Discrete-element modeling of particulate aerosol flows

Clustering of fine particles is of crucial importance in settings ranging from the early stages of planet formation [1][2][3] to the coagulation of industrial powders and airborne pollutants [4][5][6][7] . Models of such clustering typically focus on inelastic deformation and cohesion 1,4,6,8 . However, even in charge-neutral particle systems comprising grains of the same dielectric material, tribocharging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces [9][10][11] . The e ects of such forces on cluster formation are not well understood and have so far not been studied in situ. Here we report the first observations of individual collide-and-capture events between charged submillimetre particles, including Kepler-like orbits. Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces. This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation 1,12 . Moreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing new insights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids [13][14][15][16] . One of the key difficulties in studying the interplay between repulsive contact forces, short-range cohesion and long-range electrostatic forces during cluster formation has been to obtain sufficiently detailed experimental data. Seeing how this process unfolds demands in situ observation of the collision trajectories among charged grains to extract quantitative information about their interactions. This requires the grains to be freed from gravity and tracked with high spatial and temporal resolution to capture individual collision events 17,18 . We overcome these obstacles with the set-up shown in Fig. 1a (refs 19,20). The granular material, in our experiments fused zirconium dioxide-silicate grains a few hundred micrometres in diameter, is contained in a vessel mounted inside a 3.0-m-tall cylindrical chamber. We evacuate this chamber to mitigate air drag. When a shutter covering a small orifice at the bottom of the vessel is opened, particles fall out freely, forming a dilute stream. Outside the chamber, a high-speed video camera falls alongside the grains, guided by low-friction rails. In the co-moving frame seen by the camera, the effect of gravity is eliminated, making it possible to track particle interactions in detail for about 0.2 s until the camera is decelerated by a foam pad. The same apparatus also allows determination of the net charge on individual grains: during free fall a horizontal electric field can be applied and the resulting horizontal acceleration observed by the camera gives the charge to mass ratio, q/m.Using particles with a narrow size d...

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Direct observation of particle interactions and clustering in charged granular streams

et al. 2015

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“…To measure the resulting residual cohesion between grains, atomic force microscopy (AFM) has proven useful [4,9,10], but this technique mimics the static limit of central, headon collisions. Under dynamic conditions, energy is also dissipated due to the inelastic nature of collisions, including rolling and sliding friction during non-central impacts [5,[11][12][13][14]. Simulations have started to address the competition between inelasticity and cohesion during collisions in freely cooling granular systems, but so far have focused on the limit of a dilute granular 'gas' [6,13,15].…”

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confidence: 99%

“…Under dynamic conditions, energy is also dissipated due to the inelastic nature of collisions, including rolling and sliding friction during non-central impacts [5,[11][12][13][14]. Simulations have started to address the competition between inelasticity and cohesion during collisions in freely cooling granular systems, but so far have focused on the limit of a dilute granular 'gas' [6,13,15]. Little is known about the complex dynamics that lead to clustering in the dense limit, where many weakly cohesive particles collide and interact in rapid succession.…”

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confidence: 99%

Droplet and cluster formation in freely falling granular streams

et al. 2011

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Particle beams are important tools for probing atomic and molecular interactions. Here we demonstrate that particle beams also offer a unique opportunity to investigate interactions in macroscopic systems, such as granular media. Motivated by recent experiments on streams of grains that exhibit liquid-like breakup into droplets, we use molecular dynamics simulations to investigate the evolution of a dense stream of macroscopic spheres accelerating out of an opening at the bottom of a reservoir. We show how nanoscale details associated with energy dissipation during collisions modify the stream's macroscopic behavior. We find that inelastic collisions collimate the stream, while the presence of short-range attractive interactions drives structure formation. Parameterizing the collision dynamics by the coefficient of restitution (i.e., the ratio of relative velocities before and after impact) and the strength of the cohesive interaction, we map out a spectrum of behaviors that ranges from gas-like jets in which all grains drift apart to liquid-like streams that break into large droplets containing hundreds of grains. We also find a new, intermediate regime in which small aggregates form by capture from the gas phase, similar to what can be observed in molecular beams. Our results show that nearly all aspects of stream behavior are closely related to the velocity gradient associated with vertical free fall. Led by this observation, we propose a simple energy balance model to explain the droplet formation process. The qualitative as well as many quantitative features of the simulations and the model compare well with available experimental data and provide a first quantitative measure of the role of attractions in freely cooling granular streams

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“…The non-dimensional Navier-Stokes equations in the presence of RBCs may be written as (without asterisks) based on [44],…”

Section: The Modified Navier-stokes Equations and Power Lawmentioning

confidence: 99%

Red blood cell (RBC) aggregation and its influence on non-Newtonian nature of blood in microvasculature

Murali

Nithiarasu

2017

Journal of Modeling in Mechanics and Materials

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Abstract:A robust computational model is proposed to investigate the non-Newtonian nature of blood flow due to rouleaux formation in microvasculature. The model consists of appropriate forces responsible for red blood cell (RBC) aggregation in the microvasculature, tracking of RBCs, and coupling between plasma flow and RBCs. The RBC aggregation results have been compared against the available data. The importance of different hydrodynamic forces on red blood cell aggregation has been delineated by comparing the time dependent path of the RBCs. The rheological changes to the blood flow have been investigated under different shear rates and hematocrit values and quantified with and without RBC aggregation. The results obtained in terms of wall shear stress (WSS) and blood viscosity indicate a significant difference between Newtonian and powerlaw fluid assumptions.

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Discrete-element modeling of particulate aerosol flows

Cited by 236 publications

References 45 publications

Direct observation of particle interactions and clustering in charged granular streams

Direct observation of particle interactions and clustering in charged granular streams

Droplet and cluster formation in freely falling granular streams

Red blood cell (RBC) aggregation and its influence on non-Newtonian nature of blood in microvasculature

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