The wetting angle of very small and large drops

On the basis of Born-Green-Yvon integral equations for the density distribution functions, an approximate integral equation is established for the profile of the surface of the drop. Numerical solutions and analytical solutions for limiting cases are obtained for this profile. Equations relating the angle at the leading edge and in its vicinity to parameters characterizing the interaction forces between the molecules of the liquid and between those of the liquid and solid are derived for large and for very small drops on a horizontal solid surface. One concludes that there is a rapid spatial variation of shape near the leading edge, that for large drops the measured macroscopic wetting angle is reached at a distance of about 20 to 40 Å from the leading edge, and that for very small drops the wetting angle is weakly size dependent. A condition for drop stability is established, which if not satisfied, the liquid will spread over the surface of the solid.     

DEM simulation of small particles clogging in the packing of large beads

The percolation of small particles through a periodic random loose packing of large beads is studied numerically with the Distinct Element Method. The representativity of periodic mono-sized sphere packing of varying system size was first studied by comparing their pore size distributions and tortuosities with those of a larger system, considered as an infinite medium. The results show that a periodic packing of size as low as 4-grain diameters gives a reasonable representation of the porous medium and allows reducing considerably the number of particles that has to be used in the simulations. The flow and clogging of small particles of varying concentrations and friction coefficients flowing through the former packing are then studied numerically. Results show that a steady state is rapidly reached where the mean velocity and mean vertical velocity of small particles are both constant. These mean velocities decrease with an increase in friction coefficient and in small particle concentration. The influence of the friction coefficient μ is much less marked for values of μ larger than or equal to 0.5. The distribution of small particles throughout the crossed packing becomes rapidly heterogeneous. Small particles concentrate in some pores where their velocity vanishes and where the density can reach values larger than the density of the random loose packing. The proportion of particles blocked in these pores varies linearly with concentration. Finally, the narrow throats of the porous medium responsible for blocking are identified and characterized for different values of the friction coefficient.     

The Measurement of the Separation of Fine Solid Particles in Multiwir Packings

In this paper, a new type of packing for the separation of solid particles from gases is presented. The so called Multiwir (multiple vortex) [1, 2] packings (see Figure 1) consist of multiple adjacent layers of strips spaced uniformly. As the layers are placed oblique to each other, the incoming gas stream passes the packings in the form of a large number of smaller streams which cross each other spatially and thus induce rotational motion in each other. Under centrifugal forces and due to turbulent and electrostatic effects, the particles settle on the surfaces of the strips where they are collected. In this paper, some of the experimental results are described obtained in the investigation of Multiwir-packings, consisting of metal or plastic strips, with different limestone aerosols and feed particle size distributions with particles smaller than approximately 3 μm and 10 μm. The following results have been obtained: — the Multiwir separator should be used for the removal of fine particles. Particles smaller than 10 μm in size can only be collected if special precautions are used, — the efficiency of the separator is dominated by the gas velocity, the geometry of the packing, the material of the strips and the size distribution of the feed material, — the smaller the feed particles are, the smaller the distance between adjacent strips should be, — the reduction of the distance between adjacent strips is limited however by an increase of the pressure drop across the packing, — adhesion between particles and strip surface strongly influences the performance of the packing. The surface of the strips should therefore be wetted, for example with a thin oil film, — the collection of small particles can be further improved if the feed particles are charged electrostatically in front of the packing, — the total grade efficiency of the packings investigated lies between T₀ = 0.6 and 0.9. Extremely good results have been obtained with perforated plastic strips. Simplified mathematical models have been used to describe the phenomenon of particle deposition in Multiwir packings due to inertia effects and turbulent diffusion.     

Phospholipid lung surfactant and nanoparticle surface toxicity: Lessons from diesel soots and silicate dusts

Because of their small size, the specific surface areas of nanoparticulate materials (NP), described as particles having at least one dimension smaller than 100 nm, can be large compared with micrometer-sized respirable particles. This high specific surface area or nanostructural surface properties may affect NP toxicity in comparison with micrometer-sized respirable particles of the same overall composition. Respirable particles depositing on the deep lung surfaces of the respiratory bronchioles or alveoli will contact pulmonary surfactants in the surface hypophase. Diesel exhaust ultrafine particles and respirable silicate micrometer-sized insoluble particles can adsorb components of that surfactant onto the particle surfaces, conditioning the particles surfaces and affecting their in vitro expression of cytotoxicity or genotoxicity. Those effects can be particle surface composition-specific. Effects of particle surface conditioning by a primary component of phospholipid pulmonary surfactant, diacyl phosphatidyl choline, are reviewed for in vitro expression of genotoxicity by diesel exhaust particles and of cytotoxicity by respirable quartz and aluminosilicate kaolin clay particles. Those effects suggest methods and cautions for assaying and interpreting NP properties and biological activities.     

Optical second-harmonic generation by supported metal clusters: Size and shape effects

Optical Second Harmonic Generation (SHG) by metal clusters has been investigated. For this purpose clusters were generated by the deposition and nucleation of metal atoms on a LiF(100) single crystal surface under ultrahigh vacuum conditions. The size and shape of the metal particles was characterized by optical transmission spectroscopy. The SHG intensity was detected in situ as a function of cluster size during the nucleation. Fundamental wavelengths of =1064 and 532 nm were used and the SHG signal was measured for different polarization combinations of the incident and registered light. SH radiation is detectable for particles as small as approximately 1 nm. The signal grows monotonically as a function of particle size, passes a maximum and finally drops off. This behavior is discussed in terms of resonant enhancement of the signal by surface plasmon excitation and changes of ⁽²⁾ as a function of particle size and shape. In further experiments the chemisorption of oxygen on the surface of the metal particles was studied. The SH signal decreases as a function of oxygen coverage and amounts to only about 15% of the initial value upon chemisorption of one monolayer. This indicates that the SH signal originates almost exclusively from the surface of the clean clusters and that higher order bulk contributions are negligible.     

溶液液滴蒸发变干的环状沉积

液体蒸发驱动的颗粒自组装现象在许多的工业技术中有重要应用. 本文利用显微镜观测含有颗粒物质的液滴变干后留在固体表面的颗粒形成的环状沉积图案. 采用微米粒径的SiO₂小球水溶液液滴蒸发变干模拟咖啡环的形成过程, 结果发现液滴蒸发过程中接触线的钉扎是环状沉积的必要条件. 在液滴蒸发过程中颗粒随着补偿流不断的向液滴边缘移动, 聚集在接触线处形成环. 液滴蒸发变干后残留在液滴内部的颗粒数随颗粒质量分数的增加而增加, 可以达到单层的颗粒排列. 而玻璃衬底上的颗粒环在颗粒质量分数很小时, 形成单层排列, 且一排一排地生长. 蒸发过程中颗粒环由于液滴边缘的尺寸限制向液滴中心缓慢移动. 这会导致液滴中不同大小颗粒的分离. 关键词： 液滴 接触线 蒸发 颗粒     

Mössbauer effect in small particles

Mössbauer spectroscopy is very sensitive to the special magnetic properties of ultrafine particles. Studies of particles in the superparamagnetic state allow determination of the particle size and the magnetic anisotropy energy constant A strong magnetic interaction between the particles may result in ordering of the magnetic moments of particles which would be superparamagnetic if they were non-interacting. This so-called superferromagnetic state can also be characterized by Mössbauer spectroscopy measurements. Furthermore, because a large fraction of the atoms in very small particles are in the surface layer, Mössbauer spectroscopy can be applied for studies of surface magnetism in small particles.     

Tensile properties and interfacial interactions of bimodal hard/soft latex blends

In this work, the effect of composition, particle size and particle size ratio on the tensile properties of well-characterized hard/soft latex blends was investigated. Four blends of hard/soft latices, with varying particle sizes (either small or large), and volume fractions of 100/0, 80/20, 60/40, 50/50, 40/60, 20/80 and 0/100 were studied. The stress at break increased and the strain at break decreased as the amount of hard particles in the blend increased. A simple model, introduced by Pukanszky for filled polymers and polymer blends, proved to be a very useful tool for evaluating the tensile properties of the latex blends. Parameter B of the model could be related to the specific surface of the dispersed hard particles and the particle size ratio. Increasing the specific surface of the dispersed hard particles resulted in an increase in parameter B. The influence of particle size ratio on parameter B was shown to depend on the formation of aggregates.     

Computing Particle Surface Areas and Contact Areas from Three‐Dimensional Tomography Data of Particulate Materials

Microtomography is an emerging technique for particle and particulate‐materials characterization. To use this technology effectively, robust and accurate computational algorithms are needed to compute relevant particle properties, including particle surface area and particle‐particle contact area. However, the most accurate algorithms that have been developed for computing the exposed (void/solid) surface area in a microtomography image cannot be used directly for computing surface areas or particle‐particle contact areas for individual particles in a dense packing. This paper presents an algorithm for extracting particle contact areas from a digitized, segmented image of a packed granular material, which in turn can be used to find individual particle surface areas (even if the complete surfaces are not exposed because of contacts in the packing). Results show that small errors in the binary surface‐area computations are magnified in the course of determining particle contact areas; the total error in the computation depends mainly on the size of the contact area in voxel units.     

Size effect on the conduction electron spin resonance of small sodium particles in sodium azide NaN3

The conduction electron spin resonance from small (10 to 1000 Å sodium metal particles has been measured as a function of the particle size. The particles were formed inside single crystals of sodium azide by X-irradiating and annealing them at 280°C. The particle sizes were deduced from the optical absorption measurements reported in the previous article. According to the theory of the quantum size effect in small metal particles, very narrow resonances should have been observed. No such resonances were seen. The measured widths were between 7.5 and 20 G and could be analysed in terms of the bulk width plus a temperature independent but size dependent surface term. The reason why no quantum size effect was observed is not understood.     

The influence of size effects in Pt and Au nanoparticles studied by 197Au Mössbauer spectroscopy

We present a detailed Mössbauer study of a series of Au and Pt particles in the size range from 1 to 17 nm. All measured spectra can be explained consistently with a refined model, in which the Mössbauer isomer shift varies in the inner core of a small metallic particle due to size effects. The large surface/volume ratio makes surface effects, like screening of considerable importance and even the so-called quantum-size effect may have an influence. The size evolution of the electronic properties of the particles is also discussed.     

Dust in Plasma I. Particle Size and Ion‐Neutral Collision Effects

A fully kinetic self‐consistent model of an absorbing particle immersed in stationary isotropic weakly collisional plasma has been developed. The combined effects of particle size and ion‐neutral charge exchange collisions have been investigated for intermediate regimes, where no analytic theories are available. It is shown that collisional effects related to the ion orbital destruction (presence of extrema in ion flux collected on the particle surface and in particle potential and charge) are important for small particles, while they are totally absent for large particles. The potential distribution around the particle is quite well represented by a Yukawa form, but with an effective screening length that shows different dependences from the gas pressure for small and large particle size. Analytical fitting formulas of particle charge and potential and screening length depending on the particle radius parameter and on the Knudsen number have been obtained (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rationale and principle of an instrument measuring lung deposited nanoparticle surface area

The risk of nanoparticles by inhalation for human health is still being debated but some evidences of risk on specific properties of particles <100 nm diameter exist. One of the nanoparticle parameters discussed by toxicologists is their surface area concentration as a relevant property for e.g. causing inflammation. Concentrations of these small particles (~ <100 nm) are currently not measured, since the mass concentrations of these small particles are normally low despite large surface area concentrations. Airborne particles will always be polydisperse and show a size distribution. Size is normally described by an equivalent diameter to include deviations in properties from ideal spherical particles. Here only nanoparticles below a certain size to be defined are of interest. Total concentration measures are determined by integration over the size range of interest. The ideal instrument should measure the particles according to the size weighting of the wanted quantity. Besides for the geometric surface area the wanted response function can be derived for the lung deposited surface area in the alveolar region. This can be obtained by weighting the geometric surface area as a function of particle size with the deposition efficiency for the alveolar region for e.g. a reference worker for work place exposure determination. The investigation of the performance of an Electrical Aerosol Detector (EAD) for nearly spherical particles showed that its response function is close to the lung deposited surface areas in different regions of the human respiratory system. By changing the ion trap voltage an even better agreement has been achieved. By determining the size dependent response of the instrument as a function of ion trap voltage the operating parameters can be optimized to give the smallest error possible. Since the concept of the instrument is based on spherical particles and idealized lung deposition curves have been used, in all other cases errors will occur, which still have to be defined. A method is now available which allows in principle the determination of the total deposited surface area in different regions of the lung in real time. It can easily be changed from one deposited region to another by varying the ion trap voltage. It has the potential to become a routine measurement technique for area measurements and personal control in e.g. work place environments.     

Determination of Particle Size Distribution by Par-Tec® 100: Modeling and Experimental Results

Some particle size analyzers, such as the Par-Tec® 100 (Laser Sensor Technology, Redmond, WA, USA), measure the so-called cord length distribution (CLD) as the laser beam emitted from the sensor randomly crosses two edges of a particle (a cord length). The objectives of this study were to develop a model that can predict the response of the Par-Tec® 100 in measuring the CLD of a suspension for spherical and ellipsoidal particles and to infer the actual particle size distribution (PSD) using the measured CLD output. The model showed that the measured CLD is reasonably accurate for the spherical particles. However, this measurement progressively deteriorates as the shape of particles changes from spherical to ellipsoidal with large ratios of major to minor diameters. Experimental results obtained with spherical particles having a normal and a non-normal PSD indicated that the Par-Tec® 100 measurements deteriorate as the PSD deviates from a normal distribution. The information obtained from these experiments also showed that the model can reasonably predict the Par-Tec® response. Use of the inferred PSD rather than the measured CLD made a major improvement in estimating the actual PSD. Mean particle size analysis revealed that the Par-Tec® 100 volume-weighted mean particle size is closest to the unweighted mean particle size measured by sieve analysis.     

Numerically exact computer simulations of light scattering by densely packed, random particulate media

Direct computer simulations of electromagnetic scattering by discrete random media have become an active area of research. In this progress review, we summarize and analyze our main results obtained by means of numerically exact computer solutions of the macroscopic Maxwell equations. We consider finite scattering volumes with size parameters in the range [20] and [59], composed of varying numbers of randomly distributed particles with different refractive indices. The main objective of our analysis is to examine whether all backscattering effects predicted by the low-density theory of coherent backscattering (CB) also take place in the case of densely packed media. Based on our extensive numerical data we arrive at the following conclusions: (i) all backscattering effects predicted by the asymptotic theory of CB can also take place in the case of densely packed media; (ii) in the case of very large particle packing density, scattering characteristics of discrete random media can exhibit behavior not predicted by the low-density theories of CB and radiative transfer; (iii) increasing the absorptivity of the constituent particles can either enhance or suppress typical manifestations of CB depending on the particle packing density and the real part of the refractive index. Our numerical data strongly suggest that spectacular backscattering effects identified in laboratory experiments and observed for a class of high-albedo Solar System objects are caused by CB.     

Correction factors for a total scatter/backscatter nephelometer

Total aerosol scattering and backscattering atmospheric values are typically obtained with an integrating nephelometer. Due to design limitations, measurements do not cover the full (0°–180°) angular range, and correction factors are necessary. The effect of angle cutoff is examined for a number of particle size distributions and refractive indices. Scattering data for sub-micron particles can be corrected by the use of a modified Anderson approximation, while data for larger particle distributions can be approximated by a function of the effective size parameter. Correction factors for the hemispheric backscatter ratio are found to be small if nonsphericity is assumed. Such approximations will help more accurate corrections for angle range, particularly at large size parameter values.     

Optical properties of aggregate particles comparable in size to the wavelength

The angular dependence of brightness and linear polarization of randomly oriented aggregates has been investigated in order to find rules connecting their scattering properties with their structure, packing density, complex refractive index, and number and size of the spheres forming the aggregate. Our study is based on an interpretation in terms of successive orders of scattering, in particular on the analysis of the contribution of the interference and near-field effects. Such an approach allowed us to explain and interrelate the main peculiarities of the angular dependence of the intensity and polarization displayed by aggregates. Of special interest are the aggregates showing a so-called negative branch of linear polarization of light scattered into angles close to the backscattering direction. It has been shown that the enhancement of intensity and the negative polarization in this angular range are mainly caused by the interference of multiply scattered waves as well as by near-field effects. If the number of particles in the aggregate is large enough and its size is comparable to the wavelength, the backscattering enhancement is caused by the particles in the surface layers of the aggregate, where the radiation field is mostly homogeneous, while the negative branch is mainly generated by the deeper layers of particles, where the radiation field is inhomogeneous with chaotic changes of amplitudes and phases. This results in a rather weak dependence of the negative polarization on particle location in the deeper layers of the aggregate and on particle number but not on packing density.     

Formation of superconductivity through interparticle interactions in ferrimagnetic-like Sn nanoparticle assemblies

We report on the observation of the development of superconductivity through interparticle interactions in 3, 5, 7, and 23 nm ferrimagnetic-like Sn nanoparticle assemblies. The Sn nanoparticles are fabricated using the gas condensation method. Each sample consists of a macroscopic amount of individual Sn nanoparticles without a capping molecule. Ferrimagnetism is found but no sign of superconductivity can be detected when the 3 nm particles are very loosely assembled. A reduction in the mean particle moment results when the packing fraction of the assembly is increased. Superconductivity occurs when a critical packing fraction is reached. Beyond this, the superconducting transition temperature T _C continues to increase and noticeably exceeds that of the bulk T _C. The enhancement of superconductivity by interparticle interactions has also been observed in 5, 7, and 23 nm particle assemblies, with the effect becoming less significant in larger particles. We attribute these observations to the transfer of electrons between the surface and the core regions of the nanoparticles triggered by finite size effects and interparticle interactions.     

The effect of particle size on the surface composition of microcrystalline alloys

Thin films a Ni?1at%Pd alloy, ranging in thickness from 10 to 100 Å were deposited onto alumina substrates by RF sputtering. The films were subsequently heat treated to produce small particles of the alloy with average sizes in the range 300 to 1000 Å. The surface composition of aggregates of small particles was determined by Auger electron spectroscopy after equilibration of the samples at 650°C. Average particle sizes were determined by analysis of photomicrographs taken by electron microscopy techniques. The results show a strong dependence of surface composition on particle size. A decrease of a factor of five in surface concentration of palladium is observed over the range of particle sizes studied. The dependence of equilibrium surface composition on particle size has been estimated by means of a mass balance model. While this model accounts qualitatively for the effects of particle size on surface composition, discrepancies between model predictions and the experimental results suggest the intervention of other phenomena, possibly related to capillarity effects.     

Flame structure and burning velocity of flames propagating in binary iron aerosols

A numerical study was performed for binary dispersed iron aerosols in air using different particle sizes with constant average particle size. The effects of particle size and density of the two aerosols on flame structure and speed are systematically investigated. Varying the amount of small and big particles results in separated and overlapped flame fronts. For higher values of particle size ratio (ratio between the size of big and small particles) and density of small particles, flame fronts are observed to overlap. The flame speed of the binary mixture is compared with the mono-dispersed case and the difference is analyzed for different particle size ratios. The addition of a small fraction of small particles in the binary mixture is found to result in a substantial increase in the flame speed if the particle size ratio is large. Detailed analyses on the variation of the total amount of fuel shows the particle size ratio determines the equivalence ratio at which the maximum flame speed occurs. The maximum flame speed as a function of equivalence ratio was observed to move from the lean to the rich side for particle size ratio sufficiently large enough.