Solute Diffusion Process Research Articles

The disturbance effect of a circular cylinder on the diffusion of matter by a plume from a point source in grid-generated turbulence. 1st report Characteristics of velocity and mean concentration field.

The effect of the disturbance of a circular cylinder on the diffusion process of dye solution by a non-buoyant plume from a point source in grid-generated water turbulence has been examined experimentally. The cylinder is fixed so that its axis intersects the plume axis perpendicularly at the far downstream location from a source where the plume has developed to have a comparable width with the cylinder diameter. In this paper, the experimental results concerning the mean concentration field are presented. It is found that the vertical profiles of the mean concentration behind the cylinder show flatter shapes, and the profile's half width grows downstream more rapidly than those without the cylinder. These characteristics are apparently caused by the disturbance effect of the cylinder. The vertical and horizontal profiles of the mean concentration appear to be nearly similar at various downstream locations beyond about 20 diameters from the cylinder.

Pattern formation during the directional solidification of binary systems

Interface pattern formation during the directional solidification of succinonitrile-acetone system is studied in situ and the dynamics of the pattern formation are presented for the cases in which an initially unstable planar interface reorganizes into a stable cellular or dendritic shape. Two different mechanisms of pattern formation have been shown to exist. One of these mechanisms leads to the cellular pattern, whereas the other to the dendritic pattern. The wave vectors of the transient patterns have been found to lie within the unstable band predicted by the linear stability analysis. However, only two initial characteristic wave vectors are observed experimentally and the values of these observed wave vectors are significantly smaller than the theoretically predicted value which gives the largest amplification rate. These two characteristic wavelengths have been shown to exhibit simple scaling laws with all steady-state dendrite lengths, i.e. dendrite tip radius, primary spacing, initial secondary spacing and the first sidebranch perturbation location. The magnitudes of the smaller and the larger characteristic wavelengths are shown to be controlled by the solute and thermal diffusion process, respectively.

Modelling of the pervaporation of binary mixtures through moderately swelling, non-reacting membranes

The pervaporation of binary mixtures through moderately swelling membranes is analyzed as a solution—diffusion process, on the assumption that the diffusion coefficient of each permeant is an exponential function of both concentrations. A model is derived in which changes in selectivity and fluxes are related to major external conditions: the upstream mole fraction in the feed and the downstream total pressure of the pervaporate. This model leads to the computation of the pervaporation selectivity and the fluxes as a function of the parameters of the model. Numerical examples are given, relative to different shapes of curves for selectivity and fluxes. A flowsheet is given and allows, through the comparison with experimental results, the determination of the parameters of the model. Internal concentration profiles in the membrane may also be computed. In order to test the validity of this model, it has been applied to a set of experimental data for the pervaporation of hydrocarbon binary mixtures through nitrile—butadiene and styrene—butadiene copolymers.

37] Membrane systems: Theoretical aspects

This chapter discusses two types of membrane systems: membrane-coated reservoirs and monolithic matrices. These two systems involve diffusion of drug through a polymer membrane. Principles of diffusion in polymers are presented. The most important type of membrane system for controlled release consists of a nonporous, homogeneous polymer. Transport through membranes occurs by a solution-diffusion process in which the solute (drug) dissolves in the membrane and diffuses through the membrane in the direction of lower solute concentration. Membrane-coated reservoirs consist of a reservoir of active ingredient (drug) enclosed by a polymeric membrane. Drug release occurs by diffusion from the reservoir through the membrane to the external medium. A monolithic system consists of a polymeric matrix containing a uniformly dispersed or dissolved drug. Drug release occurs by diffusion through the polymeric matrix to the surface of the device.

A study on the diffusion of matter from a continuous point source in uniform mean shear flows (2nd report, Characteristics of the concentration fluctuation intensity)

The turbulent diffusion process of dye solution has been examined experimentally in the case of a continuous point source in the water turbulent flows with three different uniform mean shear rates. In this paper, the experimental results of the concentration fluctuatin intensity are investigated. Already, the characteristics of the mean concentration field have been reported in ref. (1). The data show that the horizontal and vertical profiles of the r.m.s. values of the concentraiton fluctuation appear nearly similar and especially, vertical profiles skew to the side with the smaller flow velocity. The above characteristics are the same as the results of the mean concentration field. Further, by using the random rectangular wave model for the concentration fluctuating signal, an analysis for the concentration fluctuation intensity is made. The caluculated profiles expressed quite well the experimental results.

Effect of the morphology of hydrophilic polymeric matrices on the diffusion and release of water soluble drugs

Drug release mechanisms from, and diffusion processes in, hydrophilic crosslinked polymeric systems were investigated in two macromolecular states: in the glassy and rubbery states during the early part of countercurrent water diffusion, and in the rubbery state after thermodynamic equilibrium between the network and the surrounding dissolution medium (water) was attained. Dilute, aqueous poly(vinyl alcohol) (PVA) solutions containing theophylline were crosslinked with glutaraldehyde. The crosslinking ratio, X, varied between 0.01 and 0.20 moles glutaraldehyde per mole of PVA repeating unit. Theophylline release from these rubbery matrices was followed as a function of time. It was determined that, within the range of crosslinking ratios studied, the crosslinked macromolecular structure affected the solute diffusion process. Theophylline release from crosslinked PVA slabs, which were originally dehydrated at 30°C, was also measured. The drug release process was significantly impeded in these systems, especially for samples with crosslinking ratio X ≥ 0.10. This behavior was explained in terms of relaxation of the macromolecular chains and possible existence of ordered chain structures. Glass-to-rubber transitions, a result of the countercurrent diffusion of water into the originally dried (glassy) polymer, shifted the fractional release of theophylline from a f( t 1/2) to a f( t n) time dependence, with n taking values between 0.50 and 0.76. This type of release behavior indicates anomalous diffusion mechanisms. These results may be helpful in the development of swelling controlled drug delivery systems.

Surface diffusion and spin polarization: Two dimensional CIDEP

An asymptotic Green’s function method that proved successful for spin polarization–diffusion processes in bulk solution is extended to similar processes on surfaces. An approximate solution indicates that the dependence on the exchange interaction is similar in two and three dimensional processes, the dependence on time and magnetic interactions, dissimilar. The latter difference is attributed to a fundamentally different feature of two dimensional diffusion-controlled processes: two particles undergoing random walk motion that have collided once are certain to collide again.

Approximate solution to the one carrier problem of a semiconductor-electrolyte system

The dependence upon voltage of the current through a redox electrolyte-insulator interface is considered. Marcus' theory of thermal activation processes in solution is employed to estimate the rate at equilibrium of the electron transfer process between the electrolyte and the solid. Hence a boundary condition at the interface is derived, and incorporated into the boundary value problem for charge carriers within the insulator. Approximate solutions to this problem are derived for Schottky barrier, and perfect insulator conditions; regions are delineated in which the current is controlled by processes occurring in each of the solid, the interface, and the electrolyte. Current limitation at high overvoltage may be traced either to diffusion processes in solution as at a metal electrode, or to reorientation of the solvation shell of a reactant such that its electronic level becomes isoenergetic with that of the appropriate band in the surface of the solid.