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1. Quasi-steady-state modelling and characterization of diffusion-controlled dissolution from polydisperse spheroidal particles, II: characterization

   https://doi.org/10.1098/rspa.2023.0768  

   Wang, Yanxing; Wan, Hui; Barka, Cody; Wei, Tie; Shu, Fangjun (March 2024, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   A quasi steady-state model (QSM) for accurately predicting the detailed diffusion-dominated dissolution process of polydisperse spheroidal (prolate, oblate, and spherical) particle systems was presented Part I of this study. In the present paper, the dissolution characteristics of typical polydisperse spheroidal particle systems have been extensively investigated. The effects of the distributions of particle size and shape have been studied by examining the detailed dissolution processes, such as the size reduction rates of individual particles, the increase in bulk concentration, and the dissolution time of the polydisperse systems. Some important factors controlling the dissolution process, including initial particle concentration, smallest and largest particle sizes, and the smallest and largest Taylor shape parameters, have been identified. 

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2. Quasi-steady-state modelling and characterization of diffusion-controlled dissolution from monodisperse prolate and oblate spheroidal particles

   https://doi.org/10.1098/rspa.2022.0283  

   Wang, Yanxing; Wan, Hui; Wei, Tie; Nevares, Dominick; Shu, Fangjun (November 2022, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   A quasi-steady-state model of the dissolution of a single prolate or oblate spheroidal particle has been developed based on the exact solution of the steady-state diffusion equation for mass transfer in an unconfined media. With appropriate treatment of bulk concentration, the model can predict the detailed dissolution process of a single particle in a container of finite size. The dimensionless governing equations suggest that the dissolution process is determined by three dimensionless control parameters, initial solid particle concentration, particle aspect ratio and the product of specific volume of solid particles and saturation concentration of the dissolved substance. Using this model, the dissolution processes of felodipine particles are analysed in a broad range of space of the three control parameters and some characteristics are identified. The effects of material properties indicated by the product of specific volume and saturation concentration are also analysed. The model and the analysis are applicable to the system of monodisperse spheroidal particles of the same shape. 

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3. Impact of phase state and non-ideal mixing on equilibration timescales of secondary organic aerosol partitioning

   https://doi.org/10.5194/acp-23-221-2023  

   Schervish, Meredith; Shiraiwa, Manabu (January 2023, Atmospheric Chemistry and Physics)

   Abstract. Evidence has accumulated that secondary organic aerosols (SOAs) exhibit complex morphologies with multiple phases that can adopt amorphous semisolid or glassy phase states. However, experimental analysis and numerical modeling on the formation and evolution of SOA still often employ equilibrium partitioning with an ideal mixing assumption in the particle phase. Here we apply the kinetic multilayer model of gas–particle partitioning (KM-GAP) to simulate condensation of semi-volatile species into a core–shell phase-separated particle to evaluate equilibration timescales of SOA partitioning. By varying bulk diffusivity and the activity coefficient of the condensing species in the shell, we probe the complex interplay of mass transfer kinetics and the thermodynamics of partitioning. We found that the interplay of non-ideality and phase state can impact SOA partitioning kinetics significantly. The effect of non-ideality on SOA partitioning is slight for liquid particles but becomes prominent in semisolid or solid particles. If the condensing species is miscible with a low activity coefficient in the viscous shell phase, the particle can reach equilibrium with the gas phase long before the dissolution of concentration gradients in the particle bulk. For the condensation of immiscible species with a high activity coefficient in the semisolid shell, the mass concentration in the shell may become higher or overshoot its equilibrium concentration due to slow bulk diffusion through the viscous shell for excess mass to be transferred to the core phase. Equilibration timescales are shorter for the condensation of lower-volatility species into semisolid shell; as the volatility increases, re-evaporation becomes significant as desorption is faster for volatile species than bulk diffusion in a semisolid matrix, leading to an increase in equilibration timescale. We also show that the equilibration timescale is longer in an open system relative to a closed system especially for partitioning of miscible species; hence, caution should be exercised when interpreting and extrapolating closed-system chamber experimental results to atmosphere conditions. Our results provide a possible explanation for discrepancies between experimental observations of fast particle–particle mixing and predictions of long mixing timescales in viscous particles and provide useful insights into description and treatment of SOA in aerosol models. 

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4. Effects of oxygen concentration on nanoparticle formation during combustion of iron powders

   https://doi.org/10.1016/j.fuel.2025.135366  

   Chang, Di; Thijs, Leon C; Panahi, Aidin; Mi, Xiaocheng; Bergthorson, Jeffrey M; Levendis, Yiannis A (October 2025, Fuel)

   Egolfopoulos, Fokion (Ed.)

   Powdered iron is being investigated for its potential use as a carbon-free fuel due to its ability to burn heterogeneously and produce oxide particles, which can be collected, reduced back to iron and burned again. However, high temperature oxidation of iron particles can induce partial vaporization/decomposition and evolution of nanometric iron oxide particles. To investigate the formation process of nanoparticles in iron combustion, iron powders (consisting of spheroidal 45–53 μm particles) were injected in an electrically-heated drop tube furnace, operated at a maximum gas temperature of 1375 K, where they experienced high heating rates (104 K/s). The particles reacted with oxygen at concentrations of 15, 21, 35, 50 and 100 % by volume in nitrogen diluent gas. Particles ignited and burned brightly, with peak temperatures reaching 2344–2884 K, depending on the oxygen concentration. The observed distribution of the combustion products of iron was bimodal in size and composition, containing (a) dark gray spherical micrometric particles bigger than their iron particle precursors composed of both magnetite and hematite, and (b) highly agglomerated orange-reddish nanometric particles composed of hematite. The mass fraction of nanometric particles accounted for up to 1.7–7.4 % of the collected products, increasing with the oxygen partial pressure. The nanometric particles were spherules, 30–100 nm in diameter. However, they were highly agglomerated with aggregate aerodynamic diameters peaking at 180–560 nm. The yield of nanoparticles increased with increasing oxygen concentration in the furnace. A heuristic model was used to investigate the impact and sensitivity of various strategies for modeling evaporation, aiming to identify key mechanisms that limit the evaporation rate. This study highlights that understanding the type of liquid at the particle surface is crucial, as evaporation can increase significantly with a homogeneous liquid Fe-O particle compared to a core–shell morphology. Additionally, the analysis suggests that evaporation likely occurs in an intermediate regime where gaseous Fe-containing species oxidize in the boundary layer. Understanding these boundary layer processes is essential for accurately modeling the evaporation rate while maintaining computational efficiency. 1. 

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5. Particle migration of suspensions in a pressure-driven flow over and through a porous structure

   https://doi.org/10.1122/8.0000505  

   Mirbod, Parisa; Shapley, Nina C. (January 2023, Journal of Rheology)

   Laboratory experiments were conducted to study particle migration and flow properties of non-Brownian, noncolloidal suspensions ranging from 10% to 40% particle volume fraction in a pressure-driven flow over and through a porous structure at a low Reynolds number. Particle concentration maps, velocity maps, and corresponding profiles were acquired using a magnetic resonance imaging technique. The model porous medium consists of square arrays of circular rods oriented across the flow in a rectangular microchannel. It was observed that the square arrays of the circular rods modify the velocity profiles and result in heterogeneous concentration fields for various suspensions. As the bulk particle volume fraction of the suspension increases, particles tend to concentrate in the free channel relative to the porous medium while the centerline velocity profile along the lateral direction becomes increasingly blunted. Within the porous structure, concentrated suspensions exhibit smaller periodic axial velocity variations due to the geometry compared to semidilute suspensions (bulk volume fraction ranges from 10% to 20%) and show periodic concentration variations, where the average particle concentration is slightly greater between the rods than on top of the rods. For concentrated systems, high particle concentration pathways aligned with the flow direction are observed in regions that correspond to gaps between rods within the porous medium. 

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