Showing papers in "Particle & Particle Systems Characterization in 1996"

Global Analysis of Dynamic Light Scattering Autocorrelation Functions

Abstract: Dynamic light scattering has become a standard technique for investigating colloidal suspensions and polymer solutions. The experimental field autocorrelation function g 1 (t) can often be well modelled by a Laplace transform relating g 1 (t) to a distribution of decay times A(τ). In simple systems A(τ) can usually be directly related to a distribution of molecular weights, particle sizes, diffusion coefficients or other physically relevant quantities. With constrained regularization methods, the parameter-free estimation of A(τ) has become straightforward. In complex systems, the resulting A(τ) may contain several components the identification of which is not always obvious. The problem often originates in a superposition of diffusive and angle-independent components that have different variations of their respective decay times with the scattering vector. A method is presented based on a simultaneous fit of several autocorrelation functions measured at several different scattering angles, which, using simple and reasonable assumptions, yields a robust analysis of the spectra of decay times. The application of the method is illustrated on simulated autocorrelation functions and also on real experimental data obtained on a variety of different polymer systems.

Calculating Shape Factors from Particle Sizing Data

Abstract: It is well established that particle size and shape substantially influence the bulk properties of powdered materials. Although these characteristics are closely interrelated, the tendency has been to analyse the particle size independently from particle shape. The aim of this work was the assessment of particle shape through particle sizing data. For this purpose, three different particle shape materials - glass beads (spherical), crushed glass (moderately irregular) and mica (lamellar) - were tested, employing four particle sizers. The shape factor selected was the Wadell's sphericity, which was evaluated utilizing distinct equations. The comparison of the results reveals problems arising from the use of approximations rather than more fundamental equations.

Characterising Semi‐Cohesive Powders using angle of repose

Abstract: When properly measured and interpreted angle of repose (AOR) can be a useful method of characterising a powder. Unfortunately, a multitude of definitions and measuring equipment is used. A review of the inherent problems is presented in the first part of this paper. The second part of the paper describes the improvements made to a measuring device developed originally for measuring the angle of repose of soda ash, so that a wider range of powders can be tested. In the improved design, the tendency of cohesive and semi-cohesive powders to block the device's funnel has been eliminated. A total of 73 powders consisting of four different materials, and covering Groups A, A/C and C of Geldart's classification, have been tested using the new equipment. The results show that AOR increases systematically with decrease in mean particle size. Combination of AOR measured in the new device with aerated bulk density provides a quick, sensitive and effective method for characterising a wide range of powders.

Dual-Mode Phase-Doppler Anemometer

Abstract: The accurate measurement of mass flux is identified as being essential for the successful use of phase-Doppler anemometry (PDA) in many applications, in particular with sprays. Thus measured sizes of individual droplets and the cross-sectional area to which the flux measurement is referenced must be reliable. This paper investigates sources of measurement error in each of these quantities and provides means to eliminates or minimize these errors. A new PDA configuration, termed dual–mode PDA, is introduced as an implementation of these suggestions. Example measurements illustrate the advantages of the system over conventional arrangements.

Some Recent Developments in Twin-Fluid Atomization

Abstract: The results of recent researches on twin-fluid atomization are briefly reviewed, with particular emphasis on the effects of air and liquid properties on mean drop size. It is stressed that no single equation for the mean drop sizes produced in twin-fluid atomization can satisfactorily correlate all the experimental data from any given atomizer. This is because for all types of atomizers the mechanism of liquid breakup changes from one basic mode to another with changes in atomizer operating conditions and/or liquid properties. These different modes of atomization exhibit different dependencies on atomizer dimensions and various physical properties such as liquid viscosity and air density. Equations for estimating the mean drop sizes produced in the various modes of twin-fluid atomization are presented and discussed.

Monodisperse Sprays for Various Purposes — Their production and characteristics

Abstract: The production of liquid sprays represents a key technology for a wide range of industrial proceses. Most applications currently use pressure or air-assisted atomization, resulting in the production of polydisperese sprays. Recent advances in experimental and numerical techniques for investigating liquid spraying processes, however, have enabled a closer examination of parameter optimization, leading to the conclusion that in may cases, a much narrower size distribution, or even a monodisperse spray, may exhibit many advantages. Currently monodisperse droplet generators, or drop-on-demand generators, do not meet this challenge of producing monodisperse sprays, primarily owing to the very low volume flow rate of liquid which is atomized. In the present work, a monodisperse sprya generator is introduced, which overcomes this difficulty for many applications.

Phase‐Doppler Anemometry with the Dual Burst Technique for measurement of refractive index and absorption coefficient simultaneously with size and velocity

Abstract: The principle of the dual burst technique (DBT) based on phase-Doppler anemometry (PDA) is proposed for simultaneous particle refractive index, size and velocity measurements. This technique used the trajectory effects in PDA systems to separate the two contributions of the different scattering processes. In the case of forward scattering and refracting particles, it is shown that from the phase of the reflected contribution, the particle diameter can be deduced, whereas from the refracted contribution the particle refractive index and velocity can be obtained. Furthermore, the intensity ratio of these two scattering processes can be used for absorption measurements. Simulations based on generalized Lorenz-Mie theory and experimental tests using monodispersed droplets of different refractive indices and absorption coefficients have validated this technique.

Lorenz‐Mie Theory for Spheres Immersed in an absorbing host medium

Abstract: Light scattering by particles is often used to determine velocities or concentrations of particles in gaseous or liquid streams. Within the Lorenz-Mie theory, light scattering is well understood both for a single compact spherical particle and a single multilayered particle in a non-absorbing surrounding medium. However, in some cases of practical importance the Lorenz-Mie theory in its present form may fail to describe the scattering because the host medium is absorbing (e.g. water droplets in oil). In this case, a new treatment of the scattering theory is required. In previous work, solutions were obtained in the far-field of the scattering sphere. In this paper, a rigorous solution is derived from the calculation of the total absorption rate of the particle in the host medium, which is valid for all distances from the surface of the encapsulated particle. It is shown that it is necessary to consider finite sizes R of the integrating sphere when dealing with absorbing host media. Cross-sections are defined which are characteristic quantities not only for the particle, depending on the size of a conceptual sphere around the scatterer and the imaginary part of the refractive index of the host medium. The results obtained are discussed for the case of non-absorbing host media and in the far-field approximation. Some numerical examples are given which are also related to experimental results.

Particle Size Determination by Laser Reflection: Methodology and problems

Abstract: The particle size distribution of crystalline solids has progressively become a key parameter in manufacturing processes, as important as chemical purity. Among the particle size determination and counting systems available on the market, very few offer the possibility of continuous in situ monitoring of the particle size evolution during crystallization. For this reason, much interest has been aroused by the appearance of the Par Tec 100, patented by Laser Sensor Technology [1, 2]. A study has been carried out in a stirred vessel to verify the precision and reproducibility of particle size measurement and elucidate the influence of experimental parameters on data accessible with this instrument. Optimum reproducibility has logically been achieved by fixing the highest possible cycle time and taking the mean of several cycles. Determinations with the Par Tec 100 are influenced variously, according to whether they relate to the total number of particles counted or to the mean size. Thus, the number of counts measured by a particle size probe largely depends on the operating conditions and more particularly on the hydrodynamic conditions, solvent, temperature and focal point position. Its dependence relative to the concentration of the solid in suspension is normal and linear for a solid and for a given monodisperse sample. To establish the relationship between the number of counts and the population density would therefore necessitate delicate calibration on a case-by-case basis. The mean size determined does not depend on suspension homogeneity, provided that the stirring speed is sufficient for a statistically significant total count. On the other hand, for a given sample, a displacement of the focal point can lead to considerable variations in the size determined. The optimal focal point position for small sizes is in fact highly sensitive. Lastly, the optimal position of the focal point is considerably dependent on the true size of the particles, which means that this counter is unsuitable for the precise analysis of a dispersed sample since each particle size class would require a different setting of the focal point. In addition, the sizes determined, irrespective of the products studied, appear to be underestimated for large particles and over estimated for small particles.

Deformation of liquid droplets during collisions with hot walls: Experimental and Numerical Results

Abstract: Measurements of droplet deformation during wall impingement were performed for ethanol droplets and water droplets with diameters ranging from 100 to 200 μm. The wall temperature is well above the Leidenfrost temperature of the droplet liquid. With monodisperse droplet streams and a special illumination technique, slow motion images of the phenomena can be obtained. Measurements with high temporal resolution below 1 μs are possible using a standard video camera. The experimental results are compared with numerical results, which were obtained by solving the three-dimensional Navier-Stokes equations for incompressible fluids including surface tension effects. The fluids are treated with the volume-of-fluid method and the free surface is modeled according to the continuum-surface-force model. Numerical and experimental results show good agreement.

History of Development of the Phase‐Doppler Particle‐Sizing Velocimeter

Abstract: The ability to routinely make simultaneous measurements of the size and velocity of individual spherical particles is identified as a crucial component of advances in the science and technology of multiphase systems and processes. Since it is the dual beam interferometer identified in this article as the phase-Doppler particle-sizing velocimeter (PD) that has provided this capability, and since 1995 marks twenty years from a seminal paper by Durst and Zare, a historical review is warranted and is presented. The roots and context of the development of PD are traced from the days of laser Doppler velocimetry (LDV) in the early 1970's through work culminating at several laboratories in the mid 1980's. A number of important contributions and the engineers and scientists responsible for those contributions are recognized.

Correction of the effect of particle shape on the size distribution measured with a laser diffraction instrument

Abstract: This paper gives a theoretical basis for calculating Fraunhofer diffraction patterns of arbitrary polyhedron particles. It is shown that this solution can be used for calculating a scatter matrix adapted to the particle shape in a straightforward manner. Some simulations were made to show the difference between the size distribution by volume obtained with a scatter matrix for spheres and with that for the appropriate shape. Finally, some experimentally measured signatures from platelets and rods are evaluated in order to show that the spherical equivalent diameter could be accurately retrieved.

Experimental Determination of the Transfer Function of a Differential Mobility Analyzer (DMA) in the nanometer size range

Abstract: This paper describes the evaluation of the transfer function for the TSI-short differential mobility analyzer (DMA) in the nanometer particle size range. The TSI-short DMA is identical with the TSI-long DMA (Model 3071) but has shortened inner and outer electrodes and the insulator material Teflon was replaced by black Delrin. The DMA transfer function was determined by operating two identical DMAs in series. The DMA was investigated at a sheath to aerosol flow rate ratio of between 10 and 1. Usually for particles above 50nm the parameters of the assumed triangular transfer function, i.e. height and half-width, approach the ideal values, which depend only on the flow ratio. For particle size below 50 nm the height decreases from 0.9 at 50nm to 0.18 at 5 nm. The half-width increases from the ideal value 0.1 to about 0.17. The observed changes of the transfer function are due to diffusional effects resulting in losses mainly in the inlet and outlet and also broadening of the aerosol stream inside the DMA. The determined transfer functions were also compared with transfer functions calculated using a theoretical model. The agreement proved to be satisfactory. The half-widths and the transmission efficiencies of the conventional TSI-DMA (Model 3071), TSI-short with Teflon and also black Delrin insulator as a function of particle size are compared to demonstrate the improvements in performance.

On the Optical Diagnosis and Sizing of Spherical Coated and Multilayered Particles with phase-Doppler anemometry

Abstract: After having recalled basic theoretical results concerning the extension of generalized Lorenz-Mie theory to the case of multilayered spheres, results connected with phase-Doppler anemometry are considered, showing the influence of Gaussian beam intensity profiles on the light scattering properties of these particles. Particular emphasis is placed on the case of water-coated carbon core particles, for which the possibility of obtaining simultaneous size measurements of the core and outer diameters is discussed. The sensitivity of the technique to particles with a refractive index profile is also considered, showing that this technique is more sensitive to changes in the average refractive index of the particles than to refractive index profiles, such as produced by high pressure and temperature stresses, at least for the studied geometry.

On the measurements of particles by imaging methods: Theoretical and Experimental Aspects

Abstract: Imaging techniques in particle sizing have always been important. Nevertheless, owing to the progress in CCD cameras, these techniques have found new potential which strongly depends on the capability to describe the image formation. Based on a combination of the generalized Lorenz-Mie theory and the diffraction theory, the image characteristics are described, taking into account the beam size, particle location, direction of observation and collecting optics characteristics. In the case of forward detection, a finite coherence is introduced and the results are compared with those obtained by a convolution approach, with the point spread function assumed to be Gaussian. Then some experimental results are displayed which underline the improvement achieved in particle size measurement.

Calculation of the Stokes and Aerodynamic Equivalent Diameters of a short reinforcing fiber

Abstract: A concern about all reinforcing fibers is the extent to which they are respirable. The degree of respirability of a particle is frequently estimated in terms of its aerodynamic equivalent diameter being less than a certain value (typically 3.5 to 7 μ). However, the aerodynamic equivalent diameter, like the Stokes diameter, is only defined exactly for spheres, which is far from the shape of acicular fibers. Thus, it is useful to be able to calculate, based on actual fiber diameter and aspect ratio, the effective Stokes and aerodynamic equivalent diameters of various fibers. The present paper derives simple expression for relating the two effective diameters to actual fiber dimensions. Calculated results are compared with experimental data for glass fiber and three fractions of phosphate fiber [1], which is a short, reinforcing inorganic fiber. Agreement is seen to be good. Phosphate fiber was fractionated using a sedimentation technique described in the paper. The resulting fractions were characterized by semi-manual measurements of micrographic dimensions and compared with results obtained from particle size instruments based on sedimentation principles. Based on these results, a large weight fraction of phosphate fiber is predicted not to be respirable.

Influence of Refractive Index Gradients within droplets on rainbow position and implications for rainbow refractometry

Abstract: Rainbow refractometry allows the determination of the refractive indices of droplets if a homogeneous refractive index can be assumed throughout the whole droplet. The influence of gradients on the angular position of the rainbow was examined for refractive index distributions with constant gradients and for distributions as they occur during cooling, heating and burning of droplets. Geometrical optics were used to determine rainbow positions. For a given value of refractive index at the surface, the rainbow position is in many cases a measure for the gradient itself. It was found that information obtained for constant gradients can often be used in cases with more complex distributions, which are found, for instance, for droplets undergoing heat-transfer processes. Significant errors may be obtained only for a very short initial phase of the process. Experimental results obtained with monosized droplet streams, which confirm theoretical predictions of the influence of gradients on the rainbow position, are presented.

Laser Diffraction – Unlimited?

Abstract: Within the past 20 years, particle size analysis with laser diffraction (LD) has been subject to rapid development, extending the size range stepwise from 1-200 μm to about 0.1 3500 μm. The limits of LD are discussed in terms of light sources, the influence of the beam diameter, special Fourier optics and a new detector design. It is shown that the size range is not only restricted by the wavelength of the laser and the transmission limits of the medium. Its extension is mainly related to improvements in the measurement of the angular intensity distribution. Influences from stability and flow dominate on the coarse side of the measuring range. On the fine side, the spatial extension of aerosols and the resulting demand for extended working distances can be covered only in a parallel laser beam. Extended Fourier optics in combination with an adaptable beam expansion technique and a detector with virtual borders between semicircular elements overcome the existing limits and extend the size range to a lower limit of about 0.05 μm and an upper limit above 10 mm. The sensitivity limit of LD is approaching that of single particle counting techniques. For medical spray and inhaler applications, a 0.1% optical concentration can be converted to particle size distributions even for time-resolved analyses with sample intervals of a few milliseconds. The reproducibility of the sensor, with a standard deviation typically much less than 0.5%, is no longer the limiting factor. The reproducibility of the results is mainly dominated by the reproducibility of sampling, sample splitting, dispersion and the contamination of the optical path. The latter can be improved by the control of flow, especially for in-line and inhaler applications.

Ostwald ripening of concentrated alkane emulsions : a preliminary report

Abstract: Fiber optic dynamic light scattering (FODLS) was used to study the kinetics of aging processes in emulsions of n-alkanes stabilized by a surfactant. The method is particularly useful for this purpose because it permits measurements in concentrated emulsions. Complications that may occur in traditional DLS due to the extreme dilution, such as solubilization of the particles in the medium, can be easily avoided in this way. The experimental results show that the main aging process is Ostwald ripening. The results are in agreement with theoretical predictions based on the Lifshitz-Slyozov-Wagner (LSW) theory as far as it concerns the relation between the molecular solubility of the n-alkanes (in the aqueous phase) and the Ostwald ripening rate. This illustrates that the main rate-determining factor is the molecular diffusion of the alkane molecules through the continuous (aqueous) phase. The Ostwald ripening rate is affected by the concentration of surfactant but in all situations is higher than expected from LSW theory.

Experimental Studies of Electrically Active Fibrous Filter Loading

Abstract: Electrically active fibrous filters, that is fibrous filters whose fibres carry a permanent electric charge, are an increasingly popular alternative to conventional fibrous filters in applications where low pressure drop and high collection efficiencies are critical. The advantage of these materials is the additional collection efficiency, due to electrostatic mechanisms, that can be achieved without pressure drop increase. The efficiency of these materials can fall as they are loaded with aerosol particles, so it is necessary that proper account be taken of this process during use. A complete understanding of the mechanisms responsible for this reduction in efficiency has not yet been reached. An experimental study has been undertaken at Loughborough University of Technology investigating the loading behaviour of a mixed fibre type electrically active material. The experimental techniques used in this work and experimental findings with a range of aerosol particle sizes are reported here.

Sizing of Irregular Particles Using a Phase Doppler System

Abstract: Response of a phase Doppler system to irregularly shaped particles is examined and shown to deviate qualitatively as well as quantitatively from the spherical particles. Nevertheless, the measured phase distributions based on an ensemble of particles exhibit a high degree of order and simplicity. The experimental data and the stochastic modeling of the process have shown that the phase Doppler technique can be used successfully for in-situ sizing and velocimetry of irregular particles. In the case of irregular crystalline particles, mean size and standard deviation can be deduced without requiring any assumptions regarding the functional form of the size distribution. As opposed to other optical techniques, phase Doppler can be used, in principle, near the backscattering location, so that a single optical window would be employed for transmission of laser light and collection of the scattered signals. Furthermore, size measurements can be velocity-resolved, i.e. a size distribution can be associated with each bin of the velocity histogram.

Coherent Transmission and Reflection by a monolayer of discrete scatterers

Abstract: Coherent transmission and reflection of a plane wave through a monolayer of discrete particles are considered on the basis of simple and physically transparent formulae for the single scattering approximation (SSA) corrected by introducing a multiple scattering permittivity factor. This factor allows for multiple scattering of waves between monolayer particles, opposite to the SSA. The multiple scattering permittivity factor is considered on the basis of the quasi-crystalline approximation (QCA) via T matrix formalism. The multiple scattering permittivity factor and parameters for obtaining coherent transmission and reflection coefficients (the effective extinction coefficient and the transmission and reflection coefficients due to rescattering) are calculated within the scope of QCA and plotted for comparison with SSA results. The expressions for these values are simplified for small Rayleigh particles to simple analytical formulae.

Influence of the measurement volume on the phase error in phase Doppler anemometry. Part 2: Analysis by extension of geometrical optics to the laser beam ; refractive mode operation

Abstract: The influence of the measurement volume can be investigated by using extended geometrical optics, which is based on geometrical optics by including the amplitude and phase distribution in the laser beam. The dynamics in phase Doppler anemometry can be analysed, in addition to effects of the particle size-dependent detection volume. Extended geometrical optics has been developed as a powerful tool to investigate these influences for each order of light scattering separately. Phase errors caused by Gaussian-beam intensity distribution and the curvature of the wave fronts beyond the beam waist can easily be calculated. According to Part 1 (Reflective Mode Operation), the influence of the particle trajectories on measured phase and mass concentration is simulated for refractive mode operation.

Aqueous droplet sizing by inertial classification

Abstract: The particle size distribution of an aerosol generated from an aqueous system is difficult to analyse because of the shrinkage of the droplets due to solvent evaporation. These problems are very important for the characterization of medical nebulizers, since most of the drugs delivered via inhalation are water soluble. In situ methods for droplet size analysis, such as laser diffraction, phase Doppler anemometry and light scattering, do not determine either the initial or the equilibrium size distribution. With the residual technique, which means evaporating the droplets and measuring the size and concentration of the residuals, the instability of the aqueous droplets plays no role and the necessary radioactive labelling of the sprayed material allows a direct determination of the mass flow rate at the mouthpiece of the nebulizer. In this way it is possible to discriminate between the delivered drug solution and the water necessary to humidify the incoming air. The output of nebulizers of different designs is given for various operating conditions, filling volumes and solution concentrations. The measured droplet size distribution of a nebulizer is found to be fixed mainly by its internal impaction system.

The Measurement of Particles Suspended in a stirred vessel using microphotography and digital image analysis

Abstract: A novel technique to determine the size of particles suspended in a stirred vessel is investigated. The method uses microphotography to obtain a still image of the particles in situ. The equivalent circular diameter of the particles is obtained from the photographs using digital image analysis. The particles used for the test were certified particle size standards of a very small tolerance in diameter deviation. The size comparison was determined as a percent error between the measured particle diameter (equivalent circular diameter) and the diameter established by the particle manufacturer. To determine the limiting ranges of particle size and concentrations, spherical particles in a size range of approximately 1 to 10.0 μm (microns) in diameter were employed in the test. Concentrations varied from 0.00005% to 0.1% (mass basis). For each particle size, an aqueous solution of the particles was prepared at the desired concentration and placed in a stirred vessel. Photographs of the solutions were made at 25.5x, 40x, 60x and 80x magnification. For all sizes of particles, the entire range of concentration was examined. The results indicate a minimum size resolution of approximately 3 μm, in a corresponding concentration range of 0.0001% to 0.05%. Similar limits on concentration were found for the larger diameter particles, although a true upper limit for the largest particle sizes was not established. The functional concentration range appears to shift towards higher mass concentrations with increasing particle size. For 2.92 μm diameter particles, the error in size measurement was found to be below 10% for a concentration range of 0.0005% to 0.05%. Additionally, a relationship between turbidity and the error was seen at low particle concentration levels where decreasing levels of turbidity generated increasing levels of error.

Barium Sulfate Precipitation: Crystallization kinetics and the role of the additive PMA-PVS

Abstract: The precipitation of barium sulfate was investigated by photon correlation spectroscopic (PCS) measurements of the growing crystals combined with electrical conductivity measurements of the mother solution. Small silica seeds (36nm) were added to the solution. Therefore, heterogeneous nucleation did not take place on the walls and on the stirrer, which is usually the case, but on the seeds. Hence the growth of the crystals which had nucleated on the seeds could be studied by PCS. In order to avoid the ill-conditioned inversion of the measured raw data, the autocorrelation functions, a population balance model was used to calculate these functions with the help of a kinetic model taken from the literature. The measured and the calculated autocorrelation functions should coincide and adjustable parameters were found from a least-squares fit. The measurements revealed that nucleation is strongly enhanced by PMA-PVS (polymaleic acid-polyvinylsulfonic acid). However, a simple model which assumes Langmuir adsorption of PMA-PVS on the crystal surface did not describe the kinetics very well. The nucleation rate in the absence of the additive was found to be J=1.3x10 10 exp(-27/ln 2 S) (m -3 s -1 ), while the growth rate was G=2.6x10 -8 (S-1) 2/3 S 1/3 exp(-29/3lnS) (m/s). These results match results taken from the literature fairly well.

Production of Fine Particles from Melts of Metals or Highly Viscous Fluids by ultrasonic standing wave atomization

Abstract: Ultrasonic standing wave atomization (USWA) is a new process capable of atomizing both high surface energy liquids and highly viscous liquids. Atomization is achieved through acoustic forces acting upon a liquid jet which is guided into the central pressure node of a standing wave field. Spherical metal powders with minimum mass median diameters of less than 15 μm have been produced from metal melts with surface tensions of about 0.5 N/m. Organic liquids with viscosities between 1 and 10 Pas have been atomized, yielding mass median diameters from 20 to 330 μm. The influence of different operating parameters on the mass median diameter of metal melts and highly viscous liquids was evaluated. Parameters which were varied were ambient gas pressure, vibration amplitude of the transducers, mass flow rate, density of liquid, viscosity of the liquid, surface tension and the outlet diameter. The powders and sprays were analyzed with laser diffraction particle sizers. The physical background of the atomization process is discussed and an equation for the prediction of the mass median diameter is derived.

Basic Studies of Electrohydrodynamic Atomization Process using phase Doppler measurement technique

Abstract: A planar phase Doppler system is used to measure submicron droplets generated by an electrospray. Measured drop dia-meters are correlated with the liquid properties and the condition for transition of the spray from the single-jet mode to the multi-jet mode is introduced. In another set of measurements using a standard phase Doppler system, combined size and velocity data are employed to deduce the drag force on the drops. In a situation where the drag force is balanced primarily by the electric force, the phase Doppler measurements allow to estimate the power-law relationship between the charge on a drop and its diameter and hence, many provide insights into the underlying atomization mechanisms.

Development of Agglomerate Size and Structure during spherical agglomeration in suspension

Abstract: During the spherical agglomeration process, a suspended solid is agglomerated by adding a binding liquid. First, microagglomerates or flocs are produced, which are compacted in the course of the process. Agglomerate size was evaluated by laser diffraction spectrometry, image analysis was used to determine the size and some adequately defined shape parameters calculated by Fourier analysis of the particle contour. The shape analysis confirms the visual observations; the compaction of the flocs is expressed by the corresponding change of the shape parameters. The influence of several process parameters on changes in agglomerate shape can be described quantitatively and help to gain an insight into agglomeration mechanisms. The particle size distributions determined by image analysis and laser diffraction spectrometry hardly differ for fairly spherical flocs or agglomerates. Concerning the size distribution of the irregular flocs, laser diffraction spectrometry measures larger particles than image analysis.

Generalized Lorenz-Mie Theory for Non-Spherical Particles with applications to phase-Doppler anemometry

Abstract: The scattering of focused laser beams by arbitrarily shaped dielectric bodies is investigated theoretically. The beam description is based on Davis third-order beam approximation for the field components. The scattering problem can be solved on a spherical basis by the extended boundary condition method or by the so-called modified version of the extended boundary condition method. For spheroidal particles with small eccentricities, a perturbation technique for the internal subproblem of the extended boundary condition method is described. This procedure is applied to analyse the influence of the particle shape and orientation on the response of a phase-Doppler system.