Small Particle Separations Research Articles

Electrostatic interactions between rough dielectric particles.

From colloid suspension to particle aggregation in protoplanetary formation, electrostatic attraction and repulsion between particles is a key mechanism behind the aggregation and clustering of particles. Although most studies have focused on canonical spherical particles, it remains unclear how nonspherical and rough dielectric particles interact and whether the complicated interplay between roughness and charge distribution affects their force couplings. Here a boundary-element method model was leveraged to study electrostatic interactions between charged dielectric particles with modular, axisymmetric surface features. Charge accumulation at convex surface asperities decreases the strength of electrostatic interactions between particles, and the sensitivity of the electrostatic force to the particle surface roughness and orientation is especially apparent at small particle separations. Surface interactions between the particle near-contact regions were isolated to determine the degree that near-contact interactions dictate the relationship between the net electrostatic force and the particle roughness and orientation. A correction factor ΔF is introduced to recover higher order dielectric effects from a low order analytical model. Finally, implications of surface charge asymmetries produced for different particle orientations and surface roughnesses on the long-standing problem of triboelectrification are discussed.

Universal alignment in turbulent pair dispersion

Countless processes in nature and industry, from rain droplet nucleation to plankton interaction in the ocean, are intimately related to turbulent fluctuations of local concentrations of advected matter. These fluctuations can be described by considering the change of the separation between particle pairs, known as pair dispersion, which is believed to obey a cubic in time growth according to Richardson’s theory. Our work reveals a universal, scale-invariant alignment between the relative velocity and position vectors of dispersing particles at a mean angle that we show to be a universal constant of turbulence. We connect the value of this mean angle to Richardson’s traditional theory and find agreement with data from a numerical simulation and a laboratory experiment. While the Richardson’s cubic regime has been observed for small initial particle separations only, the constancy of the mean angle manifests throughout the entire inertial range of turbulence. Thus, our work reveals the universal nature of turbulent pair dispersion through a geometrical paradigm whose validity goes beyond the classical theory, and provides a framework for understanding and modeling transport and mixing processes.

Yukawa-Friedel-tail pair potentials for warm dense matter applications.

Accurate equationsof state (EOS) and plasma transport properties are essential for numerical simulations of warm dense matter encountered in many high-energy-density situations. Molecular dynamics (MD) is a simulation method that generates EOS and transport data using an externally provided potential to dynamically evolve the particles without further reference to the electrons. To minimize computational cost, pair potentials needed in MD may be obtained from the neutral-pseudoatom (NPA) approach, a form of single-ion density functional theory (DFT), where many-ion effects are included via ion-ion correlation functionals. Standard N-ion DFT-MD provides pair potentials via the force matching technique but at much greater computational cost. Here we propose a simple analytic model for pair potentials with physically meaningful parameters based on a Yukawa form with a thermally damped Friedel tail (YFT) applicable to systems containing free electrons. The YFT model accurately fits NPA pair potentials or the nonparametric force-matched potentials from N-ion DFT-MD, showing excellent agreement for a wide range of conditions. The YFT form provides accurate extrapolations of the NPA or force-matched potentials for small and large particle separations within a physical model. Our method can be adopted to treat plasma mixtures, allowing for large-scale simulations of multispecies warm dense matter.

An efficient spiral microchannel for continuous small particle separations

We report a new two-step microfluidic device that performs efficient, continuous, and high throughput small particle separation. The separation mechanism is based the on combined effects of inertial focusing in a spiral microchannel followed by particle deflection in a straight microchannel. The particle trajectory was studied using 1 and 5.5μm fluorescent polystyrene microbeads as model particles to be separated. We demonstrate that at the condition of 0.4ml/min inlet, 0.15ml/min acting, and 0.02ml/min protecting flow rates, the separation efficiencies of 1μm particles and 5.5μm particles were 99.7% and 98.3%, and the corresponding purities were 95.7% and 99.6%, respectively. This new design shows a significant separation performance for small particles, and therefore can be potentially integrated as a promising tool to separate rare of-interest particles, such as bacteria from homogenized food matrix as part of sample preparation procedure for food safety inspections.

An analytical method for determining the interaction energy between multiple identical spherical colloidal zinc oxide nanoparticles

A modification of the linear superposition approximation for the linearized Poisson- Boltzmann equation, describing the interaction of colloidal nanoparticles, is proposed based on the implementation of a phenomenological charge regulation model to allow for the more accurate description of the interaction at small particle separations and multi-particle interactions.

Relative velocities of inertial particles in turbulent aerosols

We compute the joint distribution of relative velocities and separations of identical inertial particles suspended in randomly mixing and turbulent flows. Our results are obtained by matching asymptotic forms of the distribution. The method takes into account spatial clustering of the suspended particles as well as singularities in their motion (so-called ‘caustics’). It thus takes proper account of the fractal properties of phase space and the distribution is characterised in terms of the corresponding phase-space fractal dimension D2. The method clearly exhibits universal aspects of the distribution (independent of the statistical properties of the flow): at small particle separations R and not too large radial relative speeds |VR|, the distribution of radial relative velocities exhibits a universal power-law form , provided that D2 ≤ d + 1 and that the Stokes number St is large enough for caustics to form. The range in VR over which this power law is valid depends on R, on the Stokes number and upon the nature of the flow. Our results are in good agreement with results of computer simulations of the dynamics of particles suspended in random velocity fields in one and two spatial dimensions with finite correlation times. In the white-noise limit, the results are consistent with those of Gustavsson and Mehlig [Phys. Rev. E84 (2011) 045304].

Molecular Dynamic Study of the Swelling Behavior of Na-Montmorillonite

Research Article| May 01, 2013 Molecular Dynamic Study of the Swelling Behavior of Na-Montmorillonite PRIYANTHI M. AMARASINGHE; PRIYANTHI M. AMARASINGHE 1 Department of Civil Engineering, Johns Hopkins University, MD 1Corresponding author email: pamaras1@jhu.edu. Search for other works by this author on: GSW Google Scholar A. ANANDARAJAH A. ANANDARAJAH Department of Civil Engineering, Johns Hopkins University, MD Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2013) 19 (2): 173–183. https://doi.org/10.2113/gseegeosci.19.2.173 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PRIYANTHI M. AMARASINGHE, A. ANANDARAJAH; Molecular Dynamic Study of the Swelling Behavior of Na-Montmorillonite. Environmental & Engineering Geoscience 2013;; 19 (2): 173–183. doi: https://doi.org/10.2113/gseegeosci.19.2.173 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Swelling of clays due to water saturation causes structural damage in civil engineering structures. At the microstructural level, swelling is caused by interlayer and interparticle (double-layer) forces. The present article is focused on the interlayer force with the specific objective of developing a force-displacement relation. On the basis of the variation of the pressure tensor with distance, the interlayer repulsive force is evaluated. The interlayer force-displacement relation is thus established. It is found that (a) the interlayer force for small particle separations can be hundreds of times the double-layer force at the corresponding separations, and (b) the interlayer force decays very quickly with increasing particle separation. The results indicate that the fundamental mechanism controlling swelling changes as the specimen's density changes, implying that mitigation strategies should depend on the density of the soil. The interlayer force-displacement relation is needed in assembly analyses such as the discrete element analysis. An ongoing study indicates that the interlayer force does not contribute to the overall swelling pressure when the void ratio is larger than a certain value. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Wigner crystallization in two dimensions: Evolution from long- to short-ranged forces

We study fermions in two dimensions interacting via a long-ranged 1/r potential for small particle separations and a short-ranged 1/r^3 potential for larger separations in comparison to a length scale \xi. We compute the energy of the Wigner crystal and of the homogeneous Fermi liquid phases using a variational approach, and determined the phase diagram as a function of density and \xi at zero temperature. We discuss the collective modes in the Fermi liquid phase, finite temperature effects on the phase diagram, and possible experimental realizations of this model.

Rapid Analysis of Dried Blood Spot Samples with Sub-2-µM LC–MS/MS

The use of dried blood and dried plasma spots for storage and transportation of samples derived from clinical trials holds the promise to reduce cost, simplify storage and shipping as well as reducing animal usage. From the bioanalysts' point of view, these dried-paper samples add an extra layer of complexity to the analysis introducing extra matrix effects from the paper itself and sometimes from antiviral treatments applied to the card. In this article we demonstrate the use of the sub-2-µm particle LC-MS/MS for the bioanalysis of samples derived from a dried blood spot. The higher resolution provided by these small-particle separations allowed for greater resolution of the analyte from the endogenous components in blood samples and from the card-treatment chemicals. The method-development process was enhanced by the use of MS, which could simultaneously acquire full scan and multiple reaction monitoring data, allowing resolution from metabolites and endogenous matrix components. The use of this approach produced sensitivity levels in the 50-100 pg/ml range and analysis times in the 1-2 min range, which was five-times more sensitive and three-times faster than HPLC. This throughput and sensitivity makes this approach ideal for the analysis of preclinical and clinical studies derived from dried blood spots.

Electrostatic interaction between two spherical soft particles at small separations

An approximate analytic expression is given for the potential energy of the electrostatic interaction between two identical spherical soft particles (ion-penetrable porous spheres) in a symmetrical electrolyte solution with the help of Derjaguin's approximation. This method uses accurate analytic expressions for the corresponding interaction energy between two parallel similar soft plates (ion-penetrable membranes) obtained previously (J. Colloid Interface Sci., 350 (2010) 249). Agreement with numerical data (J. Colloid Interface Sci., 36 (1989) 523) is excellent for arbitrary potentials and small particle separations, where the Debye–Hückel linearization approximation and the linear superposition approximation are not allowed.

The van der Waals interaction between two torus-shaped colloidal particles

An expression for the van der Waals interaction energy between two parallel torus-shaped colloidal particles is derived. It is shown that for small particle separations the torus–torus interaction energy is well approximated by the interaction energy between two parallel cylinders, each being produced by cutting a torus and unrolling it. In particular, in the limit of very small particle separations, the interaction energy between two parallel tori and that between the corresponding two parallel cylinders tend to the same limiting value. This limiting value also agrees with the result of applying Derjaguin's approximation to the interaction between two parallel cylinders.

Particle encapsulation due to thread breakup in Stokes flow

The capillary instability of a liquid thread containing a regular array of spherical particles along the centreline is considered with reference to microencapsulation. The thread interface may be clean or occupied by an insoluble surfactant. The main goal of the analysis is to illustrate the effect of the particle spacing on the growth rate of axisymmetric perturbations and identify the structure of the most unstable modes. A normal-mode linear stability analysis based on Fourier expansions for Stokes flow reveals that, at small particle separations, the interfacial profiles are nearly pure sinusoidal waves whose growth rate is nearly equal to that of a pure thread devoid of particles. Higher harmonics suddenly enter the normal modes for moderate and large particle separations, elevating the growth rates and yielding a stability diagram that consists of a sequence of superposed pure-thread lobes. A complementary numerical stability analysis based on the boundary integral formulation for Stokes flow reveals the strong stabilizing effect of particles whose radius is comparable to the thread radius. Numerical simulations of the finite-amplitude motion based on the boundary integral method demonstrate that thread breakup leads to particles coated with annular layers of different thicknesses.

Diffuse Double-Layer Interaction between Two Identical Spherical Colloidal Particles

A simple method is given for calculating the potential energy of the diffuse double-layer interaction between two identical spherical colloidal particles in a symmetrical electrolyte solution with the help of Derjaguin's approximation. This method uses accurate analytic expressions for the corresponding interaction energy between two parallel similar plates obtained previously (Colloids Surf. A Physicochem. Eng. Asp.146, 213 (1999); J. Colloid Interface Sci.212, 130 (1999)). Agreement with numerical data provided by Honig and Mul (J. Colloid Interface Sci.36, 258 (1971)) is excellent particularly for small particle separations.

The Applicability of Acoustic Wave Propagation Models to Silica Sols and Gels

Acoustic attenuation and phase velocity in the frequency range 2–50 MHz have been measured in a series of silica sols and gels with particle sizes in the range 12–30 nm, and concentrations in the range 5–40% (w/w). Results have been compared with both scattering and hydrodynamic models of acoustic propagation in colloids. Differences between measured and simulated results indicate that present models are inadequate for very small particle sizes and small particle separations (<50 nm), where very high number concentrations of scatterers are present.

Variational and diffusion Monte Carlo techniques for quantum clusters.

The H[sub 2]-[sup 4]He dimer and small [sup 4]He clusters are studied using Monte Carlo sampling techniques. We consider alternative wave-function forms in order to obtain high accuracy efficiently. For the smaller systems, both guided and unguided Metropolis walks are used and the effciencies are studied. Of particular concern is accurate sampling at small particle separations and the behavior of the local energy in this regime. As a final step, we compute exact energies by a diffusion Monte Carlo method. We obtain converged energies significantly below the Green's-function Monte Carlo values, which employed an earlier He-He potential with a slightly shallower well. For He[sub 3] and He[sub 20], the Green's-function Monte Carlo energies are reproduced when employing the same potential. However, for the 112-atom cluster, our converged energy lies below the Green's-function Monte Carlo value. Second-order estimates of the exact density profiles and particle separation distributions, [ital p], are also determined. For the 14- and 20-atom clusters, second-order estimates of [ital p] show enhanced structure in comparison to variational Monte Carlo results. Statistically meaningful oscillations in the second-order estimates of the exact density profiles are not observed.

Influence of drying on the liquid limit behaviour of a marine clay

The possible mechanisms of particle aggregation and reduction in liquid limit of the Cochin marine clay on drying are investigated. Mineralogical analysis showed the absence of halloysite in the marine specimen. Experimental results also ruled out the possibility of cementitious material being responsible for particle aggregation and reduction in clay plasticity on drying. The presence of calcium and magnesium as the predominant exchangeable ions and of a high pore salt concentration facilitates strong interparticle attraction and small particle separations; the latter leads to development of significant capillary stresses that permits an intimate contact of particles and growth of strong van der Waals' and Coulombic bonds.

Electron correlations in a two-dimensional electron gas

The pair-correlation function $g(r)$ and the correlation energy of a two-dimensional electron gas have been investigated in the Hartree-Fock and the Lundqvist-Overhauser theory of the random-phase approximation. At carrier densities lower than 4 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ in the inversion layer of Si, $g(r)$ is found to be negative for small separation between particles. For higher densities, $g(r)$ remains positive for all values of $r$ although it tends to become unreasonably large for small particle separations at very high carrier densities. The consequences of these unrealistic values of $g(r)$ on the various other properties calculated within these approximations are discussed.

Stability of dispersions in non-aqueous media. Part 8.—Theoretical analysis of heterocoagulation

The potential energy of charged colloidal particles in media of low dielectric constant has been derived by application of Maxwell's equations for the potential energy of charged conducting spheres. In this treatment the presence of an electric double layer is ignored. Data computed using the Maxwell expressions are compared with those obtained from the simple Coulombic law for repulsion between point charge. The differences are particularly significant for small particle separations. Serious errors are introduced by the use of Coulomb's law for particles interacting at constant potential, but for the constant charge case are significantly less although usually > 5 %.

Cluster Development Method in the Quantum Mechanics of Many Particle System, I

In order to investigate the many particle system by the variational principle, the cluster develop­ ment method propcsed by Jastrow is sys~cmatically formulated. It is a dircct generalization of the Hartree method, in which two particle functions are included in the trial function to represent the correlations between particlcs. The cnergyxpectation value is expar..ded in a series, each term of which is expressed in cluster integra's. It is found that the series convergcs rapidly when the cor· relations are marked only fot· small particle separations, so that this method is expected to be useful especially for the system of particles interacting through short range potentials with repulsive cores. § I. Introduction The Hartree method ·for a many particle system, in which each particle is supposed to move independently 2.nd to be influenced by other p2.rticles only through their average field of force, has been an importa.nt approximation a.nd successful in various problems. However, in some cases where the correlations between particles play an important role, this simple approximation is no longer valid. In one case, when the range of the interaction between particles is long compared with the mean distance between them, many particles may be coupled togeth:::r to move in some organized mode of motion as a whole, as is seen in the typical example of the plasma oscillation in- a dense electron gas. The recent study by Tomonaga 1l made the mechanism of this so-called collective motion quite clear_ In :>.nothe: case, when the interaction is strong but of short range, the actual potential determi11cd by the instantaneous configuration of the other . particles fluctuates from the average one considerably, and the image of the independent particle motion becomes far from the truth. Such a situation occurs especially when the interaction potential has a repulsive core at short distance as in liquid helium .or in the atomic nucleus. (In the latter case the existence of the repulsive core seems very likely from the analysis of the nucleon-nucleon scattering/) the binding energy differ~nce between H'l and He3, 3l and so forth.) In fact, if .we apply the Hartree method to these systems, the rc3ulting average potential diverges to infinity. Actually, the wave function of the system must vanish in the region of the configuration space where at least two particles arc fotmd. within the hard core range. In such cases we have to take into· account this strong short range correlation in some way.