Diffusivity, bulk effective

It must be pointed out that in a diffusion layer where the ions are transported not only by migration but also by diffusion, the effective transport numbers t of the ions (the ratios between partial currents ij and total current t) will differ from the parameter tj [defined by Eq. (1.13)], which is the transport number of ion j in the bulk electrolyte, where concentration gradients and diffusional transport of substances are absent. In fact, in our case the effective transport number of the reacting ions in the diffusion layer is unity and that of the nonreacting ions is zero. 

The first reported study of a reaction of wood with an epoxide appears to be that of McMillan (1963). This involved the use of gaseous ethylene oxide (Figure 4.9, R=H) at a temperature of 93 °C and a pressure of 3 atmospheres (0.3 MPa). In some cases, the wood was diffusion pre-treated with trimethylamine vapour as the catalyst. A 65 % ASE at 20 % WPG was obtained, attributed to a bulking effect due to in situ polymerization of the epoxide. There was no effect on the static bending strength of samples, and the modified wood became distinctly brown at higher levels of treatment. 

Rose and Benjamin studied the water dipole and the water H-H vector reorientation dynamics at the water/Pt( 100) interface and the results are reproduced in Fig. 4. As in the case of the translational diffusion, the effect of the surface is to significantly slow down the adsorbed water layer. We note that the effect is very short range, and that the rotational motion of water molecules in the second layer is already very close to the one in bulk water. 

Molecular bulk diffusion coefficient Effective diffusivity... 

The best-known of the early developments addressing mass transfer effects on the BCF model was that of Chernov (1961), setting diffusion of solute through a boundary layer as the rate-limiting step. Other, more complex descriptions taking both surface and bulk diffusion into effect have been presented by Bennema (1969) and Gilmer et al. (1971). 

In addition to diffusion, bulk flow can occur within a reacting pellet (Beveridge and Goldie, 1968 Gower, 1971 Sohn and Sohn, 1980). This effect is considerably magnified for reactions with volume change such as... 

The first two mechanisms represent bulk effects, and the latter is related to the surface. The contribution of these mechanisms into the total diffraction efficiency depends on many parameters, including the extinction coefficient at a particular wavelength of a laser beam, the quantum yield of the photochemical isomerization, the temperature-dependent relaxation time of the cis-isomers, the angular diffusion, or even the film confinement conditions. To elucidate the mechanism of photorecording, and to optimize the conditions for readout of recorded data, different polarization geometries of the interfering pump beams can be chosen. ... 

Our discussion of the various types of diffusion has presumed that the diffusion takes place in a well-characterized pore stmcture, that is, straight cylindrical pores. However, the catalysts used in industry have extremely complex stmctures with interconnecting pores, tortuous pores, and wide variations in pore diameter as one moves along the length of the pore. Consequently, we need to convert the combined diffusivities that are appropriate for Knudsen and/or bulk diffusion into effective... 

It was explained earlier that condensation in the catalyst pores leads to loss of surface area and volume available for vapor-phase reaction and diffusion. This effect may be greater than anticipated from the liquid-filled fraction alone if isolated areas of vapor-filled pores become blocked off from the bulk gas at the catalyst surface by regions of condensed liquid. Such problems can be solved using the techniques of percolation theory [28,29]. 

Further analysis (de Gennes, 1976b) leads to a concept of hydrodynamic blobs of the size inside which the hydrodynamic interaction manifests itself, and a set of such blobs becomes independent both of bulk effects (subsection 3.1.1) and of the hydrodynamic interactions. That is the reason the diffusion coefficient D proves to be, in fact, the blob diffusion cocflicient Do. ... 

There are three forms of mass transport migration, diffusion and convection. Most of the current is carried by migration and diffusion. The effect of bulk chloride concentration on predicted time for onset of crevice corrosion is shown in Fig. 4.16. The prediction for onset is shortened with increased chloride concentration. 

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