the spectrum of architectures of soft nanostructured materials. 

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5. Design and operation of a von Kármán reactor for droplet breakage experiments

   https://doi.org/10.1016/j.flowmeasinst.2023.102517  

   Ravichandar, Krishnamurthy; Vigil, R. Dennis; Olsen, Michael G. (February 2024, Flow Measurement and Instrumentation)

   The dispersion of an immiscible fluid in a turbulent liquid flow is a frequent occurrence in various natural and technical processes, with particular importance in the chemical, pharmaceutical, mining, petroleum, and food industries. Understanding the dynamics and breakup of liquid droplets is crucial in many scientific and engineering applications, as poor control and optimization of droplet systems results in significant financial losses annually. Although a theoretical background for describing droplet breakup exists, many assumptions still require experimental verification. Numerous mathematical models have been proposed to describe the rate coefficient of droplet breakup and child distribution functions. However, the validation and discrimination between models have been hindered by the lack of experimental data gathered under well-controlled and well characterized conditions. Thus, to validate the current models, novel equipment and methodology for optical droplet breakage research are required. In this work, a von K´arm´an swirling flow apparatus was designed and constructed to carry out optical based droplet breakage experiments under low-intensity, homogeneous turbulent flow. The methodology presented here describes the procedure for generating and controlling the size of the droplets being injected into the homogeneous turbulent flow field. The experiments involved introducing single droplets into the test section, using peanut oil to be the droplet phase and the continuous phase is water. Automated image analysis algorithms were utilized to determine breakage time, breakage probability, and child droplet size distribution for different turbulence intensities. 

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