Jamming in granular shear flows of frictional,polydisperse cylindrical particles

In this work, frictional, cylindrical particle shear flows with different size distributions (monodisperse, binary, Gaussian, uniform) are simulated using the Discrete Element Method (DEM). The influences of particle size distribution and interparticle friction coefficient on the solid phase stresses, bulk friction coefficient, and jamming transition are investigated. In frictional dense flows, shear stresses rise rapidly with the increasing solid volume fraction when jamming occurs. The results suggest that at the jamming volume fraction, stress fluctuation and granular temperature achieve the maximum values, and the rate of the stress increase with increasing solid volume fraction approaches the peak value. Meanwhile, the degree of cylindrical particle alignment approaches a valley value. In the polydisperse flows, the jamming volume fraction exhibits significant dependences on the fraction of the longer particles and the particle size distribution. Two models considering the effect of particle size distribution are discussed for predicting the jamming volume fractions of polydisperse flows with frictional, cylindrical particles.     

Validating the assumption to the interference approximation by use of measurements of absorption efficiency and hindered scattering in dense suspensions

Frequency domain photon migration (FDPM) measurements were employed to accurately quantify optical properties of both the suspending fluid and particles within dense polystyrene suspensions of 143- or 226-nm mean diameter at varying concentrations (5-30% by volume). The measured absorption coefficients varied linearly with particle volume fraction whereas the isotropic scattering coefficients varied nonlinearly in agreement with the prediction that utilizes the hard-sphere structure factor model. These results validate the interference approximation of light scattering to describe light propagation accurately within dense suspensions. Furthermore, owing to the accuracy of FDPM absorption measurements, the imaginary refractive indices for both particles and their suspending fluid were determined and were found to compare favorably with literature values.     

Determination of Particle Sizes and Crystalline Phases on Colloidal Silicon Nanoparticle Suspensions

1.IDtroductionLightscatteringpropertiesofsphericalparticlescanbedescribedbyMietheoryl1]whichcanbeaP-pliedtocolloidalparticlessuspendedinthegasorliquidphase.Withsomecarethetheorycanevenbeap-pliedtodrypowdersorparticlesonasolidsupport[2]lthoughitisstrictlyvalidonlyforisolatedsphericalparticles.Informationonparticlesizer,sizedistribu-tionandnumberdensitiesN(r),andrefractiveindexhcanberetrievedbyexperimentaldata.Theex-tinctioncoefficient7ofacolloidalsuspension,forex-ample,canbedeterminedfromthea…     

Stream spreading in multilayer microfluidic flows of suspensions

The goal of this experimental study is to quantify the spreading of parallel streams with viscosity contrast in multilayer microfluidic flows. Three streams converge into one channel where a test fluid is sheathed between two layers of a Newtonian reference fluid. The test fluids are Newtonian fluids with viscosities ranging from 1.1 to 48.2 cP and suspensions of 10-mum-diameter PMMA particles with particle volume fractions phi = 0.16-0.30. The fluid interface locations are identified through fluorescence microscopy. The steady-state width of the center stream is strongly dependent on the viscosity ratio between the adjacent fluids and exhibits a near power-law relationship. This dependence occurs for both the Newtonian fluids and the suspensions, although the slopes differ. The high-concentration suspension (phi = 0.30) diverges from Newtonian behavior, while the low-concentration suspensions (phi = 0.16, 0.22) closely approximate that of the Newtonian fluids. The observed suspension behavior can be attributed to shear-induced particle migration.     

Temperature‐Sensitive Hydrogel‐Particle Films from Evaporating Drops

A simple method to prepare temperature‐sensitive films composed of micrometer‐sized colloidal hydrogel particles using evaporating drops of colloidal suspensions is demonstrated. The films range in thickness from a monolayer to approximately fifty particle diameters depending on initial particle volume fraction. Sessile droplets of hydrogel‐particle suspensions are evaporated on silicon wafers. The film is formed from particles spread densely over the air–water interface which then cross‐link and are deposited on the surface during the evaporation process. The resultant thin films exhibit a temperature‐responsiveness characteristic of the individual particles permitting modulation of size, shape, porosity, and optical transmission.     

Electrorheological fluids — structure and dynamics

Electrorheological fluids are colloidal suspensions that solidify under the influence of electric fields, due to the fact that electric fields induce interactions between particles arising from either the dielectric or the conductivity response of the particles. These interactions are principally dipolar at long distances. However, because of the image forces induced by constant potential electrodes, the long range dipolar repulsion is suppressed. It follows that the ground state of the system consists of a macroscopic phase separation into regions of high and low particle concentrations. The mechanism by which the suspension approaches this phase separation may be strongly dependent on thermal fluctuations. In hydrodynamical flows, these suspensions behave as shear-thinning “Bingham plastics”.     

Measurement of particle-size distribution and volume fraction in concentrated suspensions with photon migration techniques

The determination of the particle size distribution and the volume fraction in concentrated suspensions from the multiwavelength measurement of isotropic-scattering coefficients by use of frequency-domain photon migration techniques is demonstrated for three different polydisperse polystyrene suspensions. When a Newton-type inverse algorithm is used, the successful recovery of the particle size distribution, in the form of a Weibull function, and the volume fraction of polystyrene suspensions is achieved. Our results are in excellent agreement with dynamic light-scattering size distribution measurements. On consideration of the particle mass conservation as an additional constraint penalty term in the inverse algorithm, it is shown that the quality of the particle size distribution reconstruction can be improved. Because no calibration is needed, photon migration techniques are especially suited for on-line measurement of the particle size distribution and the volume fraction in the chemical- and the pharmaceutical-based industries.     

A DEM-based analysis of the influence of aggregate structure on suspension shear yield stress

This study theoretically examined the effect of aggregate structure on the suspension shear yield stress. The aggregation process of colloidal particles was simulated using the discrete element model (DEM) combined with the well-known DLVO theory. The predicted aggregate structural characteristics, namely the coordination number and inter-particle forces were then used in a modified version of the Flatt and Bowen mechanistic model [6] to calculate the corresponding suspension yield stress. The effect of key parameters such as solid volume fraction, suspension pH and ionic strength on the aggregate structure and hence the yield stress of the suspension was investigated.The results showed that the yield stress increased significantly under conditions that were favourable for formation of complex net-like aggregate structures, such as high solid volume fractions, pH values near the iso-electric point, and high ionic strengths. In such cases, the mean coordination number reached a maximum value which was considered to be dependent on the particle size and size distribution. The suspension yield stress exhibited a power law dependency on the solid volume fraction. The interconnected network structure developed at high solid volume fractions was found to be the major contributing factor to the observed high suspension yield stress. As the particle–particle repulsion became significant, a decrease in both the number of bonds and the mechanical bonding strength of the aggregate structure was observed. That was considered to be responsible for the reduction in the suspension yield stress. The suspension yield stress became independent of the suspension ionic strength when the ionic strength exceeded the critical coagulation concentration. Satisfactory agreements were obtained between simulation results and the published experimental data.     

基于颗粒动力学演化的磁弹体力磁耦合数值模型

基于颗粒动力学演化的磁致微观结构建立了横观各向同性磁弹体（MREs）三维几何模型,在考虑了磁场和变形耦合作用的基础上,依据当前MREs研究较热的两种磁颗粒作用模型构建了颗粒的控制方程,从而建立MREs多颗粒的力磁耦合数值模型,从细观角度研究MREs的力磁耦合性能。数值模型和剪切实验对比表明,点偶极子作用力模拟的MREs磁流变效应远低于实验数据,而多极作用力在量级上更接近实验数据。基于构建的数值模型,还详细探究了磁感应强度和颗粒浓度对磁致剪切模量的影响,模拟结果和实验趋势吻合较好,颗粒体积分数在20%附近时,相对磁流变效应达到最大。     

球形NQ对DNAN基熔铸炸药流变性能的影响

为研究硝基胍（NQ）球形化对2,4-二硝基苯甲醚(DNAN)/NQ体系流变性能的影响,采用旋转黏度仪研究了固体质量分数、剪切速率、颗粒形状及级配、工艺温度等因素引起的DNAN/NQ悬浮体系表观黏度的变化规律。结果表明:随着悬浮液固体质量分数的增加,表观黏度呈先缓慢增加至拐点再急剧增加的趋势,球形NQ与级配球形NQ在DNAN中的临界固体质量分数分别为55%和70%;固体质量分数越高,DNAN/NQ悬浮体系表观黏度受剪切速率的影响愈发明显,假塑性程度越高;相同条件下,颗粒越不规则,悬浮液体系越偏离牛顿流体,颗粒级配可有效降低悬浮液体系的假塑性程度;Arrhenius方程可精确描述DNAN/球形NQ悬浮液在96~115 ℃时表观黏度与温度的关系,悬浮液固体质量分数由0增加至41.18%时,流动活化能由36.69 kJ/mol增加到47.59 kJ/mol,悬浮液体系表观黏度相对温度变化越敏感。     

Analytical model of thermal stresses in two- and three-component materials

The paper deals with an analytical model of thermal stresses in isotropic solid continuum represented by periodically distributed isotropic spherical particles in an isotropic infinite matrix, with or without an isotropic spherical envelope on the particle surface. The multi-particle-(envelope)-matrix system to represent a model system regarding the analytical modelling is applicable to two and three types of two- and three-component materials, respectively. The thermal stresses as functions of microstructural parameters (particle volume fraction, particle radius, inter-particle distance, envelope thickness) originate during a cooling process as a consequence of the difference in thermal expansion coefficients and as a consequence of dimension changes of mutually transforming crystalline lattices due to a phase transformation. Additionally, an analytical-(computational)-experimental method of the lifetime prediction based on the analytical model of the thermal stresses in a three-component material is also presented.     

A parametric study of dispersed laminar gas-particle flows through vertical and horizontal channels

Particle diameter, particle phase material density and inlet particle volume fraction are three important parameters governing the flow physics of dispersed gas-particle flows. In this work, an inhouse numerical solver is developed to investigate the effects of particle diameter (Stokes number), particle phase material density, inlet particle volume fraction and inlet phase velocities in the flow characteristics of gas-particle flows through vertical and horizontal channels and also in open domains. It is found that, for a constant inlet particle volume fraction, lower diameter particles attain a higher steady state velocity at any section inside the channel than the higher diameter particles; while the corresponding steady state gas velocity at any section increases with increase in particle diameter. On the other hand, for a constant particle diameter, the steady state gas phase velocity at any section decreases with increase in inlet particle volume fraction. Significant changes in both gas and particle velocity and volume fraction profiles have also been observed with inlet slip, i.e., when the velocities of both the phases at inlet are distinct as opposed to being equal, keeping all other flow and physical parameters invariant.     

Stochastic generation of particle structures with controlled degree of heterogeneity

The recently developed void expansion method (VEM) allows for an efficient generation of porous packings of spherical particles over a wide range of volume fractions. The method is based on a random placement of the structural particles under addition of much smaller “void-particles” whose radii are repeatedly increased during the void expansion. Thereby, they rearrange the structural particles until formation of a dense particle packing and introduce local heterogeneities in the structure. In this paper, microstructures with volume fractions between 0.4 and 0.6 produced by VEM are analyzed with respect to their degree of heterogeneity (DOH). In particular, the influence of the void- to structural particle number ratio, which constitutes a principal VEM-parameter, on the DOH is studied. The DOH is quantified using the pore size distribution, the Voronoi volume distribution and the density-fluctuation method in conjunction with fit functions or integral measures. This analysis has revealed that for volume fractions between 0.4 and 0.55 the void-particle number allows for a quasi-continuous adjustment of the DOH. Additionally, the DOH-range of VEM-generated microstructures with a volume fraction of 0.4 is compared to the range covered by microstructures generated using previous Brownian dynamics simulations, which represent the structure of coagulated colloidal suspensions. Both sets of microstructures cover similarly broad and overlapping DOH-ranges, which allows concluding that VEM is an efficient method to stochastically reproduce colloidal microstructures with varying DOH.     

Optical waveguiding in suspensions of dielectric particles

An optical waveguide formed by a suspension of dielectric nanoparticles in a microchannel is described. The suspensions, chosen for their guiding and scattering properties, are silica and polystyrene particles that have diameters of 30-900 nm and are dispersed in water with volume fractions up to 10%. Changing the diameter and concentration of the particles causes the suspensions to transition from Rayleigh to Mie scattering and from single to multiple scattering. The threshold for optical guiding in a waveguide core composed of these suspensions is set by the numerical aperture of the effective refractive-index difference introduced by the suspension and not by the average interparticle distance.     

Influence of particles on the transition to turbulence in pipe flow

We investigate by experiment the influence of suspended solids upon the transition to turbulence in pipe flow. The particles are monodisperse and neutrally buoyant with the liquid. The role of the particles on the transition depends upon both the volume fraction, phi, and particle size. Below a critical particle diameter, particles alter the transition to larger critical Reynolds numbers for all phi. In contrast to this, larger particles move the transition to smaller Reynolds numbers for small phi, but they delay the transition at larger concentration.     

MEASUREMENT OF PARTICLE SIZE DISTRIBUTION IN COAL OIL WATER FUELS BY FOUR METHODS

Four methods were used to measure the solids particle size distribution in coal-ol1-water fuel. Both dry and wet screening were utilized for the coarser particles while four different instrumental methods were used to measure the finer particles in diluted liquid suspension. The wet stages of the analyses included both solvent-diluted organic suspensions and measurement after inversion to an aqueous system.

Consistent differences in the absolute values of particle size were observed between the four procedures. Possible reasons for these differences are discussed.     

Study of particle size effects on an optical fiber sensor response examined with Monte Carlo simulation

Optical fiber sensors based on the total light transmittance are widely used to measure the volume fraction of particles in suspensions. However, the sensor response depends not only on the volume fraction but also on the particle size. The particle size effect is studied for a sensor configuration consisting of two linear arrays of fibers on each of two blocks: the emitting and receiving blocks. These two linear arrays are arranged with three adjacent fibers (one fiber on the first array, two fibers on the second array) forming a perfect triangle. The almost superimposition of the calculated sensor response versus the extinction factor for different particle sizes allows for the application of single- curve models. Two single-curve models that describe the sensor response for all particle sizes ranging from 36 to 200 microm are proposed. The models are validated by Monte Carlo simulation for different particle sizes and are valid within a detectable volume fraction. The single-curve models proposed provide an easier approach to creating a database for sensor calibration for suspended sediment concentration measurements.     

Analytical model of thermal stresses in two- and three-component materials II

The paper deals with an analytical model of thermal stresses in isotropic solid continuum represented by periodically distributed isotropic spherical particles in an isotropic infinite matrix, with or without an isotropic spherical envelope on the particle surface. The isotropic multi-particle–(envelope)–matrix system to represent a model system regarding the analytical modelling is applicable to two and three types of two- and three-component isotropic materials, respectively. The thermal stresses as functions of microstructural parameters (particle volume fraction, particle radius, inter-particle distance, envelope thickness) originate during a cooling process as a consequence of the difference in thermal expansion coefficients. The analytical modelling based on fundamental equations of mechanics of solid continuum represents a combination of different mathematical techniques applied to the equilibrium and compatibility equations, where a solution for a radial stress, and consequently for tangential and shear stresses is obtained. Finally, as an application example, the analytical model of the thermal stresses in the multi-particle–matrix system is applied to the SiC–Si₃N₄ ceramics representing a two-component material which consists of SiC particles and Si₃N₄ matrix.     

Monitoring particle adsorption by use of laser reflectometry near the critical angle

We investigate the use of laser reflectometry near the critical angle to monitor particle adsorption onto a flat glass surface. Experimental results show that positive particles are adsorbed onto the glass surface and that their adsorption kinetics depend strongly on the volume fraction occupied by the particles in suspension but not appreciably on the particle size. The reflectance near the critical angle is dominated by the particles on the surface, with the contribution of the particles in suspension being very low. We compare the reflectance change near the critical angle with the change in reflectance near the Brewster angle when particles are adsorbed onto the glass surface. We find that reflectometry near the critical angle is 3000 times more sensitive than it is near the Brewster angle. Some optical images are presented to validate our results.     

Numerical simulation of sedimentation in the presence of 2D compressible convection and reconstruction of the particle-radius distribution function

Numerical simulation of the sedimentation of a polydisperse suspension in a convectively unstable medium is presented. For the simulation of 2D compressible convection, the full system of hydrodynamic equations is solved by the explicit MacCormack scheme. Velocities and positions of suspension particles are calculated simultaneously with the solution of the equations. Initially, the particles are randomly distributed in the computational region. The total weight of sedimented matter is recorded during the numerical experiment. The results are compared with the sedimentation of the same suspension without convection. To reconstruct the particle-radius distribution function from the sedimentation curve, a new method is used. This method is based on the solution of the sedimentation integral equation by the Tikhonov regularization method and was recently developed by the author. To illustrate this technique, sedimentation of cement powder in air is simulated. The suspension contains 50000 particles. The particle radii are assumed to be log-normally distributed. Heat-driven convection is completely determined by the top and bottom boundary temperatures of the computational region and lateral boundary conditions. It is shown that convective motions of a medium with sedimented particles lead to the following effect: the fine disperse fraction of the suspension remains suspended much longer than without convection. Some particles will not sediment at all. The maximum radius of the particles of this fraction depends on the convection parameters (e.g. on convection cell size and convection velocities). These parameters, in their turn, depend only on the temperature difference of the top and bottom boundaries. The results of these calculations can be applied in geology and meteorology for studying dust sedimentation in air as well as in technology. Heat-driven convection can be used for separation of suspensions with the cut-off particle radius depending on temperature difference only.