Variation of chaotic motion of silicon particles suspended in liquid under field

A decrease in the characteristic correlation time of light scattered by silicon particles in transformer oil and an increase in their effective diffusion coefficient when the particle suspension in oil is exposed to a dc electric field were experimentally shown. The current in the suspension did not exceed 100 nA and its temperature was unchanged. It was also shown that the shape of the correlation function of light scattered by particles changes, i.e., a Gaussian factor appears in it. The dependences of the exponential and Gaussian components on the electrode voltage are presented.     

Relaxation of an electrooptical effect in colloids induced by a field of short pulses

The possibility of studying the size distribution of particles suspended in a liquid irradiated with short pulses of strong electric fields, which almost do not change the properties of the disperse system is considered. The particle size is varied from 10 to 1000 nm. Relaxation dependences of the optically anisotropy induced by such pulses are investigated. The relaxation curves of the induced electrooptical effect are shown to behave similarly to the relaxation curves of the effect in the system with completely oriented particles. For aqueous polydisperse systems of diamond, graphite, and palygorskit, whose particles significantly differ in shape and physicochemical properties, the relaxation dependences of the electrooptical effect induced by fields of different duration and amplitude are measured. The correlation coefficients between the relaxation dependences are calculated. If the field is sufficiently strong, the correlation coefficients remain close to unity for all the three systems studied irrespective of the duration of the field pulse. This indicates that these curves are similar and that short powerful pulses can be used for determining the size distribution of particles in nanodisperse systems.     

Formation of the structure of gold nanoclusters during crystallization

The structure formation in gold nanoparticles 1.6–5.0 nm in diameter is studied by molecular dynamics simulation using a tight-binding potential. The simulation shows that the initial fcc phase in small Au clusters transforms into other structural modifications as temperature changes. As the cluster size increases, the transition temperature shifts toward the melting temperature of the cluster. The effect of various crystallization conditions on the formation of the internal structure of gold nanoclusters is studied in terms of microcanonical and canonical ensembles. The stability boundaries of various crystalline isomers are analyzed. The obtained dependences are compared with the corresponding data obtained for copper and nickel nanoparticles. The structure formation during crystallization is found to be characterized by a clear effect of the particle size on the stability of a certain isomer modification. Nickel and copper clusters are shown to exhibit common features in the formation of their structural properties, whereas gold clusters demonstrate much more complex behavior.     

Particular features of circular dichroism spectra of resin-asphaltene compounds in solvent and crude oil

The circular dichroism (CD) spectra of solutions of asphaltenes, resins, and crude oils in toluene, as well as of oil residue, which is free of resin-asphaltene compounds (RACs), were studied in the spectral range 400–600 nm. It was demonstrated that, unlike, oil and RACs solutions, oil residue does not show the CD signal. This result proves that, in this spectral range, the occurrence of the CD signal in oil is determined by the presence of RACs. The concentration dependences of CD signal for the solutions under study were obtained for the maximum of the CD band in the considered spectral range. For pure toluene solutions of asphaltenes and resins, these dependences can be approximated with good accuracy by a linear function, whereas, for solutions of crude oils, the concentration curves deviate from the linear dependence at large concentrations of RACs due to the formation of particles of the disperse phase in oil. Therefore, in a certain concentration range, the structure of oil undergoes significant changes and becomes disperse, whereas solutions of molecules of RACs remain molecular.     

Development of particle image velocimetry for multiphase flow diagnostics

This paper discusses the suitability of PIV for measuring simultaneously the velocity field of every phase and the size and concentration field of the disperse phases in multiphase flows. Velocity and disperse phase information are both inferred either directly from the Young’s fringe pattern (far field diffraction function) or from its 2-D Fourier transform (autocorrelation function). In the first case, the velocity is inferred from the orientation and spacing of the fringes while the disperse phase size is inferred from the size of the diffraction halo that modulates the fringes. In the second case, the velocity and particle shape are related to the position and shape of the strongest autocorrelation peaks, respectively. Particle sizes are used to discriminate between phases on the velocity measurements. The technique has been demonstrated on a high speed air-particle flow, where the potential for determining air velocities and particle size and velocities are shown.     

Classification of phase transitions in small systems

We present a classification scheme for phase transitions in finite systems like atomic and molecular clusters based on the Lee-Yang zeros in the complex temperature plane. In the limit of infinite particle numbers the scheme reduces to the Ehrenfest definition of phase transitions and gives the right critical indices. We apply this classification scheme to Bose-Einstein condensates in a harmonic trap as an example of a higher order phase transition in a finite system and to small Ar clusters.     

Computer simulations for multiple scattering of light rays in systems of opaque particles

A new computer model for multiple light scattering in arbitrary systems of opaque diffusely scattering particles is considered. For ray tracing and scattering in such systems, the geometric optics approximation is used. Semi-infinite media and clusters with spherical and irregular shaped particles are investigated. The irregular particles are approximated with a discrete set of small triangular facets attached to each other. The particle surface is supposed to scatter by the Lambertian indicatrix. Scattering of the first six orders is considered, but the model can be effectively used for calculations of higher orders too. Phase-angle curves of scattering for media and clusters with different packing density are calculated. It is shown that the contributions of scattering orders rapidly diminish as the order grows even for non-absorbing particulate surfaces. Only the first scattering order shows the opposition effect and is rather sensitive to packing density. Higher orders do not show any features near zero phase angle. The contributions of high orders increase slightly, when the packing density increases. The form of particles is important mostly for the second scattering order. For clusters of particles both packing density and number of particles are important for phase function behavior. Clusters consisting of 100 particles show weak phase-angle dependences of high orders of scattering. These dependences become more prominent with increase of number of particles. Phase curves for spherical and cubic clusters are compared. It turns out that the influence of cluster shape is only a minor factor.     

Laser diagnostics of the Bubston phase in the bulk of aqueous salt solutions

Stable gas nanobubbles in the bulk of NaCl aqueous solutions and clusters of these nanobubbles have been investigated at different ion concentrations by four independent laser techniques (phase microscopy, dynamic light scattering, optical breakdown, and measurement of angular dependences of the light scattering matrix). The results obtained by these radically different techniques are in good agreement. It is found that the nanobubble size is practically constant and amounts to approximately 100nm in the range of ion concentrations 10^?6<C <1M. It is shown that a necessary condition for nanobubble nucleation is the saturation of solution with dissolved air. It is revealed that nanobubble clusters form a thermodynamically nonequilibrium phase with a lifetime of several months.     

Hadron resonance gas and nonperturbative QCD vacuum at finite temperature

Nonperturbative QCD vacuum with two light quarks at finite temperature was studied in a hadron resonance-gas model. Temperature dependences of the quark and gluon condensates in the confined phase were obtained. It is shown that the quark condensate and one-half (chromoelectric component) of the gluon condensate are evaporated at the same temperature corresponding to the quark-hadron phase transition. With allowance for the temperature shift of hadron masses, the critical temperature was found to be T _c ?190 MeV.     

On Measuring and Modeling Local Average Particle Velocities for particle ensembles in classifying systems

In most multi-phase flow problems, the particulate phase is exposed to an external field which causes dispersion. Therefore, local velocity measurements of the disperse phase are no longer equivalent with respect to averaging in time and averaging in volume. While the local time-averaged velocity still characterizes the transport of the ensemble in the Eulerian sense, one has to be be careful in modeling this velocity average by considering the ensemble's composition. It is shown for different particle ensembles that the conventional particle velocity average M_1,0 calculated with respect to the dispersion relationship and a particle size number density distribution is far below the measured ensemble average; the deviation depends on the width of the particle size distribution. It is deduced that Eulerian particle velocity values referring to a certain time interval can be modeled by a ratio of velocity moments M_2,0/M_1,0 calculated with particle size number distributions referring to a certain probe volume. This relationship was confirmed by measurements.     

Thermal ignition of a heterogeneous system in a stirring semibatch reactor

The thermal ignition of a heterogeneous two-phase system of two immiscible liquids in a stirring semibatch reactor was modeled. The model took into account the dispersion of one phase in the other, the mass transfer of a reagent from the disperse phase into the dispersion medium, and the bimolecular exothermal reaction that occurred in the latter. The kinetic dependences of reagent concentrations, the dynamics of reactor heating, and the critical conditions of thermal ignition were studied. It was found that the initial temperature of the reactor had a considerable effect on its dynamic behavior.     

Equilibrium equation of small-size phases and some its applications

The equilibrium equation of small-size phases for a one-component system, taking into account the size dependences of the pressure, temperature, and interface energy at the phase boundaries in the highly dispersed state, is presented. It is shown that the phase-transition and ternary-point temperatures decrease with a decrease in the particle size. Expressions are derived, which relate the surface tensions at the solid-vapor and solid-liquid interfaces (which are difficult to experimentally determine) to the liquid-vapor surface tension and other physical quantities, which can be experimentally determined with high accuracy. There is satisfactory agreement between the obtained results and experimental data.     

Particle Sizes of Fumed Oxides: A New Approach Using PCS Signals

Fumed oxides produced in gas‐phase processes, such as silicas and aluminum oxide, consist of a cluster of aggregated primary particles. The aggregate size of these particles is an important variable in many applications. However, current procedures for measuring particle sizes all assume that the particles have a spherical shape and are thus not truly capable of determining aggregate size. The results of such particle size measurements are consequently called “equivalent spherical diameter” (ESD), but these results vary from method to method. This publication shows that it is feasible to use the number of primary particles per aggregate, rather than the ESD, as a measure for the particle size of clusters of this type. The method is based on dynamic light scattering (photon correlation spectroscopy, PCS), which has proven itself in the analysis of fumed oxides. A numerical simulation based on random, computer‐generated model aggregates is used to modify the well‐known Stokes‐Einstein equation so that the number of primary particles can be determined.     

铁电体中极化长程涨落的光子关联谱实验研究

从极化团簇的随机涨落出发, 利用维纳过程模型, 推导了铁电体中极化长程涨落的弛豫规律以及光强自相关函数所可能的表现形式. 阐述了微观极化团簇的弛豫过程与宏观测量弛豫规律之间的联系. 通过对原有氦氖激光光子关联谱实验装置进行改进, 观测了BaTiO₃和0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃单晶中极化团簇长程涨落在居里点和立方到四方相变点附近的弛豫过程. 在BaTiO₃中发现极化团簇长程涨落在居里点之上4 K存在双弛豫现象, 此现象与其有序无序相变机理相联系. 在Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃中发现极化团簇长程涨落在相变点两侧都存在双弛豫现象. 利用推导的理论结果很好地拟合了实验结果并提取了极化团簇长程涨落的弛豫时间. 两种样品中极化团簇长程涨落的弛豫时间都在相变点出现突变, 并呈现临界慢化现象.     

Shear induced anisotropy in two-dimensional liquids

The behavior of a dense two-dimensional soft disc liquid under shear is studied via nonequilibrium molecular dynamics. The structure factor for the liquid at a given shear rate is evaluated directly by plotting the particle positions, taken at random from the NEMD simulation at that shear, onto photographic film and using light scattering to obtain a diffraction pattern. The pair correlation function of this system is also extracted directly by histogramming the particle positions with respect to a given central particle as a function of separation and angle. The pair correlation function is compared to that approximated by a Fourier series expansion to rank ten. Results are reported as a function of shear rate from a shear rate of 0.1 (when the fluid is essentially Newtonian) to 10 (when the fluid can display a string phase). The appearance of the string phase is discussed and shown to be a consequence of the definition of temperature in the simulation algorithm. A modification of the algorithm is proposed. Comparisons between this work and previous work with three-dimensional liquids are given. The two-dimensional structure factor is compared with that obtained from a real colloidal suspension via light scattering.     

Diffuse x-ray scattering in the Hg2I2 model incipient ferroelastics

This paper reports the first detection and study of diffuse maxima in x-ray scattering in Hg₂I₂ incipient ferroelastics. These maxima originate from the formation of clusters of the incipient orthorhombic phase in the paraelastic tetragonal matrix. The nucleation and growth of the clusters are caused by spatial and temporal fluctuations of the order parameter (which correspond to the TA soft mode at the X point of the Brillouin zone edge) and are induced by the incipient phase transition. Information is obtained on the temperature behavior of the susceptibility and correlation length and on the shape and anisotropy of clusters, and the critical indices are determined.     

Efros-Shklovskii law and localized states in weakly doped lanthanum manganites

The existence of clusters in weakly doped lanthanum manganites at temperatures about twice as high as their Curie temperature T _C has been shown. Electrical resistance in weakly doped lanthanum manganites obeys the Efros-Shklovskii law. The temperature and magnetic-field dependences of a cluster size determined from the magnetotransport properties have been described using the model of phase separation into small metallic droplets within the dielectric paramagnetic and antiferromagnetic matrices. The results agree with the existence of the Griffiths phase.     

Particle Sizing Using Frequency Domain Photon Migration

The transport properties of particulate process streams and their final product quality, are directly affected by critical parameters of particle size distribution, f(x), and volume, mass, or number density of particles or dispersed phase droplets. A method is proposed for the potential on-line monitoring of particle size distribution and volume fraction in real time, using frequency-domain photon migration measurements (FDPM). Theory, experimental measurements, and results for the determination of particle size distributions for both a polystyrene latex and a titanium dioxide suspension determined using the photon migration technique are presented. The critical issues associated with the application of photon migration to particulate and dispersed phase processes are discussed, including the effects of interparticle interactions on the transport of light.     

Fractal-cluster kinetics in phase transformations in the relaxor ceramic PLZT

Fractal formalism has been used to study the evolution of a heterophase state (consisting of polar nanoregions in a nonpolar matrix) subjected to polarization in an electric field within the diffuse phase transition in the transparent relaxor ferroelectric ceramic PLZT 8/65/35. The time dependences of the fractal dimensions of polar clusters under polarization switching and spontaneous backswitching at different temperatures have been derived from measurements of the elastic scattering of transmitted light. Various scenarios for the evolution of the heterophase and nanodomain states in relaxors with variation of the temperature and electric field are proposed. Fiz. Tverd. Tela (St. Petersburg) 41, 505–509 (March 1999)