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1. Designing non-segregating granular mixtures

   https://doi.org/10.1051/epjconf/202124903011  

   Duan, Yifei; Umbanhowar, Paul B.; Lueptow, Richard M. (January 2021, EPJ Web of Conferences)

   Aguirre, M.A.; Luding, S.; Pugnaloni, L.A.; Soto, R. (Ed.)

   In dense flowing bidisperse particle mixtures varying in size or density alone, large particles rise (driven by percolation) and heavy particles sink (driven by buoyancy). When the two particle species differ from each other in both size and density, the two segregation mechanisms either enhance (large/light and small/heavy) or oppose (large/heavy and small/light) each other. In the latter case, an equilibrium condition exists in which the two mechanisms balance and the particles no longer segregate. This leads to a methodology to design non-segregating particle mixtures by specifying particle size ratio, density ratio, and mixture concentration to achieve the equilibrium condition. Using DEM simulations of quasi-2D bounded heap flow, we show that segregation is significantly reduced for particle mixtures near the equilibrium condition. In addition, the rise-sink transition for a range of particle size and density ratios matches the predictions of the combined size and density segregation model. 

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2. Modelling segregation of bidisperse granular mixtures varying simultaneously in size and density for free surface flows

   https://doi.org/10.1017/jfm.2021.342  

   Duan, Yifei; Umbanhowar, Paul B.; Ottino, Julio M.; Lueptow, Richard M. (July 2021, Journal of Fluid Mechanics)

   null (Ed.)

   Flowing granular materials segregate due to differences in particle size (driven by percolation) and density (driven by buoyancy). Modelling the segregation of mixtures of large/heavy particles and small/light particles is challenging due to the opposing effects of the two segregation mechanisms. Using discrete element method (DEM) simulations of combined size and density segregation we show that the segregation velocity is well described by a model that depends linearly on the local shear rate and quadratically on the species concentration for free surface flows. Concentration profiles predicted by incorporating this segregation velocity model into a continuum advection–diffusion–segregation transport model match DEM simulation results well for a wide range of particle size and density ratios. Most surprisingly, the DEM simulations and the segregation velocity model both show that the segregation direction for a range of size and density ratios depends on the local species concentration. This leads to a methodology to determine the combination of particle size ratio, density ratio and particle concentration for which a bidisperse mixture will not segregate. 

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3. Impacts of packed bed polydispersity and deformation on fine particle transport

   https://doi.org/10.1002/aic.18499  

   Vyas, Dhairya_R; Gao, Song; Umbanhowar, Paul_B; Ottino, Julio_M; Lueptow, Richard_M (June 2024, AIChE Journal)

   Abstract Static granular packings play a central role in numerous industrial applications and natural settings. In these situations, fluid or fine particle flow through a bed of static particles is heavily influenced by the narrowest passage connecting the pores of the packing, commonly referred to as pore throats, or constrictions. Existing studies predominantly assume monodisperse rigid particles, but this is an oversimplification of the problem. In this work, we illustrate the connection between pore throat size, polydispersity, and particle deformation in a packed bed of spherical particles. Simple analytical expressions are provided to link these properties of the packing, followed by examples from Discrete Element Method (DEM) simulations of fine particle percolation demonstrating the impact of polydispersity and particle deformation. Our intent is to emphasize the substantial impact of polydispersity and particle deformation on constriction size, underscoring the importance of accounting for these effects in particle transport in granular packings. 

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4. Programmable Crowding and Tunable Phases in a Binary Mixture of Colloidal Particles under Light-Driven Thermal Convection

   https://doi.org/10.1021/acs.jpcb.4c02301  

   Lopez-Ceja, Jose; Flores, Vanessa; Juliano, Shirlaine; Machler, Sean; Smith, Stephen; Mansingh, Gargi; Shen, Meng; Tanjeem, Nabila (September 2024, The Journal of Physical Chemistry B)

   We employ photothermally driven self-assembly of colloidal particles to design microscopic structures with programmable size and tunable order. The experimental system is based on a binary mixture of “plasmonic heater” gold nanoparticles and “assembly building block” microparticles. Photothermal heating of the gold nanoparticles under visible light causes a natural convection flow that efficiently assembles the microscale building block particles (diameter 1–10 μm) into a monolayer. We identify the onset of active Brownian motion of colloidal particles under this convective flow by varying the conditions of light intensity, gold nanoparticle concentration, and sample height. We realize a crowded assembly of microparticles around the center of illumination and show that the size of the particle crowd can be programmed using patterned light illumination. In a binary mixture of gold nanoparticles and polystyrene microparticles, we demonstrate the formation of rapid and large-scale crystalline monolayers, covering an area of 0.88 mm2 within 10 min. We find that the structural order of the assembly can be tuned by varying the surface charge of the nanoparticles and the size of the microparticles, giving rise to the formation of different phases–colloidal crystals, crowds, and gels. Using Monte Carlo simulations, we explain how the phases emerge from the interplay between hydrodynamic and electrostatic interactions, as well as the assembly kinetics. Our study demonstrates the promise of self-assembly with programmable shapes and structural order under nonequilibrium conditions using an accessible setup comprising only binary mixtures and LED light. 

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5. Air entrapment, internal structure, and density of hydrophobic particle–water–air mixtures

   https://doi.org/10.1680/jenge.25.00003  

   Ma, Wenpei; Tomac, Ingrid (November 2025, Environmental Geotechnics)

   This paper investigates internal structure-driven density changes of post-wildfire and natural debris flows resulting from sand hydrophobicity and shearing. Hydrophobic sand particles entrap air by way of an armoured bubble/gas marble mechanism in water. Although individual armoured bubbles have already been broadly investigated, the effects of fluid drag and collisions in multiphase water–air–sand mixtures remain largely unexplored. The armoured bubbles’ stability in water depends on the force balance on the air bubble–particle boundary, which largely defines the mixture’s internal structure. Gravity, relative armoured bubble and fluid velocities govern the collision forces, local changes in mixture concentration, and the separation or attachment of hydrophobic particles to air bubbles in water. The initially large entrapped air volume decreases due to degassing and large armoured bubble breakdowns downstream. Experimental and theoretical approaches quantify the air entrapment under different sand-water volumetric concentrations, as well as the effects of mixing speed, duration, and sand particle size on the final mixture’s internal structure. Since hydrophobic sand particles can effectively entrap many air bubbles in the final debris flow-like mixture, the densities of debris flows that sweep over hydrophobic soil will accordingly reduce. Therefore, this paper proposes empirical estimates of density reductions resulting from air entrapment. 

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