Optical properties of interacting particles

The radiative cross sections of random homogenous dense systems of interacting Rayleigh particles are studied analytically in terms of a proposed model for the radial distribution function. The model provides a closed-form expression for the radial distribution function that captures the essential features of the function's behavior as described by experimental results and numerical calculations.     

Asymptotic solutions for optical properties of large particles with strong absorption

The transverse wave condition is not applicable to the refracted electromagnetic wave within the context of geometrical optics when absorption is involved. Either the TM or the TE wave condition can be assumed for the wave to locally satisfy the electromagnetic boundary condition in a ray-tracing calculation. The assumed wave mode affects both the reflection and the refraction coefficients. As a result, nonunique solutions for these coefficients are inevitable. In this study the appropriate solutions for the Fresnel reflection-refraction coefficients are identified in light-scattering calculations based on the ray-tracing technique. In particular, a 3 x 2 refraction or transmission matrix is derived to account for the inhomogeneity of the refracted wave in an absorbing medium. An asymptotic solution that completely includes the effect of medium absorption on Fresnel coefficients is obtained for the scattering properties of a general polyhedral particle. Numerical results are presented for hexagonal plates and columns with both preferred and random orientations.     

A study of the electrophoretic properties of montmorillonite particles in aqueous electrolyte solutions

Experimental results on the electrophoretic velocity and mobility on Ca- and Na-montmorillonite in water and aqueous electrolyte solutions are reported. The effects of particle concentration, electric field strength and, particularly, pH and concentration and type of the electrolyte present in the dispersion medium are considered. The very weak variation of the mobility with the volume fraction of solids is discussed in the light of the predictions of three theoretical models of collective electrophoresis. The null effect of the variations of the applied field strength is considered on a qualitative basis assumming a simple double layer model. Finally, the general trends of the interesting behaviour of montmorillonite in different conditions of electrolyte concentration and valency are discussed taking into account the structural characteristics of the clay.     

Electrorheological and dielectric properties of polypyrrole dispersions

The electrorheological (ER) response and dielectric properties of the polypyrrole (PPy) dispersions in mineral oil were investigated. Various PPy particles were synthesized controlling the amount of oxidant and surfactant to investigate the effect of these variables on the ER response. The ER response increases with the oxidant and surfactant amount, passes through a maximum, and then decreases with the oxidant and surfactant amount. However, the decrease in the ER response at large surfactant amounts is not so significant compared to that at large oxidant amounts. The increase in the ER response with the oxidant and surfactant amount arises from the enhanced particle polarization originated from the increased PPy conductivity. The decrease in the ER response at large oxidant amounts seems to arise from the increased conduction between the PPy particles, while that at large surfactant amounts arises from the decreased particle polarization.     

SANS analysis of aqueous dispersions of Eu- and Gd-grafted nanodiamond particles

Abstract

The structure of detonation nanodiamond aqueous dispersions grafted by europium or gadolinium atoms was investigated by small-angle neutron scattering over a wide size scale from 1 to 3000?nm. Similar to the previous studies of nanodiamond suspensions, a strong association of the particle into developed aggregates was revealed. While the characteristic aggregate size depends on the modification of the dispersions and varies in a wide interval of 35–1500?nm, the fractal character of clusters of nanodiamond particles (packed in a specialized branched form with a fractal dimension of 2.4) remains unchanged independent of the cluster size. The effect of the aggregate size increase upon grafting is considered as a basis for the stability reduction mechanism at the microstructural level.     

Characterization of magnetic dispersions: Rheological, mechanical and magnetic properties

The rheological properties of magnetic oxide dispersions have been studied, varying dispersing agent, magnetic oxide, milling time and oxide loading level. Magnetic properties of handcoats as well as mechanical and magnetic properties of free films prepared from these dispersions have also been studied. The results show a good correlation between rheological, magnetic and physical properties. As a result, rheological measurements can be a convenient and powerful tool for evaluating the dispersibility of a magnetic oxide and the efficiencies of formulation components such as dispersing agents. Rheological measurements can also determine optimum oxide loading, optimum dispersant level, and optimum milling time. Results show that for a given oxide, there exists an oxide-binder ratio where mechanical, rheological and magnetic properties are optimized. This optimum suggests the existence of a well-defined oxide-binder structure. Results at oxide loadings lower and higher than the optimum can be explained, respectively, on the basis of individual particles with no long range structure formation and on the basis of possible clustering of particles.     

Optical properties caused by periodical array structure with colloidal particles and their applications

High quality opal films have been attracting much attention due to their novel properties and applications, such as smart materials with structural color, novel photonic/optical devices and three-dimensional photonic crystals. In this article, the author reported a colloidal crystal consisting of cubic closely packed (ccp) polystyrene particles and filled with polydimethylsiloxane (PDMS) elastomer. The array of ccp (111) planes diffracts light of selective wavelengths according to Bragg’s law. The PS-PDMS hybrid opal films exhibit dynamic tuning structural colors. The lattice distance of ccp (111) planes is variable by swelling PDMS elastomer with hydrophobic liquid or by applying mechanical deformation. The hybrid opal films have potential applications in wide fields, for example, in smart sensing materials, color imaging without pigments and strain mapping of plastic deformation.     

Optical, structural and microhardness properties of KDP crystals grown from urea-doped solutions

Potassium dihydrophosphate single crystals were grown from aqueous solutions onto a point seed using temperature reduction method by doping with different molar values of urea. The characterization of the grown crystals was made by visible and Fourier transform infrared spectroscopy, Vicker's hardness studies, X-ray powder diffraction, non-linear optical and laser damage threshold measurements. By comparing these crystals with the ones grown from the pure solution, it is shown that 0.2-2.0 M of the urea additive enhances the laser damage threshold and the second harmonic efficiency more than by 25 and 20%, respectively. By means of the Bond method using a multipurpose three-crystal X-ray diffractometer it is shown that the presence of urea additive increases the crystal lattice parameter c of the grown crystals, whereas the lattice parameter a is by an order less sensitive to the changing urea concentration in the solution. The Vicker's hardness studies at room temperature carried out on (1 0 0) and (0 0 1) crystallographic planes show an increased hardness of the doped crystals (grown in the presence of urea additive) on the plane (0 0 1) in comparison with that of pure potassium dihydrophosphate crystal.     

Optical properties of silver nanoparticles in R-phycoerythrin nanochannels in aqueous solutions and films

The optical properties of Ag⁰ nanoparticles in R-phycoerythrin nanochannels have been studied in liquid and solid phases by confocal fluorescence microscopy, absorption spectroscopy, and fluorescence spectroscopy. The optical properties of the Ag⁰ nanoparticles are shown to depend on their phase state at the levels of an individual nanoparticle and many nanoparticles. UV irradiation has different effects on Ag⁰ nanoparticles in deionized water and in thin films on glass.     

Degradation of omeprazole induced by enteric polymer solutions and aqueous dispersions: HPLC investigations

Two reversed-phase high-performance liquid chromatography (RP-HPLC) methods were developed to investigate the degradation of the acid-labile proton-pump-inhibitor omeprazole in organic polymer solutions and aqueous dispersions of enteric coating polymers (Eudragit L-100, S-100, CAP, HP-55, HPMCAS-HF, -LF, and shellac). The overall goal of the study was to determine the influence of the polymer structure on the degradation of omeprazole, i.e., whether the acid structure of the enteric coating polymers caused an instability of the proton pump inhibitor. Moreover, it was investigated whether a difference in omeprazole degradation could be detected between organic polymer solutions and aqueous dispersions. pKa values of the polymers and pH values of the aqueous dispersions were determined to see whether there was a correlation with the extent of degradation of omeprazole induced by enteric polymers. As the polymers containing phthalate moieties are very susceptible to hydrolysis, the influence of free phthalic acid on omeprazole stability was investigated. Finally, the degradation kinetics of omeprazole in organic polymer solutions were determined. Omeprazole degradation is more pronounced in aqueous polymer dispersions than in organic polymer solutions. The influence of organic polymer solutions on the stability of omeprazole depends on the amount of acidic groups in the polymeric structure, whereas the influence of aqueous polymer dispersions depends on the pH value of the dispersion. The amount of free acids present in some polymers as by-products also cause a degradation of the proton pump inhibitor. Among all investigated polymers, shellac showed the least influence on the stability of omeprazole. The decomposition of omeprazole in organic polymer solutions followed first-order kinetics. The decrease of omeprazole peak area in organic polymer solutions was in the order Eudragit L-100> HPMCAS-HF>shellac.     

Mechanical properties of dispersions of thoria in beryllia

The behaviour of the two-phase ceramic system BeO-ThO₂ as a function of thoria (ThO₂) particle size and concentration was studied by measuring changes in strength and Young's modulus. Modulus of rupture measurements were made by the four-point bending of cylindrical specimens, and Young's modulus was calculated from the deflection of rectangular beams. Examination of matching fracture faces showed that cracks passed through thoria particles, indicating they were bonded to the matrix. At 1.6 vol % thoria, the strength of a dispersion depended on the thoria particle size; 5 m particles caused little change from 33000 lb/in.² (23.1 kg/mm²) for pure beryllia (BeO), while with 110 m particles the strength was 20100 lb/in.² (14.1 kg/mm²). The strength of 1.6 vol % 200 m dispersions increased with test temperature in the range 20 to 400° C, indicating the presence of internal tensile stresses, caused by the differing coefficients of thermal expansion of beryllia and thoria. Increasing the concentration of thoria caused Young's modulus and strength to decrease; the observed values for Young's modulus agreed closely with those predicted for BeO-ThO₂ dispersions using Hashin and Shtrikman's method. It is concluded that the strength of BeO-ThO₂ dispersions is controlled by the strength of the weak thoria particles unless the particle size is restricted to less than 5 m.     

Optical properties of atmospheric particles: complete parameter sets obtained through polar photometry and an improved inversion technique

Complete sets of optical parameters of dry particles sampled on a Nuclepore filter are derived through interpretation of photometric data with an improved inversion technique. The parameters are the volume-extinction and absorption coefficients, the single-scattering albedo, the asymmetry parameter of the volume scattering function, the apparent complex refractive index, and the apparent soot content. They may serve as input data for solar radiation-budget considerations. Results from preliminary measurements taken in Central Europe and Italy show an extreme variability of the optical parameters. Both large regional and temporal variabilities have been observed caused by the fluctuating midlatitude weather systems and human activities.     

Optical imaging fiber-based single live cell arrays: a high-density cell assay platform

A high-density, ordered array containing thousands of microwells is fabricated on an optical imaging fiber. Each individually addressable microwell is used to accommodate a single living cell. A charged coupled device (CCD) detector is employed to monitor and spatially resolve the fluorescence signals obtained from each individual cell, allowing simultaneous monitoring of cellular responses of all the cells in the array using reporter genes (lacZ, EGFP, ECFP, DsRed) or fluorescent indicators. Yeast and bacteria cell arrays were fabricated and used to perform multiplexed cell assays with resolution at the single-cell level. Monitoring gene expression in single yeast cells carrying a two-hybrid system was used to detect in vivo protein-protein interactions. The single-cell array technology provides a new platform for monitoring the unique multiple responses of large populations of individual cells from different strains or cell lines. The rich data acquired by the cell array has the potential to be employed as a new tool for cell biology research as well as to improve cell-based high-throughput screening (HTS) applications, such as the validation of new disease-associated cellular targets and the early-stage evaluation of potential drug candidates.     

Optical properties of plasma ion-assisted deposition silicon coatings: application to the manufacture of blocking filters for the near-infrared region

I report on the preparation and characterization of optical constants of silicon coatings deposited by an electron beam gun with plasma ion-assisted deposition. With the fabrication of long-wave-pass filters the reliability of the optical constants is assured.     

Cast aluminium alloy containing dispersions of TiO2 and ZrO2 particles

Up to 4 wt% of ultrasonically-pretreated titania and zirconia particles (5m average size) were dispersed in Al-11.8 wt% Si-1 wt% Mg alloy castings. The technique employed to prepare the cast composites involved mechanically mixing pretreated oxide particles in aluminium alloy melts in the presence of magnesium, followed by casting the resulting mix in permanent moulds. Pretreatment of the oxide particles consisted of agitating the particles in acetone by ultrasonic vibrations to cause the disintegration of agglomerates and the removal of adsorbed water molecules from the particle surfaces. Infra-red (i.r.) spectroscopy of powders showed desorption of water molecules from oxide surfaces by ultrasonic pretreatment in acetone. Ultrasonic pretreatment of oxide particles before their incorporation into the melt and presence of magnesium in the melt were found to be necessary to disperse TiO₂ and ZrO₂ particles in molten Al. Microscopic studies of the composites indicated dispersed TiO₂ and ZrO₂ particles which exhibited a reaction zone at the periphery of the particles, and microprobe X-ray elemental analysis confirmed preferential segregation of magnesium at the particle-matrix interface. The composites exhibited an increase in hardness as a result of dispersions of titania and zirconia particles, while the tensile-strength values of composites were comparable with that of the base alloy.     

Optical properties of β-FeSi2 single crystals grown from solutions

We studied the optical absorption, polarized reflectance (PR) and photoluminescence (PL) of β-FeSi₂ single crystals grown from solution. In low-absorption measurements, we found a phonon emission and absorption structure, which suggests an indirect transition. The exciton energy gap of 0.814 eV was determined from the absorption spectrum at 3.5 K. We also found a direct transition with the gap energy of 0.939 eV. PR measurements for Ea, Eb and Ec revealed the anisotropy of reflectivity of β-FeSi₂. We observed the PL with a peak wavelength of about 1.56 μm at 20 K.     

Using solution thermodynamics to describe the dispersion of rod-like solutes: application to dispersions of carbon nanotubes in organic solvents

We report a simple model describing the solubility of rods in solvents, expressing the final result explicitly in terms of the surface entropy and the enthalpy of mixing. This model can be combined with any expression for the mixing enthalpy depending on the requirements. For example, in one instance it predicts the dispersed concentration of rods to decrease exponentially with the Flory-Huggins parameter of the dispersion. Using a different enthalpy function, it predicts a Gaussian peak when concentration is plotted versus solvent surface energy. The model also suggests specific solvent-rod interactions to be important and shows the dispersed concentration to be very sensitive to ordering at the solvent-rod interface. We have used this model to describe experimental results for the concentration of dispersed nanotubes in various solvents. Qualitative agreement with these predictions is observed experimentally. However, we suggest that the fact that quantitative agreement is not found may be explained by solvent ordering at the nanotube surface.