Stability and magnetorheological behaviour of magnetic fluids based on ionic liquids

This paper reports the preparation of magnetic fluids consisting of magnetite nanoparticles dispersed in an ionic liquid. Different additives were used in order to stabilize the fluids. Colloidal stability was checked by magnetic sedimentation, centrifugation and direct observation. The results of these tests showed that a true ferrofluid was only obtained when the nanoparticles were coated with a layer of surfactant compatible with the ionic liquid. These experiments also showed that stability could not be reached just by electrostatic repulsion. The conclusions of the stability tests were confirmed by calculations of the interparticle energies of interaction. The rheological behaviour of the magnetic fluids upon magnetic field application was also investigated. The experimental magnetoviscous response was fitted by a microstructural model. The model considered that the fluids consisted of two populations of particles, one with a magnetic core diameter of 9?nm, and another with a larger diameter. Upon field application chain-like structures are supposed to be induced. According to estimations particles of 9?nm are too small to aggregate upon field application. The results of the calculations showed that the intensity of the magnetoviscous response depends on the concentration and size of the large particles, and on the thickness of the surfactant layers.     

微尺度稀薄效应下颗粒沉降的格子Boltzmann方法模拟

基于多松弛格子Boltzmann模型,对竖直细长微通道内颗粒自由沉降过程进行模拟,分析气体稀薄效应、初始位置以及颗粒间相互作用对微颗粒沉降特性的影响.研究表明：随Knudsen数增大,微通道内气体稀薄效应增强,颗粒表面气体滑移速度增大,气相流体有效粘度减小,颗粒相同运动状态下受到气体阻力相应减小,颗粒沉降平衡速度明显增大;不同初始位置颗粒沉降过程存在明显差异,初始位置偏离中心线颗粒将发生水平方向位移且呈振荡趋势,最终稳定于中心线平衡位置;在微尺度双颗粒沉降DKT现象过程中,气体稀薄效应影响颗粒运动特性,后颗粒跟随过程明显增长.     

A comparative study on the effect of hydrodynamic interactions in the non-sequential deposition of concentrated colloidal dispersions: stochastic rotation dynamics and Brownian dynamics simulations

Irreversible adsorption of particles onto a flat surface as a consequence of sedimentation colloidal suspension has been studied by two simulation techniques: Brownian dynamics (BD) and stochastic rotation dynamics (SRD). The purpose of using both methods is to investigate the effect of hydrodynamic interactions on adsorption kinetics and structure of the first monolayer of sediment obtained from the sedimentation of a concentrated and monodisperse colloidal suspension. Three systems were studied, characterised by the Péclet numbers: 0.1, 1.0 and 10. To physically understand the kinetic behaviour, simulation results were analysed using a kinetic model based on chemical reactions. High values of jamming limit (θ_∞ > 0.61) were obtained for both simulation techniques, with the SRD showing the highest figures (0.631) due to the hydrodynamics effect that takes into account the fluid backflow produced on particle sedimentation. A two-step adsorption mechanism was proposed based on the observed kinetic behaviour.     

Estimation of particle size distributions obtained by gas phase processes

Nanoparticles intended for high value added applications often require special size distributions. Based on model calculations, this article compares the particle size distributions obtained with conventional and plasma processes. The model is based on an estimation of the probability for collisions; either for neutral or equally charged particles, whereas the growth of the particles is calculated using a model derived from Markov chains. The results of these calculations confirm the empirical knowledge that, under the special conditions of particles carrying electric charges of equal sign, plasma processes deliver products with the narrowest particle size distribution. Synthesis of extremely small particles with conventional processes leads to a significant residue of unreacted precursor. This finding is important in cases of expensive educts. The results of these model calculations are in perfect agreement with experimental findings.     

An effect of turbulent clustering on scattering of microwave radiation by small particles in the atmosphere

A coherent scattering of electromagnetic waves by clusters of inertial Rayleigh particles in atmospheric turbulence is considered. A preliminary estimate based on the Maxwell-Garnett theory and the Rayleigh approximation for single clusters demonstrates an importance of the coherent scattering contribution. It is confirmed by a general solution in a combination with theoretical estimates for the two-point probability density function for low-inertia spherical particles in isotropic turbulence. An approximate analytical expression for the coefficient characterizing effect of coherent scattering by the particle clusters is derived. The calculations for small Stokes numbers typical of water droplets in cumulus clouds yield an estimate of the coherent scattering effect on the microwave radar reflection. The model suggested allows solving the inverse problem to determine the pair correlation function for cloud particles. It is expected to be important for the investigations on particle–turbulence interaction in the atmosphere. The theoretical model developed is true not only in the limit of low-inertia particles and can be potentially used at arbitrary Stokes numbers in other applications.     

Deposition of inertial particles from turbulent flow in channels at high Reynolds numbers

On the basis of the previously developed asymptotic theory of turbulent particle-laden flow with particle deposition in channels coupled with the transport model for the particle Reynolds stress, an asymptotic solution to the problem on the deposition of particles in the limit of high Reynolds numbers was obtained. The numerical calculations confirmed the presence, in the region of the transition from the diffusion-impaction regime of particle sedimentation to the inertia-moderated regime, bifurcation phenomenon of a solution found previously in earlier studies. Features of particle accumulation in the viscous sublayer are analyzed. On the basis of the numerical solution, correlations for particle deposition velocity were obtained. Boundary conditions of the wall-function type for particle concentration whose use allows widening the applicability limits of the equilibrium Eulerian models in terms of particle inertia are proposed.     

沉降对带电胶体粒子结晶过程的影响

采用密度匹配法(重水与水按一定比例混合),以及反射光谱法,研究了重力沉降作用对直径为98 nm的带电胶体粒子结晶过程的影响. 结果表明,重力沉降在晶体生长的初期提高了晶体生长速率,而后期降低了晶体生长速率,这是由于在晶体生长初期,沉降作用可使更多的粒子结合到晶体结构中,而当晶体尺寸进一步增加,其沉降速率也相应地增大,晶相与液相间的摩擦阻力导致一部分颗粒从胶体晶体上脱落. 总的来说,重力沉降在初期加剧了晶体的生长,后期阻碍了晶体的生长. 另外,在微重力环境下形成的胶体晶体比在重力环境下形成的胶体晶体更加完整紧密.     

Inelastic interactions of high energy nucleons with heavy nuclei

The intranuclear cascade model including the effect of changes in the properties of the target nucleus due to development of the shower of cascade particles is applied to calculations of the interactions of protons with fissioning nuclei in the energy region T ≈ 0.1–30 GeV. The fission cross sections, yields of different isotopes after the evaporation stage, and the characteristics of accompanying particles are calculated. The properties of the excited evaporating and fissioning nuclei are discussed. The calculated results are compared with experiment.     

Development and application of a diffusion-inertia model for simulating gas-dispersed turbulent flows

The development and application of a diffusion-inertia model are shown for computing the propagation and deposition of aerosol particles in turbulent flows. Comparison with experimental and numerical results points to the fact that the developed model adequately describes all the typical peculiarities of the process of the sedimentation of aerosol particles in straight and curved pipes.     

Preequilibrium versus thermalized light-charged particle emissions in the40Ar (1100MeV)+24Mg reaction. Dynamical calculations

In an attempt to separate preequilibrium and thermalised emissions of light particles in low impact parameter heavy-ion collisions, the⁴⁰Ar+²⁴Mg reaction has been studied at 27.5 MeV/nucleon. Exclusive measurements have permitted us to examine, in some detail, heavy fragments and charged particles (p, d, t,-particle). The fragments recognized as evaporation residues have been selected and, due to inverse kinematic conditions, the related preequilibrium and statistical emissions of light particles resulting from incomplete fusion reaction appear to be distinguishable to a fair extent. This separation is fully supported by Monte Carlo calculations. Some experimental characteristics of the light particles have been examined and compared to the predictions of dynamical calculations. These calculations, associating a preequilibrium (interpreted as prompt emitted particles) model with a statistical-decay model, follow the evolution of the collision from the point of contact between the projectile and the target to the final evaporation-residue formation on an event by event basis. The predictions of these calculations have been compared to experimental data and satisfactory agreement is achieved for fragment-mass distribution, proton-energy spectra, and proton-angular distribution.     

Spectrum of galactic cosmic rays accelerated in supernova remnants

The spectra of protons, nuclei, and electrons accelerated by shocks in supernova remnants of different types were determined. The calculations were made using a numerical code that allows us to model spherical shock evolution and particle acceleration with allowance for the back reaction of accelerating particles on a hydrodynamic flow. The effect of Alfvenic particle drift in the amplified magnetic field in the regions upstream and downstream of the shock was taken into consideration. The maximum energy of accelerated particles is as high as ∼5 × 10¹⁸ eV for iron nuclei in Type IIb supernova remnants. The calculated spectrum and composition of cosmic rays in the interstellar medium are in good agreement with observations.     

Simulation of scattering of optical radiation by a metal surface with nanometer irregularity

Within the framework of the model of a limited electron free path, the scattering indicatrix for spherical nanoparticles was calculated. The calculations were made by an example of silver and supersmooth surfaces, which were modulated by a monolayer of spherical nanoparticles taking into account the correlation in the distribution of particles and the size effect within the framework of the interference approximation. As follows from the calculations, the inclusion of the size effect into consideration leads to a noticeable change of the scattering intensity, both for separate particles and a surface, near the plasma resonance wavelength. Because of this, the spectral dependences of the refractive index calculated with and without allowance for the size dependence of optical constants differ in the resonant region by more than an order of magnitude.     

α-Fe粉体浓悬浮体系的制备及磁流变效应

报道使用价格低廉的还原铁粉制备磁性粒子浓悬浮体系 ,对体系磁流变效应进行研究 ,并研究了磁性颗粒尺寸和氧化物对悬浮体系力学性质及沉降稳定性的影响 .     

Modelling of the sedimentation of polydisperse suspension particles in a plate centrifuge

The investigation of the sedimentation of particles of a dense polydisperse suspension in the plate centrifuge was carried out based on numerical simulation. It was shown that the reason for the nonmonotone behavior of the sedimentation velocity dependence on particle size is the action of different mechanisms of the particles interaction. The appropriateness of the formula U _s,i = −r _p /2 ∂lnc _i /∂t for measuring the sedimentation velocity of particles of a dense polydisperse suspension in the plate centrifuge was examined.     

热对流条件下双颗粒沉降的直接数值模拟

在任意拉格朗日-欧拉(ALE)算法模拟等温惰性颗粒两相流的基础上,增加对能量方程的联立求解,对热对流条件下双颗粒的沉降进行了直接数值模拟.结果表明:双颗粒在等温流体、热流体和冷流体中运动形式是不同的,造成不同的主要因素是热对流引起了颗粒不同的动态尾迹,在热流体中颗粒尾部形成了涡的脱落,在冷流体中颗粒尾部形成了羽流.     

An estimate of shape-distribution of small CdS particles with luminescence spectra

The luminescence spectra of CdS particles obtained by sedimentation method are studied. The comparison of measured and calculated photoluminescence spectra was used as the base of the method defining the shape distribution of the particles. For this purpose one can use the symmetry rule for absorption and luminescence spectra. Absorption spectra were calculated via dissipative function in terms of local field theory. The experimental spectra of the photoluminescence were measured during the process of sedimentation in the CdS nano-composite powder. The modification of luminescence spectra detected during sedimentation time is associated with changing the dimension and shape distributions of CdS suspension. As a result, the particles shape distribution was estimated for sizes less then 1 μm.     

Particle-size sorting system of lunar regolith using electrostatic traveling wave

A particle-size sorting system of lunar regolith using an electrostatic traveling wave is developed for In-Situ Resource Utilization on the Moon to extract indispensable resources from the regolith and realize long-term exploration. The regolith is sorted by utilizing a balance between the electrostatic and gravitational forces, which are determined depending on particle size, in vacuum conditions where the particles are not subjected to air drag. In this study, the effect of particle charge on the particle motion is confirmed by conducting model experiments and numerical calculations based on the distinct element method. In addition, it was experimentally demonstrated that particles less than approximately 20 μm in diameter were efficiently separated from the bulk of a lunar regolith simulant FJS-1 in a vacuum condition (∼1.5 × 10⁻² Pa), and the performance of the size sorting system on the Moon was predicted by the numerical calculations. The system utilizes only the electrostatic force, and it does not require any gas, liquid, or mechanical moving parts.     

A Fictitious Domain,parallel numerical method for rigid particulate flows

In this paper, we develop a Fictitious Domain, parallel numerical method for the Direct Numerical Simulation of the flow of rigid particles in an incompressible viscous Newtonian fluid. A Simultaneous Directions Implicit algorithm is employed which gives the model a high level of parallelization. The projection of the fluid velocity onto rigid motion on the particles is based on a fast computational technique which relies on the conservation of linear and angular momenta. Numerical results are presented which confirm the ability of the proposed method to simulate the sedimentation of one and many particles; the parallel efficiency of the algorithm is also assessed.     

Accounting for nonstoichiometry of niobium carbide NbC y upon milling to a nanocrystalline state

The dependence of the size of particles in the prepared nanocrystalline powders on the composition of nonstoichiometric compounds within their homogeneity intervals has been considered in terms of the high-energy ball milling model. It has been shown that the effect of nonstoichiometry on the milling manifests itself in the concentration dependences of the main characteristics (parameters of the crystal structure, energy of interatomic bonds, elastic properties) of the milled nonstoichiometric compound. The results of model calculations performed for nonstoichiometric cubic niobium carbides NbC _y have been compared with the experimental data on milling of the NbC_0.93 carbide.