An electron diffraction apparatus for small particle beams

An electron diffraction apparatus is described which has been developed for use with small particle beams. The apparatus features a pulsed 100 kV electron beam and on-line parallel measurement of the diffraction pattern using linear charge coupled devices. Preliminary results on a molecular beam of small silver particles are presented.     

Effect of small particles on the near-wall dynamics of a large particle in a highly bidisperse colloidal solution

We consider the hydrodynamic effect of small particles on the dynamics of a much larger particle moving normal to a planar wall in a highly bidisperse dilute colloidal suspension of spheres. The gap h(0) between the large particle and the wall is assumed to be comparable to the diameter 2a of the smaller particles so there is a length-scale separation between the gap width h(0) and the radius of the large particle b>h(0). We use this length-scale separation to develop a new lubrication theory which takes into account the presence of the smaller particles in the space between the larger particle and the wall. The hydrodynamic effect of the small particles on the motion of the large particle is characterized by the short time (or high frequency) resistance coefficient. We find that for small particle-wall separations h(0), the resistance coefficient tends to the asymptotic value corresponding to the large particle moving in a clear suspending fluid. For h(0)>a, the resistance coefficient approaches the lubrication value corresponding to a particle moving in a fluid with the effective viscosity given by the Einstein formula.     

Effects of particle size segregation on crossflow microfiltration performance: Control mechanism for concentration polarisation and particle fractionation

Although the principal mechanisms of crossflow microfiltration (MF) are well-known, the practical applicability of the resulting microfiltration models is still limited. This can be largely attributed to the lack of understanding of effects of polydispersity in the particulate suspensions, as relevant to concentration polarisation in MF. This paper describes an investigation of concentration polarisation behaviour of bidisperse suspensions, in the regime where shear-induced diffusion is the dominant back-transport mechanism. In the transient flux regime, the particle deposition onto the membrane was monitored by means of confocal scanning laser microscopy. As in accordance with the linear dependence of the shear-induced diffusivity on a², only the small particles in the bidisperse suspensions were found to deposit onto the membrane. The back-transport flux that was calculated from the deposition rate and the actual permeate flux, was found to be independent of the composition of the suspension, whereas it was equal to the back-transport flux of a monodisperse suspension of the small particles only, with a similar total particle fraction. These results can be explained with the occurrence of particle size segregation in the feed flow, which leads to an enrichment with small particles of the suspension near the membrane. The findings are also shown to be relevant to particle fractionation processes by MF. In such fractionation processes, particle size segregation is found to have a strong effect on the separation characteristics such as particle size and fat content of the permeate. A polydisperse suspension could be fractionated using a membrane having a pore size larger than the largest particles present. The fractionation thus results not from size exclusion in the membrane, but from segregation effects in the feed channel.     

Electrochemical characterization of the surface of a small particle and nonequilibrium electric surface phenomena

By investigating the effects that originate from nonequilibrium double layer states, it is possible to discriminate between two types of surfaces respectively corresponding and not corresponding to the standard electrokinetic model (SEM). This goal is accomplished by measuring the induced dipole moment (i.d.m.) of a particle, which enables one to determine the specific surface conductivity κ^σ, mobile charge σ_m and efficient Stern potential Ψ_ef. Porous or rough surfaces are quite abundant, but they do not correspond to SEM which implies that Ψ_ef exceeds ζ. Since ion adsorption and the stability ratio depend exponentially on Ψ_ef, substitution of the ζ potential for Ψ_ef in these exponents results in a very large error if Ψ_ef is high. The investigation of particles with a porous or a rough surface or with an anomalous conductivity confined to Stern layer ions can be quantified through integrated measurements of the nonequilibrium electric surface phenomena (NESP) used in conjunction with the extension of DLVO theory over this more complicated type of surface.NESP which are used for measurement of κ^σ are characterized and experiments are discussed which have confirmed the correctness of the theory of NESP and of the procedure for κ^σ, σ_m and Ψ_d calculation and determination of the ζ-potential under polarization conditions. A comparative evaluation of the high- and low-frequency as well as of the stationary complexes of NESP measurements is carried out, and their advantages and disadvantages are shown as they reveal themselves in electrochemical characterization. The unsuitability of the Henry-Booth equation for the interpretation of low-frequency or stationary NESP is demonstrated as well as its apparent agreement with experiment and the complete inconsistency in the case of the nonstandard model. The software, which provides a correct extraction of information about the surface of a spherical particle from data of combined electric surface investigations of real disperse systems whose particles are usually polydisperse and do not correspond to SEM, is discussed. Attention is paid to efficient methods for measuring the i.d.m. κ^σ and Ψ_ef of a nonspherical particle, based on the anisotropy of conduction and on the electro-optic phenomena.The nonlinearity of high-voltage electrophoresis caused by surface diffusion of the adsorbed ions is predicted and described. It is proposed to use this new effect for studying the dynamic Stern layer and for measuring the adsorbed ion diffusivity. Such investigations are especially important for studying the electrostatic interaction of colloid particles which has a strong effect on the kinetics of slow coagulation according to studies made by Lyklema and Dukhin. Within the framework of the Lyklema-Klein model, the effect of gel layers on the electrostatic interaction of particles is considered and on this basis the importance is shown of identification of charged gel layers and of their electrochemical characterization.     

Influence of very small bubbles on particle/bubble heterocoagulation

Very small bubbles which partially coat the surface of particles influence whether or not heterocoagulation between a particle and a bubble occurs. The electrostatic and van der Waals forces of interaction between particles and bubbles were calculated as a function of electrolyte concentration, particle size, and the size and distributions of these very small bubbles present on the particle surface. The height of the surface force barrier was compared with the hydrodynamic pressing force under conditions of flotation. The presence of these very small bubbles has a profound effect on the interaction between particles and bubbles and, in particular, strongly decreases the critical particle radius for heterocoagulation.     

Free energy of solvation of a small Lennard–Jones particle

In molecular dynamics (MD) and Monte Carlo (MC) free energy calculations, the choices of the thermodynamic paths from state a to state b affect the accuracy of the result and the efficiency of the programs. Most of the problems occur at the initial stages of growing in a new particle into a solvent. Based on statistical mechanical perturbation theory, an accurate and efficient direct calculation of inserting a small Lennard–Jones particle into solvent is derived. This eliminates the need for calculation of the initial stages of growing in a new particle by MD or MC simulation. Examples are given to show the utility of direct calculation. The recommended procedure is to use direct calculation for a small Lennard–Jones particle and then use MD or MC simulations to calculate the ΔG of changing the small Lennard–Jones particle into the target molecule. © 1994 by John Wiley & Sons, Inc.     

Fumed silica synthesis: influence of small molecules on the particle formation process

Burning silicon tetrachloride in an oxygen‐hydrogen flame produces fumed silica. This process is known for at least 50 years [1‐5], but some important details are still uncertain. We would like to study several starting steps of fumed silica synthesis on the way from molecules to products. To do this we have performed quantum‐level simulations of protoparticle and primary particle formation, from silicon dioxide molecules. Additionally, we have simulated the behavior of silica clusters in the presence of small molecules like water and hydrochloric acid. The reaction of silicon dioxide molecules leads to a silica cluster, which is covered with chemicaly highly active sites of one‐coordinated oxygen atoms and three‐coordinated silicon atoms. These clusters interact together and produce silica bulk like quartz glass. Reaction with water terminates the silica particle surface and leads to a complicated structure of the particle surfaces. The hydroxyl shell protects the particle body against the increase in particle size, but leads to aggregate and agglomerate formation.     

Electron density and atomic relaxation around a vacancy in a small metal particle

The electron density in an ideal vacancy placed at the center of a small Aluminium microparticle has been studied as a function of the number of atoms in the microparticle. In these circumstances one can conclude that the perturbation of the density is localized in the close neighborhood of the vacancy and is nearly independent of the particle size. Then relaxation of the nearest neighbours of the vacancy was allowed. This relaxation was found to be small, but its sign depends on particle size. For particles larger than ～100 atoms the relaxation is towards the vacancy, as for a vacancy in bulk Aluminium. But for smaller particles, the relaxation is away from the vacancy. This relaxation suggests a nontrivial size dependence of the energy of vacancy formation.     

Auxiliary conversion tables for sensitivity performance tests of atomic-absorption instruments

Conversion tables have been prepared which give the signal (in absorbance units and percentage of absorption) corresponding to a given sensitivity level, expressed as concentration limit, when measuring the response of a solution of a given analyte concentration. These tables permit the operator a rapid check of the sensitivity performance of an instrument by comparing signals obtained under his conditions with the signal expected for a given concentration from concentration limits already published in the literature or previously obtained by himself in his laboratory with other operating conditions.     

Polychloroprene rubbers used for adhesives: a small angle X-ray scattering preliminary investigation under stretching

Summary Three types of polychloroprene rubbers (Du Pont de Nemours Co. Neoprenes AD, WHV and W) used for adhesives and having different crystallization rates have been investigated.The mechanical properties of the Neoprenes samples were determined and the films were examined under stretching by SAXS for a preliminary investigation on their different superreticular order.

---

Zusammenfassung Drei Typen von Polychloropren-Kautschuk mit verschiedener Kristallisationsgeschwindigkeit (Neoprene AD, WHV und W von Du Pont de Nemours), die als Klebemittel dienen, wurden als gedehnte Filme mechanisch und röntgenographisch (SAXS) untersucht. Sie zeigen Unterschiede in der Überstruktur.

     

Effect of nanomagnetic particle on mechanics performance of magneto-rheological fluid

This paper presents an experimental method to investigate the effect of nanomagnetic particle on mechanics performance of magneto-rheological (MR) fluid, including the normal force and shear torque. Under different currents, the mechanics performance of a series of MR fluids with different quantities of nanomagnetic particles (0%, 4%, 7%, 12%, and 16% respectively) are measured by the plate-on-plate shearing test rig. The experimental results indicate that, with an increasing quality percentage of nanomagnetic particles, in a small range of currents (0–1.5 A), the mechanics performances are increasing, but their increment is no longer obvious when the current is above 1.5 A.     

High resolution MW spectroscopy of Rydberg clusters for the studies of small metal cluster ions: a preliminary experiment

The influence of the occupation of the single particle levels on the impact parameter dependent K-K charge transfer occuring in collisions of 90 keV Ne⁹⁺ on Ne was studied using coupled channel calculations. The energy eigenvalues and matrixelements for the single particle levels were taken from ab initioself consistent MO-LCAO-Dirac-Fock-Slater calculations with occupation numbers corresponding to the single particle amplitudes given by the coupled channel calculations.     

Experimental humidity dependency of small particle adhesion on silica and titania

The humidity present in ambient atmosphere affects the adhesion of small particles by causing capillary bridge formation between the particle and the surface. Even in moderate relative humidities this, usually attractive, force can have a significant effect on adhesion behaviour of micro and sub-micro particles. We have directly measured the pull-off forces of initially adhered oxide particles on oxide surfaces with atomic force microscope in controlled atmosphere with adjustable humidity. We demonstrate the effect of the surface roughness resulting in two different regions of capillary formation and the particle shape having a strong effect on the humidity dependency of adhesion. The experimental results are explained by theoretical framework.     

Scanning tunneling microscopy and spectroscopy for cluster and small particle research

Scanning tunneling microscopy (STM) and spectroscopy (STS) are new methods to investigate atomic arrangements and electronic structures of clusters and small particles of atoms. In this paper we review recent developments in this field, in particular the work from our laboratory. We show studies of single adatoms, small clusters and larger particles of platinum and a trimer of aluminum imaged with atomic resolution on highly-orient ed pyrolytic graphite. We find different isomeric structures for clusters of a specific size. Taking the substrate lattice as reference we determine bond lengths and angles for the clusters. We find that adsorbed Pt-particles have a strong influence on the substrate. Periodic charge density modulations on the graphite lattice surrounding the particles are observed. We also discuss recent STS experiments which showed Coulomb blockade in electron tunneling. A silicon-oxide-graphite tip-junction is used where a mesoscopic insulating area containing trap levels for temporary electron storage is responsible for the blockade of single electron transport. Such an ultra-small insulator capacitor shows large voltage steps in current-voltage characteristics and quantization of the tunneling current.     

Phonon effect on enhancement and reduction of the superconducting transition temperature of a small particle and amorphous thin films

Using an elastic vibration model for phonons of an oxidized small metallic particle, size dependence of the superconducting transition temperature T_C is derived. It is shown that the oxidization results in the reduction of T_C with decrease in particle size. The main cause is that the surface oxide enhances the phonon frequencies as the particle size decreases. Superconducting character of amorphous thin films and its relation to an oxidized small particle is also discussed.     

Investigation of the effect of a dc glow discharge on the performance of a particle beam liquid chromatography/mass spectrometry interface

A low voltage (180-V) dc glow discharge device was inserted just below the pneumatic nebulizer in a particle beam interface of a high performance liquid chromatography/mass spectrometry system. The discharge in a helium atmosphere increased the signal produced by 12 test compounds by factors of about 2–6. The increases in signal were probably produced by an increase in the efficiency of solute transmission through the interface. The signal increases caused by the glow discharge were compared to somewhat larger increases caused by 0.01-M ammonium acetate in the mobile phase. The combination of glow discharge and ammonium acetate provided no meaningful advantage over the individual techniques. The mechanism of improved transport efficiency is not proven, but the neutralization of charged particles is a viable hypothesis.     

Particle morphology effects on the performance of PMMA filled with aluminium hydroxide in a variety of fire tests

The effect of aluminium hydroxide particle size and shape has been investigated on the performance of (polymethyl methacrylate) composites in a number of flammability tests. As a subsidiary exercise correlation between the tests has been investigated. The work has shown the expected effect of particle size on performance, with surface area giving the best measure of particle size if non-spherical particles are involved. While the smallest particles always give the best results, the effect varies markedly according to the test and is most pronounced in the ISO ignitability test. Some possible explanations for these effects are proposed but more experiments are needed to choose between them. Correlation between the various tests is generally poor and reliance on any one of them for product development would be unwise.     

Relation between the particle size distribution and the kinetic performance of packed columns. Application to a commercial sub-2 microm particle material

To study the influence of the particle size distribution (PSD), we measured the chromatographic performance of a series of sub-2 microm particle high performance liquid chromatography (HPLC) columns packed with four different particle mixtures having a purposely imposed different size distribution. Using the reduced kinetic plot representation by plotting the separation impedance (E(0)) versus the plate number ratio (N(opt)/N), the different columns could be classified according to their chromatographic performance without the need to specify a mean particle diameter or a molecular diffusion coefficient, as is needed in the classical reduced plate height and flow resistance analysis. The present analysis shows that it is not so much the width or span of the particle size distribution, but rather the presence of fines that greatly determines the chromatographic performance of particulate columns.     

Enhanced particle trapping performance of induced charge electroosmosis

By increasing the number of floating electrodes or enlarging the width of single floating electrode, this work provides effective ways to strongly improve the particle trapping performance of induced charge electroosmosis (ICEO). Particle trapping with double or triple separate narrow floating electrodes increases the effective actuating range of ICEO flow and therefore enhance the optimum trapping ability to be 1.63 or 2.34 times of that with single narrow electrode (width of ), and the ideal trapping frequency is independent of the electrode number due to the mutual independence of electrochemical ion relaxation over each electrode. Furthermore, using a single wide floating electrode with the effective width equal to three separate narrow floating electrodes () instead of a single narrow one slightly lowers the ideal trapping frequency due to an increase in the characteristic polarization length, but the trapping performance is only up to 1.59 times of that with original single narrow electrode, implying that vertical channel confinement effect may severely suppresses the effective actuating range of ICEO flow and renders the trapping performance not as expected. Trapping experiments over wide floating electrode with different channel height were carried out, showing that the trapping performance increases by correctly increasing the channel height.