Macroscopic elastic properties of regular lattices

In this paper we study analytically the elastic properties of the 2-D and 3-D regular lattices consisting of bonded particles. The particle-scale stiffnesses are derived from the given macroscopic elastic constants (i.e. Young's modulus and Poisson's ratio). Firstly a bonded lattice model is presented. This model permits six kinds of relative motion and corresponding forces between each bonded particle pair. By comparing the strain energy distributions between the discrete lattices and the continuum, the explicit relationship between the microscopic and macroscopic elastic parameters can be obtained for the 2-D hexagonal lattice and the 3-D hexagonal close-packed and face-centered cubic structures. The results suggest that the normal stiffness is determined by Young's modulus and the particle size (in 3-D), and that the ratio of the shear to normal stiffness is related to Poisson's ratio. Rotational stiffness depends on the normal stiffness, shear stiffness and particle sizes. Numerical tests are carried out to validate the analytical results. The results in this paper have theoretical implications for the calibration of the spring stiffnesses in the Discrete Element Method.     

Pickering emulsions stabilized by colloidal surfactants: Role of solid particles

Pickering emulsions are emulsions stabilized by colloidal surfactants, i.e. solid particles. Compared with traditional molecular surfactant-stabilized emulsions, Pickering emulsions show many advantages, such as high resistance to coalescence, long-term stability, good biocompatibility and tunable properties. In recent years, Pickering emulsions are widely applied in scientific researches and industrial applications. In this review, we focus on the influences of particle properties on Pickering emulsions, including particle amphiphilicity, concentration, size and shape, and summarize the strategies developed for the preparation of amphiphilic Janus particles. The applications of Pickering emulsions in food industry, cosmetic industry, material science, drug delivery, biomedical research and vaccine adjuvant will also be covered. Pickering emulsions are a unique system for multi-disciplinary studies and will become more and more important in the future.     

Influence of suspension stability on wet grinding for production of mineral nanoparticles

Grinding behavior of nanoparticles in an attritor mill and the minimum achievable particle size are strongly influenced by the suspension stability. In the present work, suspension stability (i.e. (-potential) of nanoparticles was studied by measuring pH as a function of grinding time in the wet milling process. It was found that after a certain time in an attritor mill, there is no further size reduction and the average product particle size increases monotonically. One of the reasons is that the production of submicron particles leads to more particle-particle interactions and consequently pH of the suspension decreases with grinding time. Usually pH value is related to suspension stability and it can be enhanced by addition of NaOH solution. The maximum negative (-potential of -51.2 mV was obtained at pH of 12 for silica. The higher the (-potential with the same polarity, higher will be the electrostatic repulsion between the particles. Hence, the maximum electrostatic repulsion force was maintained by the adjustment of pH value in wet milling. The experiments were conducted at different pH conditions which were maintained constant throughout the experiments and nanosized particles were obtained consequently.     

PARTICLE PROPERTIES—PROMISE AND NEGLECT

We see two major trends in Particle Technology. First, the focus is shifted from unit operations towards functional products, i.e. towards product engineering. Second, modeling will become more and more important. Processes cannot yet be designed from basic molecular understanding. Nanotechnology, however, begins to bridge this gap between molecules and particles and may thus open new ways not only for the production and handling of particulate matter but also for the engineered design of advanced material properties. Starting from the concept of product engineering we investigate the basic preconditions for tailoring nanoparticulate properties, i.e. the control of the particle interactions. Nanotechnology can only be transferred to industrial production if the interactions are effectively controlled. Material and particle properties are essential for predictive models. Although strong tools like MD, DEM or population balance models are available, these models are only predictive if realistic material and particle properties are available which is often not the case. We show for selected examples how particle properties can be obtained by studying the physically relevant elementary processes. The impact breakage behavior of many different materials is described by a master curve. Particle adhesion can be modeled if the roughness of particle and substrate and the Hamaker constant are known. The latter is obtained from adsorption studies.     

DRAG ON SUBMICRON NANOPARTICLE AGGREGATES

A new procedure was developed for estimating the effective collision diameter of an aggregate composed of primary particles of any size. The coagulation coefficient of two oppositely charged particles was measured experimentally and compared with classic Fuchs theory, including a new method to account for particle non-sphericity. A second set of experiments were performed on well-defined nanoparticle aggregates at different stages of sintering, i.e. from the aggregate to the fully sintered stage. Here, electrical mobility was used to characterize the particle drag. The aggregates are being built from two different size-fractionated nanoparticle aerosols, the non-aggregated particles are discarded by an electrofilter and then they are passed through a furnace at concentrations low enough not to induce coagulation.     

基于扩展有限元法的颗粒增强复合材料静态及动态断裂行为研究

运用不含裂尖增强函数的扩展有限元法,分别研究了静态及动态载荷作用下颗粒增强复合材料的断裂行为。假定基体和颗粒都为线弹性材料并且绑定完好,研究了不同颗粒位置、数量对基体裂纹尖端断裂参数的影响。数值模拟结果清晰地显示了含刚性颗粒和柔性颗粒时不同的失效机制。结果同时证明本文采用的数值方法能够准确预报颗粒增强复合材料的断裂行为,也更易于被工程界所接受     

The uniaxial tension of particulate composite materials with nonlinear interface debonding

Debonding of particle/matrix interfaces can significantly affect the macroscopic behavior of composite material. We have used a nonlinear cohesive law for particle/matrix interfaces to study interface debonding and its effect on particulate composite materials subject to uniaxial tension. The dilute solution shows that, at a fixed particle volume fraction, small particles lead to hardening behavior of the composite while large particles yield softening behavior. Interface debonding of large particles is unstable since the interface opening (and sliding) displacement(s) may have a sudden jump as the applied strain increases, which is called the catastrophic debonding. A simple estimate is given for the critical particle radius that separates the hardening and softening behavior of the composite.     

Influence of suspension stability on wet grinding for production of mineral nanoparticles

Grinding behavior of nanoparticles in an attritor mill and the minimum achievable particle size are strongly influenced by the suspension stability. In the present work, suspension stability （i.e. ξ-potential） of nanoparticles was studied by measuring pH as a function of grinding time in the wet milling process. It was found that after a certain time in an attritor mill, there is no further size reduction and the average product particle size increases monotonically. One of the reasons is that the production of submicron particles leads to more particle-particle interactions and consequently pH of the suspension decreases with grinding time. Usually pH value is related to suspension stability and it can be enhanced by addition of NaOH solution. The maximum negative ξ-potential of -51.2 mV was obtained at pH of 12 for silica. The higher the ξ-potential with the same polarity, higher will be the electrostatic repulsion between the particles. Hence, the maximum electrostatic repulsion force was maintained by the adjustment ofpH value in wet milling. The experiments were conducted at different pH conditions which were maintained constant throughout the experiments and nanosized particles were obtained consequently.     

基于均匀化理论的混凝土宏细观力学特性研究

在细观层次上将混凝土视为由砂浆基质、骨料及其界面组成的三相复合材料,在此基础上,采用有限元方法,对混凝土弹性本构关系进行数值模拟,利用位移渐近展开技术和均匀化理论建立了多尺度力学框架下的有限元平衡方程,重点考察了单胞尺寸对宏观力学性能的影响,得到了相对最大骨料粒径的最小单胞尺寸,即5～10倍最大骨料粒径.作为例证,对混凝土三点弯梁进行了宏细观尺度数值仿真研究,结果表明:基于本文方法可以较好地反映混凝土的宏观力学行为.此外,由于骨料、砂浆的相互作用,应力分布呈现出宏观渐变平滑与细观局部突变的特征.     

FUNDAMENTALS OF TRANSMISSION FLUCTUATION SPECTROMETRY WITH VARIABLE SPATIAL AVERAGING

Transmission signal of radiation in suspension of particles performed with a high spatial and temporal resolution shows significant fluctuations,which are related to the physical properties of the particles and the process of spatial and temporal averaging.Exploiting this connection,it is possible to calculate the particle size distribution (PSD) and particle concentration.This paper provides an approach of transmission fluctuation spectrometry (TFS) with variable spatial averaging,The transmission fluctuations are expressed in terms of the expectancy of transmission square (ETS) and are obtained as a spectrum,which is a function of the variable beam diameter.The reversal point and the depth of the spectrum contain the information of particle size and particle concentration,respectively.     

Topological evolution in dense granular materials: A complex networks perspective

One of the great challenges in the science of complex materials – materials capable of emergent behavior such as self-organized pattern formation – is deciphering their “inherent” structural design principles as they deform in response to external loads. We have been exploring the efficacy of techniques from complex networks to the study of dense granular materials as a means to: (i) uncover such design principles and (ii) identify suitable metrics that quantify the evolution of structure during deformation. Herein, we characterize the developing network structure and loss of connectivity in a quasistatically deforming granular medium from the perspective of complex networks. Attention is paid to the evolution of the contact and contact force networks at the local or mesoscopic level, i.e., a particle and its immediate neighbors, as well as the macroscopic level. We explore network motifs and other topological properties at these multiple length scales, in an attempt to find that which best correlates with the constitutive properties of nonaffine deformation and dissipation, spatially and with respect to strain. Key processes or rearrangement events that cause loss of connectivity within the material domain, e.g. microbanding and force chain buckling, are investigated. Network statistics of these processes, previously shown to be major sources of energy dissipation and nonaffine deformation, are then tied to corresponding trends observed in the evolving macroscopic network. It is shown that consideration of the unweighted contact network alone is insufficient to tie dissipation to loss of material connectivity.     

Dual characters of interaction between particles and fluid

The unique characteristics of gas-solids two-phase flow and fluidization in terms of the flow structures and the apparent behavior of particles and fluid-particle interactions are closely linked to physical properties of the particles, operating conditions and bed configurations. Fluidized beds behave quite differently when solid properties, gas velocities or vessel geometries are varied. An understanding of hydrodynamic changes and how they, in turn, influence the transfer and reaction characteristics of chemical and thermal operations by variations in gas-solid contact, residence time, solid circulation and mixing and gas distribution is very important for the proper design and scale-up of fluidized bed reactors. In this paper, rather than attempting a comprehensive survey, we concentrate on examining some important positive and negative impacts of particle sizes, bubbles, clusters and column walls on the physical and chemical aspects of chemical reactor performance from the engineering application point of view with the aim of forming an adequate concept for guiding the design of multiphase fluidized bed chemical reactors.One unique phenomenon associated with particle size is that fluidized bed behavior does not always vary monotonically with changing the average particle size. Different behaviors of particles with difference sizes can be well understood by analyzing the relationship between particle size and various forces. For both fine and coarse particles, too narrow a distribution is generally not favorable for smooth fluidization. A too wide size distribution, on the other hand, may lead to particle segregation and high particle elutriation. Good fluidization performance can be established with a proper size distribution in which inter-particle cohesive forces are reduced by the lubricating effect of fine particles on coarse particles for Type A, B and D particles or by the spacing effect of coarse particles or aggregates for Type C powders.Much emphasis has been paid to the negative impacts of bubbles, such as gas bypassing through bubbles, poor bubble-to-dense phase heat & mass transfer, bubble-induced large pressure fluctuations, process instabilities, catalyst attrition and equipment erosion, and high entrainment of particles induced by erupting bubbles at the bed surface. However, it should be noted that bubble motion and gas circulation through bubbles, together with the motion of particles in bubble wakes and clouds, contribute to good gas and solids mixing. The formation of clusters can be attributed to the movement of trailing particles into the low-pressure wake region of leading particles or clusters. On one hand, the existence of down-flowing clusters induces strong solid back-mixing and non-uniform radial distributions of particle velocities and holdups, which is undesirable for chemical reactions. On the other hand, the formation of clusters creates high solids holdups in the riser by inducing internal solids circulations, which are usually beneficial for increasing concentrations of solid catalysts or solid reactants.Wall effects have widely been blamed for complicating the scale-up and design of fluidized-bed reactors. The decrease in wall friction with increasing the column diameter can significantly change the flow patterns and other important characteristics even under identical operating conditions with the same gas and particles. However, internals, which can be considered as a special wall, have been used to improve the fluidized bed reactor performance.Generally, desirable and undesirable dual characteristics of interaction between particles and fluid are one of the important natures of multiphase flow. It is shown that there exists a critical balance between those positive and negative impacts. Good fluidization quality can always be achieved with a proper choice of right combinations of particle size and size distribution, bubble size and wall design to alleviate the negative impacts.     

岩土类颗粒物质宏-细观力学研究进展

岩土类颗粒物质在自然界、工程建设以及日常生活中普遍存在,其运动特性的研究在力学界已经开展了几十年.在近20年开展的一系列小尺寸物理实验中,颗粒物质表现出许多新奇现象,人们从物理角度开展了系统研究,在统计力学中,颗粒固体的流体动力学等理论研究以及实验检测技术等方面都取得突破性进展,深刻地揭示了颗粒材料的物理机制,促使力学...     

The cohesive law for the particle/matrix interfaces in high explosives

The debonding of particle/matrix interfaces has an important effect on the macroscopic behavior of composite materials. There are extensive analytical and numerical studies on interface debonding in composite materials based on cohesive zone models which assume a phenomenological relation between the normal (and shear) traction(s) and opening (and sliding) displacement(s) across the particle/matrix interface. However, there are little or no experiments to determine the cohesive law for particle/matrix interfaces in composite materials. In this paper, we develop a method to determine the cohesive law for particle/matrix interfaces in the high explosive PBX 9501. We use the digital image correlation technique to obtain the stress and displacement around a macroscopic crack tip in the modified compact tension experiment of PBX 9501. We use the extended Mori-Tanaka method (which accounts for the effect of interface debonding) and the equivalence of cohesive energy on the macroscale and microscale to link the macroscale compact tension experiment to the microscale cohesive law for particle/matrix interfaces. Such an approach enables us to quantitatively determine key parameters in the microscale cohesive law, namely the linear modulus, cohesive strength, and softening modulus of particle/matrix interfaces in the high explosive PBX 9501. The present study shows that Ferrante et al.'s [1982 Universal binding energy relations in metallic adhesion. In: J.M. Georges (Ed.), Microscopic Aspects of Adhesion and Lubrication, Elsevier, Amsterdam, pp. 19-30.] cohesive law, which is established primarily for bimetallic interfaces, is not suitable to the high explosive PBX 9501.     

Onset velocity of circulating fluidization and particle residence time distribution:A CFD-DEM study

Until now,the onset velocity of circulating fluidization in liquid-solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superficial liquid velocity,and is reported to be only dependent on the liquid and particle properties.This study presents a new approach to calculate the onset velocity using CFD-DEM simulation of the particle residence time distribution(RTD).The onset velocity is identified from the intersection of the fitted lines of the particle mean residence time as a function of superficial liquid velocity.Our results are in reasonable agreement with experimental data.The simulation indicates that the onset velocity is influenced by the density and size of particles and weakly affected by riser height and diameter.A power-law function is proposed to correlate the mean particle residence time with the superficial liquid velocity.The collisional parameters have a minor effect on the mean residence time of particles and the onset velocity,but influence the particle RTD,showing some humps and trailing.The particle RTD is found to be related to the particle trajectories,which may indicate the complex flow structure and underlying mechanisms of the particle RTD.     

An assessment of particle methods for approximating anisotropic dispersion

We derive a smoothed particle hydrodynamics (SPH) approximation for anisotropic dispersion that only depends upon the first derivative of the kernel function and study its numerical properties. In addition, we compare the performance of the newly derived SPH approximation versus an implementation of the particle strength exchange (PSE) method and a standard finite volume method for simulating multiple scenarios defined by different combinations of physical and numerical parameters. We show that, for regularly spaced particles, given an adequate selection of numerical parameters such as kernel function and smoothing length, the new SPH approximation is comparable with the PSE method in terms of convergence and accuracy and similar to the finite volume method. On other hand, the performance of both particle methods (SPH and PSE) decreases as the degree of disorder of the particle increases. However, we demonstrate that in these situations the accuracy and convergence properties of both particle methods can be improved by an adequate choice of some numerical parameters such as kernel core size and kernel function. Copyright © 2012 John Wiley & Sons, Ltd.