Void size distribution

In another exemplary study, optical microscopy revealed that the void content of resole networks ranged from 0.13 to 0.21.82 Resole networks prepared from different F/P molar ratios showed comparable void size distributions. A bimodal distribution was observed for all networks, which was attributed to... 

The void size distribution is also altered and, as shown in Fig. 3.25 [30], the vibration mainly removes the larger diameter voids. 

Fig. 3.25 Change in void size distribution on vibration of air-entrained concrete (Johansen). 

Losses of entrained air on vibrating placed concrete can be considerable, and typical results are shown in Fig. 3.24 [30], The void size distribution is also altered and, as shown in Fig. 3.25 [30], the vibration mainly removes the larger diameter voids. 

The analysis of the adsorption branch of the isotherm permits one to obtain the void-size distribution (6). 

One can also use small-angle x-ray scattering to measure microvoids. The intensity of the diffuse scattering is measured as a function of the angle between the scattered and incident beams, and this intensity is then converted into the void size distribution by use of the Gunier method of analysis [267]. This method appears to be suitable for only the very small pore range (less than 200 A in radius) and is more useful in determining the existence and shape of the pores rather than the size and number [266]. 

Another way to obtain the desired dense-coagulated fiber structure is to add an ionic comonomer to the chain. Terada [269] has used mercury porosimetry to follow the change in the microvoid size distribution that occurs when comonomers of differing polarity and ionic structure are added to PAN. When hydrophobic comonomers such as methyl acrylate (MA) are used, the peak of the pore size distribution is shifted in the direction of increased size. However, ionic comonomers such as SSS or sodium allyl sulfonate (SAS) shift the peak in the direction of decreased size. With a three-component composition containing both the hydro-phobic and ionic comonomers, as in poly(AN-MA-SAS), the distribution has both characteristics. Examples of void size distributions are shown in Figure 12.27. The 5% MA comonomer has a fairly sharp peak near 850 A and the maximum size is extended to 200 A. Moreover, there is only a slight proportion of pores with small sizes present. 

FIGURE 12.27 Void -size distribution in freeze-dried uncollapsed and unoriented fibers as determined by mercury porosimetry. The weight fraction of comonomers methyl acrylate (MA) and sodium styrene sulfonate (SSS) are in parentheses. (From Terada, K., Sen i-Gakkaishi 29,12, T451, 1973.)... 

Qn distribution, RING SIZE DISTRIBUTION, AND VOID SIZE DISTRIBUTION... 

G. Malavasi, M.C. Menziani, A. Pedone, U. Segre, Void size distribution in MD-modelled silica glass structures. J. Non-Cryst. Solids 352, 285-296 (2006)... 

Many commercially important processes involve the transport of fluids through porous media and the displacement of one fluid, already in the media, by another. The role play by pore stnicture is of fundamental importance, and its size distribution determination necess. in order to obtain an understanding of the processes. The quality of powder compacts is also affected by the void size distribution between the constituent particles. For these reasons mercury porosimetry has long been used as an experimental technique for the characterization of pore and void structure. Although quantitative information is contained in mercury intrusion - extrusion curves it can only be elucidated fiilly by the use of a theoretical model for pore structure. 

The relation between the dusty gas model and the physical structure of a real porous medium is rather obscure. Since the dusty gas model does not even contain any explicit representation of the void fraction, it certainly cannot be adjusted to reflect features of the pore size distributions of different porous media. For example, porous catalysts often show a strongly bimodal pore size distribution, and their flux relations might be expected to reflect this, but the dusty gas model can respond only to changes in the... 

For large amounts of fillers, the maximum theoretical loading with known filler particle size distributions can be estimated. This method (8) assumes efficient packing, ie, the voids between particles are occupied by smaller particles and the voids between the smaller particles are occupied by stiH smaller particles. Thus a very wide filler psd results in a minimum void volume or maximum packing. To get from maximum packing to maximum loading, it is only necessary to express the maximum loading in terms of the minimum amount of binder that fills the interstitial voids and becomes adsorbed on the surface of the filler. 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

It is found that the viscosity of a paste made from a fixed polymer/plasticiser ratio depends to a great extent on the particle size and size distribution. In essence, in order to obtain a low-viscosity paste, the less the amount of plasticiser required to fill the voids between particles the better. Any additional plasticiser present is then available to act as a lubricant for the particles, facilitating their general mobility in suspension. Thus in general a paste polymer in which the pastes have a wide particle size distribution (but within the limit set by problems of plasticiser absorption and settling out, so that particles pack efficiently, will... 

Even the void fraction together with particle size distribution does not provide all of the necessary information on the kind of flow. The mutual forces between distinct particles depend not only on the distance between the particles but also on the surface properties of the particles. The strength of the attractive forces between particles depends on conditions. For instance, the moisture content of the solid is essential for determining the attractiv c forces between particles, especially for hydroscopic materials such as wood. Airflow between particles usually tends to separate particles, whereas the surface forces, adhesion forces, tend to bring them together. 

The porous materials that offer the narrowest possible pore size distribution are those that have cylindrical pores of uniform diameter penetrating the entire medium without branching. Branching gives polymer molecules in the junctions extra conformational entropy. An agglomerate of tiny pieces of these porous materials, interlaced with larger voids (much larger than the pore size), should also be chosen. 

Size exclusion chromatography has been used to analyse the size distribution of liposomes. For example, SUV can be separated from MLV, which elute in the void volume, by using a Sepharose 4B gel. 

Hydraulic conductivity is one of the characteristic properties of a soil relating to water flow. The movement of water in soil depends on the soil structure, in particular its porosity and pore size distribution. A soil containing more void space usually has a higher permeability. Most consolidated bedrocks are low in permeability. However, rock fractures could create a path for water movement. 

W-3 CHF correlation. The insight into CHF mechanism obtained from visual observations and from macroscopic analyses of the individual effect of p, G, and X revealed that the local p-G-X effects are coupled in affecting the flow pattern and thence the CHF. The system pressure determines the saturation temperature and its associated thermal properties. Coupled with local enthalpy, it provides the local subcooling for bubble condensation or the latent heat (Hfg) for bubble formation. The saturation properties (viscosity and surface tension) affect the bubble size, bubble buoyancy, and the local void fraction distribution in a flow pattern. The local enthalpy couples with mass flux at a certain pressure determines the void slip ratio and coolant mixing. They, in turn, affect the bubble-layer thickness in a low-enthalpy bubbly flow or the liquid droplet entrainment in a high-enthalpy annular flow. 

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