Diffusion to surface

In general, homogeneous nucleation of nanoparticles occurs when a solute (C) diffuses to surface of a cluster from a bulk solution, and then incorporates into the cluster through surface reaction until a nucleus (C ) is obtained ... 

Number of molecules diffusing to surface per second = 47rDg Ag/(l + A KJ. (XX)... 

Deposition other than in rain is termed dry deposition, and this includes sedimentation of particles, molecular and Brownian diffusion to surfaces, impaction on roughness elements and deposition under electrical or thermophoretic forces. The velocity of deposition is defined... 

Figure 11-2 Forced axial convection vdfii diffusion to surface.

The calculations have shown that in SixHvPJ clusters H atoms initially located in near-surface layers diffuse to surface forming covalent bonds with some surface Si atoms and stabilizing them. Surface atoms relax to a cluster bulk. The H atoms which are in deeper layers are located in interstitial positions. Initial amorphization of silicon layers under action of H atoms is observed. 

Rb, or Cs, is only possible in part. They can be described in terms of adsorption on a substrate whose first layer contains some fraction of a monolayer of vacancies, just as in the case of the single-layer surface alloys. These vacancies are occupied with a first layer of Na. A second layer of Na (or K, Rb, or Cs in the case of the ternary alloys) is then adsorbed on the A1 vacancy layer. However, in the case of the Al(lll)—(2 x 2) multilayer surface alloys, this second layer of Na is adsorbed in fee sites on the A1 vacancy layer, and the A1 atoms ejected from the vacancy layer are readsorbed in hep sites on the vacancy layer. By contrast, in the Al(l 10)—(4 x 1)—3Na multilayer surface alloy, the second Na layer is adsorbed in sites of low symmetry on the vacancy layer, and the A1 atoms ejected from the vacancy layer do not form a part of the structure, but presumably diffuse to surface steps where they are readsorbed. 

This is the usual boundary condition for molecular diffusion to surfaces in gases and liquids for a perfectly ab.sorbing surface. Hence the results of experiment and theory for molecular diffusion in the absence of a force field can often be directly applied to particle diffusion. However, the effect of finite particle size is very imporiaiU when diffusion boundary layers are present as discussed in the next chapter. 

Gas-diffusion to surfaces is mainly dependent on reactor configuration (geometry) and is to be taken under considerations when the membrane architecture includes a porous layer (grain size, porosity, interconnection shapes and dimensions,...). 

Despite the large difference in the apparent diffusivities derived from uptake rate measurements, measurements of the intracrystalline self-diffusivity (for C2H6-5A) by the NMR (PFG) method show little difference between the large laboratory synthesized crystals and the small Linde crystals, thus favoring the surface barrier hypothesis. The increase in activation energy, which is observed for this system on severe hydrothermal pretreatment of the smaller crystals, is also consistent with a change from intracrystalline diffusion to surface barrier control. Detailed sorption rate studies with the system n-butane-5A, however, support the opposite conclusion that the differences in... 

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

Be Diffusion-to-surface-exchange coefficient defined according to Eqs. (14.87)-(14.88). 

The substance dissolves in the solvent and diffuses to surface layer and pass... 

FU DIFFUSES TO SURFACE OF POLYMER SLOWER THAN STEP 1, BUT FAST COMPARED... 

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