Diffusion-controlled reactions among stationary sinks

We summarize results of some of our calculations in diffusion-controlled reaction theory. We derive the transport equation describing a diffusing species which can react with a set of randomly distributed spherical sinks. Both the form of the transport equation and the dependence on sink volume fraction of the reaction rate and the effective diffusion coefficient are discussed.Presented at the Symposium on Random Walks, Gaithersburg, MD, June 1982.Supported in part by the National Science Foundation under Grant No. CHE-78-07849.     

Polarization-dependent gain and saturation energy density in a TE N2-laser amplifier

The polarization-dependent gain, g₀, and saturation energy density, E_s, in a TE N₂-laser amplifier were measured, using an oscillator-amplifier laser system for different amplifier electrode gap separations, d_AMP, of 7, 9 and 4 mm and gas pressure of p = 77, 60, and 165 Torr, respectively. It was realized that for the amplifier with the gap separation of 7 and 9 mm, where the pd_AMP-value has its optimum-value of 54 Torr cm, the gain-coefficient for the input beam with the polarization parallel to the discharge electrodes (P-polarized beam) is slightly higher than the case when the beam polarization is perpendicular to the discharge electrodes (S-polarized beam). In this case, the depolarization ratio for d_AMP = 7 mm is the range of ∼0.998 to ∼0.962 as the input voltage increases from 12 to 15.5 kV, having a minimum of 0.937 around 14 kV. For the E_s-parameter, the reversed order is true. Also, it was found that the saturation energy densities for three states of polarization are linearly related to the output energy densities, having different slopes of 0.11, 0.14, and 0.17 for R (randomly), P- and S-polarization, respectively. The present measurement supports qualitatively the prediction of polarization-inhomogeneity model for the stimulated emission cross-section, showing that randomly oriented dipoles exhibit slightly larger gain on the direction of the electric field.     

Refractive index design and validation for evanescent field power maximization in optical fibre sensors

The viability and operation of evanescent field-based optical fibre sensors is largely determined by the fraction of the total supported modal field power in the evanescent field. As this fraction is highly dependent on the refractive index profile of the fibre, one design strategy for fibre sensors is to maximize this field power fraction over the class of all refractive index profiles. This paper documents this design strategy for circular geometry optical fibres, where the goal is to maximize the evanescent field power fraction for a particular mode via the selection of an optimal refractive index profile. The axially symmetric profiles obtained approximate “holey” annular structures, the performance of which can be validated using existing waveguide analysis techniques.     

Influence of surface treatment on the electroless nickel plating of textile fabric

The present study is performed with an objective to acquire a deeper understanding of the properties of nickel-plated polyester fabric after conducing low temperature plasma treatment. Low temperature plasma treatment with oxygen and argon gases was employed to render a hydrophilic property of woven polyester fabrics and facilitate the absorption of a palladium catalyst in order to provide a catalytic surface for electroless nickel plating. The properties of plasma-induced electroless nickel-plated polyester fabrics were evaluated by various standard testing methods in terms of both physical and chemical performances.     

Fibre optic target reflectivity sensor

This paper reports the principle of operation, the design aspects, experimentation and performance of a fibre optic target reflectivity sensor to examine the correlation between the detector output, variation in material type and the reflectivity properties of the materials tested. The device consists of a fibre optic transmitter, a fibre optic probe, target and a photodiode detector. The fibre optic probe consists of two well-polished PMMA (polymethyl methacrylate) fibres cemented together along some distance over the length. The principle of fibre optic lever displacement sensors is applied. Material effects are examined by preparing a variety of samples namely gold coated mirror, copper, brass, aluminium, steel and galvanized iron using the same polishing techniques. It is found that the response of the sensor changes with change of target surface. The results show that the fibre optic probe is capable of discriminating between materials. With the use of commercially available fibre, source and detector, the set-up proves to be simple, highly sensitive, low cost and versatile one, which can be adopted for on-line measurement or inspection of test components.     

On the theory of rheological properties of magnetic suspensions

The paper deals with a theoretical study of influence of magnetic field on effective viscosity of suspension of non-Brownian magnetizable particles. It is supposed that experimentally observed magnetorheological effects are provided by chain-like aggregates, consisting of the particles. Unlike previous works on this subject, we take into account that the chains cannot be identical and estimate their size distribution. The following power law (η-η₀)/η₀∼Mn^-Δ, detected in many experiments, is obtained theoretically (η and η₀ are the suspension effective viscosity and the carrier liquid viscosity, respectively, Mn is the so-called Mason number, proportional to the shear rate and inversely proportional to the square of magnetic field). The calculated magnitude of the exponent Δ increases with the applied magnetic field from approximately 0.66 to 0.8-0.9 and slowly increases with the volume concentration ? of the particles. These results are in agreement with known experiments.     

A Survey of Complementary Methods for the Characterization of Dense Colloidal Silica

The aim of this study is to enhance existing knowledge of different techniques developed for the characterization of stability and particle sizing of nanoparticles in dense dispersions subjected to interparticulate and hydrodynamic forces. Silica suspension, commercially known as Klebosol^® 30R50 and consisting of a particle size of 80 nm on average, was investigated in the study over a wide range of concentrations. The investigations were carried out using different optical and acoustic techniques such as laser diffraction, multiple light scattering, photon correlation spectroscopy and acoustic spectroscopy. The study details the capabilities and limitations of these modern techniques based on the different physical principles behind the characterization of the size distribution of particles in suspensions. The results are presented in terms of particle size ranges, solid concentration and technological aspects such as online and offline analysis. An important finding is that many of these modern techniques need to be improved for applications at higher concentrations since the standard models become practically invalid because of the complex interaction of acoustic and optical waves with particles in suspensions of silica.     

Transfer of a microfluid to a stochastic fibre network

The transfer of a microscopic fluid droplet from a flat surface to a deformable stochastic fibre network is investigated. Fibre networks are generated with different levels of surface roughness, and a two-dimensional, two-phase fluid-structure model is used to simulate the fluid transfer. In simulations, the Navier-Stokes equations and the Cahn-Hilliard phase-field equations are coupled to explicitly include contact line dynamics and free surface dynamics. The compressing fibre network is modelled as moving immersed boundaries. The simulations show that the amount of transferred fluid is approximately proportional to the contact area between the fluid and the fibre network. However, areas where the fluid bridges and never actually makes contact with the substrate must be subtracted.     

Flame retardant challenges for textiles and fibres: New chemistry versus innovatory solutions

Almost 50 years ago, the 1950-1960 period witnessed the development of the chemistry underlying most of today’s successful and durable flame retardant treatments for fibres and textiles. In today’s more critical markets in terms of environmental sustainability, chemical toxicological acceptability, performance and cost, many of these are now being questioned. “Are there potential replacements for established, durable formaldehyde-based flame retardants such as those based on tetrakis (hydroxylmethyl) phosphonium salt and alkyl-substituted, N-methylol phosphonopropionamide chemistries for cellulosic textiles?” is an often-asked question. “Can we produce char-forming polyester flame retardants?” and “Can we really produce effective halogen-free replacements for coatings and back-coated textiles?” are others.These questions are addressed initially as a historical review of research undertaken in the second half of the twentieth century which is the basis of most currently available, commercialised flame retardant fibres and textiles. Research reported during the first decade of the twenty first century and which primarily addresses the current issues of environmental sustainability and the search for alternative flame retardant solutions, the need to increase char-forming character in synthetic fibres and the current interest in nanotechnology is critically discussed. The possible roles of micro- and nano-surface treatments of fibre surfaces and their development using techniques such as plasma technology are also reviewed.