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1. Backscatter multiple wavelength digital holography for color micro-particle imaging

   https://doi.org/10.1364/AO.441509  

   Giri, Ramesh; Berg, Matthew_J (November 2021, Applied Optics)

   This work applies digital holography to image stationary micro-particles in color. The approach involves a Michelson interferometer to mix reference light with the weak intensity light backscattered from a distribution of particles. To enable color images, three wavelengths are used, 430, 532, and 633 nm, as primary light sources. Three separate backscattered holograms are recorded simultaneously, one for each wavelength, which are resolved without spectral cross talk using a three-CMOS prism sensor. Fresnel diffraction theory is used to render monochrome images from each hologram. The images are then combined via additive color mixing with red, green, and blue as the primary colors. The result is a color image similar in appearance to that obtained with a conventional microscope in white-light epi-illumination mode. A variety of colored polyethylene micro-spheres and nonspherical dust particles demonstrate the feasibility of the approach and illustrate the effect of simple speckle-noise suppression and white balance methods. Finally, a chromaticity analysis is applied that is capable of differentiating particles of different colors in a quantitative and objective manner. 

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2. Survival of newly formed particles in haze conditions

   https://doi.org/10.1039/D2EA00007E  

   Marten, Ruby; Xiao, Mao; Rörup, Birte; Wang, Mingyi; Kong, Weimeng; He, Xu-Cheng; Stolzenburg, Dominik; Pfeifer, Joschka; Marie, Guillaume; Wang, Dongyu S.; et al (May 2022, Environmental Science: Atmospheres)

   Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s −1 . We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO 3 ) and ammonia (NH 3 ), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO 3 and NH 3 can be expected to contribute to particle survival during haze. 

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3. The Inert Drift Atlas Model

   https://doi.org/10.1007/s00220-022-04589-2  

   Banerjee, Sayan; Budhiraja, Amarjit; Estevez, Benjamin (December 2022, Communications in Mathematical Physics)

   Consider a massive (inert) particle impinged from above by N Brownian particles that are instantaneously reflected upon collision with the inert particle. The velocity of the inert particle increases due to the influence of an external Newtonian potential (e.g. gravitation) and decreases in proportion to the total local time of collisions with the Brownian particles. This system models a semi-permeable membrane in a fluid having microscopic impurities (Knight in Probab Theory Relat Fields 121:577–598, 2001). We study the long-time behavior of the process (V , Z), where V is the velocity of the inert particle and Z is the vector of gaps between successive particles ordered by their relative positions. The system is not hypoelliptic, not reversible, and has singular form interactions. Thus the study of stability behavior of the system requires new ideas. We show that this process has a unique stationary distribution that takes an explicit product form which is Gaussian in the velocity component and exponential in the other components. We also show that convergence in total variation distance to the stationary distribution happens at an exponential rate. We further obtain certain law of large numbers results for the particle locations and intersection local times. 

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4. Influence of survival, promotion, and growth on pattern formation in zebrafish skin

   https://doi.org/10.1038/s41598-021-89116-4  

   Konow, Christopher; Li, Ziyao; Shepherd, Samantha; Bullara, Domenico; Epstein, Irving R. (May 2021, Scientific Reports)

   Abstract The coloring of zebrafish skin is often used as a model system to study biological pattern formation. However, the small number and lack of movement of chromatophores defies traditional Turing-type pattern generating mechanisms. Recent models invoke discrete short-range competition and long-range promotion between different pigment cells as an alternative to a reaction-diffusion scheme. In this work, we propose a lattice-based “Survival model,” which is inspired by recent experimental findings on the nature of long-range chromatophore interactions. The Survival model produces stationary patterns with diffuse stripes and undergoes a Turing instability. We also examine the effect that domain growth, ubiquitous in biological systems, has on the patterns in both the Survival model and an earlier “Promotion” model. In both cases, domain growth alone is capable of orienting Turing patterns above a threshold wavelength and can reorient the stripes in ablated cells, though the wavelength for which the patterns orient is much larger for the Survival model. While the Survival model is a simplified representation of the multifaceted interactions between pigment cells, it reveals complex organizational behavior and may help to guide future studies. 

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5. Atmospheric Nanoparticle Survivability Reduction Due to Charge‐Induced Coagulation Scavenging Enhancement

   https://doi.org/10.1029/2021GL092758  

   Mahfouz, Naser G. A.; Donahue, Neil M. (April 2021, Geophysical Research Letters)

   Abstract New‐particle formation is important to aerosol–cloud interactions and thus climate, but for newly formed particles to become cloud condensation nuclei, they must grow and avoid scavenging by larger background particles. Whereas ion‐induced new‐particle formation and growth have received attention recently, here we study an opposing effect, blunting the enhancement due to ions, that has received less attention: Small charged particles are scavenged more efficiently due to their charge, and thus their survival probability is lower than that of their neutral counterparts. Through simulations, we show that particle survival is reduced, in some cases dramatically, matching updated theoretical predictions. We also show that the survival of charged particles is enhanced if particles lose their charge via neutralization; therefore, for ion‐induced nucleation to be important, the resulting charged particles must become neutral as soon as possible. Overall, the coagulation scavenging enhancement due to charge ought to lessen the influence of ions in new‐particle formation and growth. 

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