Recent Advances in Light Scattering Calculations for Nonspherical Particles

Methods for calculating the light scattering properties of nonspherical particles are reviewed, and applications to particle characterization are considered. Angular scattering results are given for particles in fixed and random orientations. These include the Mueller matrix representation and quasi-three-dimensional computer generated plots. Spectral (wavelength-dependent) scattering results are shown for spheres, a spheroid, and a glass fiber. Applications of spectral scattering to characterizing polydisperse suspensions are discussed.     

Polarized Light Scattering by Small Particles

All of the polarized light scattering properties of single particles and suspensions of particles are contained in the 16-element Mueller matrix. For single-particle studies the particle is electrostatically suspended, the light scattering functions are measured, and the particle is then extracted from the suspension chamber by placing it on the point of a needle. The particle is examined subsequently with an electron microscope to determine precisely its size and shape. Results will be presented for single polystyrene spheres, sphere multiplets, approximately cubic NaCl crystals, and cubes of MgO. Recent studies of suspensions have concentrated on oceanic hydrosols. Some results and general features of these hydrosols will be presented.     

Multiangle Polarimetry of Thermal Emission and Light Scattering by Individual Particles in Airflow

The generalized Kirchhoff's law predicts that the polarization state of thermal emission from an individual small particle depends on the particle shape. We show for the first time experimental evidence confirming this prediction for particles smaller than the wavelength by using a newly developed laser-induced incandescence instrument and a theoretical model that can predict the signals of thermal emission and light scattering for an ellipsoid under the dipole approximation (Rayleigh ellipsoid). We use single-sphere (singlet) and two-sphere clusters (doublet) of polystyrene latex spheres as primary test particles. These are currently available, well-characterized spherical and nonspherical particles, respectively. The polarization states of thermal emission and scattered light of graphite particles smaller than the wavelength show a good agreement with model calculations for plate-like Rayleigh-spheroids, consistent with their plate-like shapes observed by a transmission electron microscope. We propose that the measurement of the polarization state of thermal emission may be applicable to real-time analysis of the shape of light-absorbing particles in air.     

Dynamic Shape Factor of Nonspherical Aerosol Particles in the Diffusion Regime

Diffusional losses of monodisperse polysterene spheres and doublets from laminar aerosol flows and stagnant aerosols in cylindrical tubes were measured. The losses of spheres were used to determine their diameter (range 69–120 nm) and the volume equivalent diameter of their doublets. The losses of the doublets were used to determine their dynamic shape factor which for randomly oriented doublets was established to be 1.127 ± 0.085 for Knudsen numbers between 1.1 and 2.0.     

Scattering and Absorption by Elongated Aerosol Particles

Recently we developed a new technique, the iterative extended boundary condition method, which is suitable for calculating the scattering and absorption by elongated particles in a broad frequency range, including at resonance. This paper briefly describes this technique, illustrates its capabilities, and presents the results of its most recent extensions. This includes the implementation of a sectioning procedure and the use of a new segmentation method for calculating scattering by very long oriented chains of aerosols. Results of absorption cross sections in two different frequency ranges, visible and infrared, and for two different polarizations, parallel and end-fire polarizations, will be presented. The range of validity of some reported relationships between scattering from a single particle and from coagulated particles are discussed. It is shown that in the end-fire polarization case the sum of the absorption by separate n particles (P _sum) is more than the absorption by a chain of n coagulated particles (P _end-fire), while in the parallel polarization case, the ratio between P _parallel/P _end-fire may be as high as a factor of 4. This observation emphasizes the importance of taking the polarization into account in calculating light scattering by elongated chains of aerosols.     

Determination of the Velocity of Constrained Motion of Nonspherical Particles in Liquid

A semi-empirical equation has been derived based on the variation principle of the minimum of the energy dissipation intensity, which describes the ratio of the velocities of the constrained and free motion of nonspherical solid particles and is in good agreement with the experimental data.     

Erosion and Damage by Hard Spherical Particles on Glass

Solid particle impact of hard spherical particles on glass is of fundamental interest because of the presence of a number of different impact regimes. Understanding the impact of spherical particles is also a step toward modeling the behavior of rounded particles. This paper verifies theoretical models for the transitions between the different impact regimes and process parameters like erosion rate and surface roughness. The work also includes rounded particles. The transitions plotted in a so-called erosion map are validated with single-impact experiments. Data from erosion experiments are used to test the relations for the erosion rate and resulting surface roughness following from the models. Although the theoretical transitions compare reasonably with experiments, the models for erosion rate and surface roughness do not describe the experimentally found behavior. The models not incorporating the interaction between an impact and the damage remaining from earlier impacts might cause this deviation.     

The Presence of Adsorbed Proteins on Particles Increases Aggregated Particle Sedimentation,as Measured by a Light Scattering Technique

Protein conformational changes are often induced when bound to surfaces and can modulate colloidal stability of protein coated particles in dispersion. We evaluated bovine serum albumin (BSA) adsorbed onto polystyrene particles at room temperature. A z-axis translating laser light scattering device (ZATLLS) measured the sedimentation velocity of protein-coated particles tracking aggregation characteristics compared with non-coated ones. Sedimentation velocities of particles moving in the dispersion, and the resulting viscosity and density of the residual solution following sedimentation determined aggregate size in the dispersion using Stoke's law. Our experiments objectively show that albumin-coated polystyrene forms aggregates. Interestingly, coating particles with protein slows the sedimentation velocity which should correspond to a more dispersed system, but it leads to higher aggregate sizes due to the larger influence of proteins in solution raising solution viscosity. Protein-bound particles were observed to fall out of solution in a more controlled and steady manner compared with uncoated particles.     

球形催化剂颗粒内气体扩散及其有效因子

研究了三元催化器处理天然气汽车尾气过程中球形催化剂颗粒内的尾气扩散系数、浓度分布以及催化剂有效因子(η)等影响因素.结果表明,适当增大催化剂颗粒的平均孔半径或增大分子的平均自由程,可使扩散系数在整个催化剂颗粒内均保持较高的值.随Thiele模数(Φs)值的减小,催化剂颗粒内的相对浓度分布更为均匀,且呈整体增大的趋势,有...     

Light Scattering and Spinodal Decomposition in Glasses

It is shown that the well-known anomalous Rayleigh scattering of phase-separated glasses, which can be accounted for by the assumption of nucleation of a new phase followed by diffusion-controlled growth, can also be qualitatively accounted for by the assumption that the phase separation is occurring within the spinodal region of the phase diagram; here there is no nucleation barrier to phase separation, only a diffusional barrier. A distinction between the two mechanisms in any actual system can be made only by the use of other experimental tools, such as electron microscopy. A quantitative theory of this process proposed by Cahn is examined; it is concluded that light scattering probably reveals only the later stages of the process, rather than the initial stages dealt with by Cahn's theory.     

Structural Relaxations in Alkali Silicate Systems by Brillouin Light Scattering

The Brillouin light scattering technique was used to investigate the viscoelastic relaxations in alkali silicate systems up to temperatures of 1500°C. The line shape analysis of the Brillouin spectra reveals information about the mechanical rigidity and the viscous dissipation in the scattering medium. Brillouin frequency shifts and the frequency-dependent viscosity coefficients, v ' were measured as a function of temperature. The shapes of the v ' vs temperature curves, and the location of their maxima on the temperature scale, reveal the activation energies and the resonance frequencies of the underlying mechanisms. In binary alkali silicates, small dissipation maxima were observed between the glass transition and the melting temperature, which are attributed to the increased cation mobility as a result of the accelerated disintegration of the silica networks. In mixed alkali systems, multiple dissipation maxima are observed. These are spread over a range of temperatures, which are distinctly higher than in binary systems.     

十六烷基三甲基溴化铵与亮绿SF作用的共振光散射研究

研究了十六烷基三甲基溴化铵(CTAB)和亮绿SF作用的共振散射光谱特征。结果表明:在p H=4.10缓冲介质中,十六烷基三甲基溴化铵与亮绿SF相互作用形成离子缔合物,产生以505 nm为特征峰的共振光散射(RLS)增强信号,其RLS增强程度与亮绿SF浓度成线性关系,检出限和线性范围分别为97.6 nmol/L和0.98~75μmol/L,据此建立了痕量亮绿SF的共振光散射分析方法。将方法用于水样中亮绿SF含量的快速检测,结果令人满意。     

Problems in Characterizing Transparent Particles by Laser Light Diffraction Spectrometry

Laser diffraction spectrometry is an optical technique that uses information from the scattered light distribution to determine particle size distribution. When laser light diffraction spectrometry is used with small transparent particles, secondary scattering effects are present. As a result, the particle size distribution determined via the Fraunhofer approximation predicts the existence of a fine particle fraction that in reality does not exist. Application of the Mie theory eliminates the false prediction of a fine particle fraction, but only if the material's refractive index and absorption index are exactly known. This phenomenon was confirmed theoretically for transparent particles by simulating the scattered light distribution and experimentally by comparing results with those obtained using non‐optical methods.     

Triacylglycerol Analysis of Fats and Oils by Evaporative Light Scattering Detection

A high performance liquid chromatography method with evaporative light scattering detection was developed for the analysis of oils and fats, which enabled excellent separation of major and minor triacylglycerol (TAG) species in 33 min, including regeneration of the column. The influence of the mobile phase and temperature on separation and analysis time were evaluated with a cocoa butter standard. The influence of the drift tube temperature and flow of the nebulising gas on the evaporative light scattering detector output signal was investigated by means of a response surface experimental design. Especially the flow of the nebulising gas had a profound effect on the detector signal. An optimal separation was obtained when using a 150 × 3.0 mm C18 column with 3 μm particle diameter at 20 °C and an acetonitrile/dichloromethane gradient at 0.72 mL/min. The maximum response was attained when the ELSD detector was set at the minimum temperature (45 °C) and a gas flow of 1.2 L/min. Finally, the linearity of the detector was investigated. It was found that at very low concentrations, the signal tends to flatten towards zero, giving an underestimation for minor TAG species, especially for oils or fats with a mixed fatty acid composition.     

酪蛋白溶解与酶解行为的动态光散射

采用动态光散射技术,以磷酸盐为缓冲液,研究了温度、离子强度和酶解行为对酪蛋白胶束平均流体力学半径(Rh)的影响规律. 结果表明,Rh值在升温过程中发生不可逆减小;随着离子强度增大,Rh值先减小再增大;酪蛋白在胰蛋白酶作用下,溶液光散射强度对应于分子量大小先急剧下降,再逐渐趋于平缓,而Rh值的变化规律与之相反;并推测了酪蛋白胶束酶解过程的结构变化模型,即由原始致密的球状体逐渐舒展为松散而规则的毛束状蛋白肽链.     

Stability of Erythrocyte-Derived Nanovesicles Assessed by Light Scattering and Electron Microscopy

Extracellular vesicles (EVs) are gaining increasing amounts of attention due to their potential use in diagnostics and therapy, but the poor reproducibility of the studies that have been conducted on these structures hinders their breakthrough into routine practice. We believe that a better understanding of EVs stability and methods to control their integrity are the key to resolving this issue. In this work, erythrocyte EVs (hbEVs) were isolated by centrifugation from suspensions of human erythrocytes that had been aged in vitro. The isolate was characterised by scanning (SEM) and cryo-transmission electron microscopy (cryo-TEM), flow cytometry (FCM), dynamic/static light scattering (LS), protein electrophoresis, and UV-V spectrometry. The hbEVs were exposed to various conditions (pH (4–10), osmolarity (50–1000 mOsm/L), temperature (15–60 °C), and surfactant Triton X-100 (10–500 μM)). Their stability was evaluated by LS by considering the hydrodynamic radius (R_h), intensity of scattered light (I), and the shape parameter (ρ). The morphology of the hbEVs that had been stored in phosphate-buffered saline with citrate (PBS–citrate) at 4 °C remained consistent for more than 6 months. A change in the media properties (50–1000 mOsm/L, pH 4–10) had no significant effect on the R_h (=100–130 nm). At pH values below 6 and above 8, at temperatures above 45 °C, and in the presence of Triton X-100, hbEVs degradation was indicated by a decrease in I of more than 20%. Due to the simple preparation, homogeneous morphology, and stability of hbEVs under a wide range of conditions, they are considered to be a suitable option for EV reference material.     

Behavior of Compact Nonspherical Particles in the TSI Aerodynamic Particle Sizer Model APS33B: Ultra-Stokesian Drag Forces

The aerodynamic behavior of aggregates consisting of uniform polystyrene latex (PSL) spheres and unaggregated cuboidal Natrojarosite particles in a TSI aerodynamic particle sizer (Model APS33B) has been studied. In initial tests, monodisperse PSL micro-spheres ranging from 0.3 to 7 μm in geometric diameter were generated from aqueous suspensions using a Lovelace nebulizer. APS33B responses for these uniform-sized particles showed multiple peaks. The major (primary) peak, which resulted from the smallest particle, corresponded to the unaggregated single spheres (singlets); the second, third, and fourth peaks were identified as doublets, triangular triplets, and tetrahedral quadruplets, respectively. Both doublets and triplets moved with their long axes in perpendicular (maximum drag) orientation to the flow direction in the APS33B. In contrast, the tetrahedral particles were isometric and had the same dynamic shape factor (drag resistance) for all three primary orientations. The particle Reynolds numbers (Re _p) for these particles were calculated and ranged from 0.2 to 30 in the sensing volume of the APS33B detector (i.e., ultra-Stokesian conditions). Ultra-Stokesian drag forces for all three types of aggregates were, therefore, estimated and expressed as a function of an empirical factor (1 + aRe ^b _p) to the Stokesian drag force. The ultra-Stokesian drag of a Natrojarosite particle was measured in the range 20 Re _p < 50 and could be described with a similar expression. This approach facilitates the study of the dynamic behavior of nonspherical particles and yields new information about the characteristics of drag forces in the ultra-Stokesian regime     

Polysaccharide Characterization by Aqueous Size Exclusion Chromatography and Low-Angle Laser Light Scattering

Abstract

Aqueous size exclusion chromatography coupled with on-line low-angle laser light scattering (SEC/LALLS) is a valuable analytical tool for characterization of polysaccharides and other important biopolymers. This work reviews the fundamental size separation mechanism of polymers chromatographed via SEC, the development of SEC/LALLS methods for characterization of eluted polymers, and applications of this technique to determine polysaccharide physical and chemical properties. Important nonsize exclusion effects encountered in aqueous SEC of polysaccharides are discussed and attributed to intramolecular and polymer-support interactions, as well as flow-related anomalies. The necessity of absolute molecular weight detection as a direct means of calibration is presented. Low-angle laser light scattering coupled to SEC provides a simple method of direct calibration and allows determination of polymer molecular weight and molecular weight distribution. Recent applications of SEC/LALLS to determine polysaccharide branching characteristics are detailed. The combined knowledge of molecular weight distributions and branching distributions provides insight into the molecular kinetic events of polysaccharide processing operations.     

Determination of the Scalar Friction Factor for Nonspherical Particles and Aggregates Across the Entire Knudsen Number Range by Direct Simulation Monte Carlo (DSMC)

The friction factor of an aerosol particle depends upon the Knudsen number (Kn), as gas molecule–particle momentum transfer occurs in the transition regime. For spheres, the friction factor can be calculated using the Stokes–Millikan equation (with the slip correction factor). However, a suitable friction factor relationship remains sought-after for nonspherical particles. We use direct simulation Monte Carlo (DSMC) to evaluate an algebraic expression for the transition regime friction factor that is intended for application to arbitrarily shaped particles. The tested friction factor expression is derived from dimensional analysis and is analogous to Dahneke's adjusted sphere expression. In applying this expression to nonspherical objects, we argue for the use of two previously developed drag approximations in the continuum (Kn → 0) and free molecular (Kn → ∞) regimes: the Hubbard–Douglas approximation and the projected area (PA) approximation, respectively. These approximations lead to two calculable geometric parameters for any particle: the Smoluchowski radius, R _S, and the projected area, PA. Dimensional analysis reveals that Kn should be calculated with PA/πR _S as the normalizing length scale, and with Kn defined in this manner, traditional relationships for the slip correction factor should apply for arbitrarily shaped particles. Furthermore, with this expression, Kn-dependent parameters, such as the dynamic shape factor, are readily calculable for nonspherical objects. DSMC calculations of the orientationally averaged drag on spheres and test aggregates (dimers, and open and dense 20-mers) in the range Kn = 0.05–10 provide strong support for the proposed method for friction factor calculation in the transition regime. Experimental measurements of the drag on aggregates composed of 2–5 primary particles further agree well with DSMC results, with differences of less than 10% typically between theoretical predictions, numerical calculations, and experimental measurements.

Copyright 2012 American Association for Aerosol Research     

LED用光扩散剂与光扩散材料研究现状

文章就目前光扩散剂、光扩散塑料、光扩散灌封胶、光扩散涂料和油墨的研究及应用现状进行介绍。