VARIATION WITH PARTICLE SIZE

The aerosol composition distribution. Figure 3, shows pronounced variation with particle size. The distribution has a distinct break in the 0.1 to 0.3 um size range. The larger particles account for the vast majority of the aerosol mass. 

Similar plots of the data from higher temperature, nearly stoichiometric combustion. Figure 6, show substantially different trends. The refractory species, i.e., A1, Si, and Ca,show little variation with particle size. Comparison with Figure 5 suggests that this results from vaporization of small amounts of these species. The apparent decrease in the fine particle iron is caused by dilution with other major ash constitutents. Both sulfur and zinc concentrations increase as size decreases in spite of the larger amount of the other species in the fines. 

Roessler, D. M., and F. R. Faxvog, 1979b. Opacity of black smoke calculated variation with particle size and refractive index, Appl. Opt., 18, 1399-1403. 

Horizontal diffusion. As the cloud expands horizontally, the particle size distribution may vary as a function of distance from the center if the diffusion coefficient is a function of particle size. This would cause the true correction factor to vary with horizontal position, and should be observable if observations are made for horizontal positions at the same altitude and time. Since the diffusion coefficient for clouds is known only within a factor of 10-100, no estimate of the variation with particle size is possible at this time. 

The particle mobility B is defined as B = U. Generally, the particle velocity is given in terms of the product of the mobility and a force F acting externally on the particle, such as a force generated by an electrical field. Under such conditions, the particle motion is called quasi-stationary. That is, the fluid particle interactions are slow enough that the particle behaves as if it were in steady motion even if it is accelerated by external forces. Mobility is an important basic particle parameter its variation with particle size is shown in Table II along with other important parameters described later. 

Figure 49. Permeability of Ural-mountain Magnetite at Various Field Strengths. Variation with Particle Size.

It is possible to explain this variation with particle size if one considers in more detail the nature of the interaction of an ionic lattice with a dipolar molecule. The ion-dipole contribution to the interaction energy is linearly dependent on the electrostatic field strength. Any modification of crystalline to amorphous character should reduce the electrostatic field strength at the surface because of the more random arrangement of positive and negative ions. 

A quantitative estimation once again would require an estimation of the electrostatic field strength and its variation with particle size. This has been estimated previously [15], but since the electrostatic field strength is such a sensitive function of the distance normal to the surface, the calculation has little quantitative significance. 

The Nusselt numbers for cylinders and rings showed more variation with particle size, but the differences from the results for spheres were not enough to justify a separate correlation. 

Variation of Specific Surface Eneigy with Particle Size"... 

Figure 13 The variation of particle size with the Hansen solubility parameter of the n-alcohols. (Adapted from Ref. 85 with the permission of John Wiley Sons, Inc.)...

The other major problem concerned with sampling is that of the sample size. The size of the sample taken from a heterogeneous material is determined by the variation in particle size, and the precision needed in the results of the analysis. 

Marked changes occur in the visible appearance of dehydrated foods with variation in particle size. It has been found that this effect is chiefly one of variation in luminous reflectance, Y (see Tables I and II). In some instances (note the data for cabbage), chromaticity (x, y) remains so nearly constant over a fairly wide range of particle size that it appears possible that for certain products and purposes the effect of particle size might be eliminated by the choice of chromaticity as a color variable. 

Color Variations in Certain Dehydrated Foods with Variation of Particle Size... 

Table I. Variation of Apparent Moisture Content of Dehydrated Carrots with Particle Size and Time of Drying in Vacuum Oven at 70° C.a...

The question arises whether an internal standard can be relied upon to eliminate physical differences among samples, the Class II deviations of Section 7.8. No clear answer is possible. Variations in intensity ratios with particle size and with length of grinding time have been observed, especially in the analysis of minerals, but these effects seem due primarily to a nonuniform distribution of the internal standard, and not to particle size as such. These two possible causes of nonuniformity are difficult to separate. 

The results from the literature are not easy to interpret due to the limited temperature ranges, small variations in particle sizes and the occurrence of side reactions with chain branching. 

Except for support effects, structure sensitivity has usually appeared in one of two aspects, variation of rate with svirface crystal face or with particle size. In ICC 1 Gwathmey reported in one of the first experiments with single crystal faces that different faces machined fi om Ni single crystal spheres catalyzed the hydrogenation of ethylene at different rates (ICC 1 paper 5). 

An ensemble effect related to the variation of the number of sites of specihc configuration with particle size. Ensemble effects have been, for example, suggested to account for the decrease in Hupd coverage, and the S A in the MOR. 

As with particle size, particle shape can influence the compaction properties of solids. While work in this area is limited, some work has been reported. Ridgway and Scotton [17], for example, investigated the effect of particle shape on die-fill weight and found that more angular materials had a greater weight variation. Also, Rupp [18] investigated the effects of particle shape on tablet... 

Practical metal catalysts frequently consist of small metal particles on an oxide support. Suitable model systems can be prepared by growing small metal aggregates onto single crystal oxide films, a technique whereby the role of the particle size or of the support material may be studied. [37] A quite remarkable example of the variation of the catalytic activity with particle size has recently been found for finely dispersed Au on a Ti02 support, which was revealed to be highly reactive for combustion reactions. [38] On the basis of STM experiments it was concluded that this phenomenon has to be attributed to a quantum size effect determined by the thickness of the gold layers. 

Cunningham, P. T., S. A. Johnson, and R. T. Yang. Variations in chembtiy of airborne particulate material with particle size and time. Environ. Sci. Technol. 8 131-135, 1974. 

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