Dispersion formation

Dispersion Formation by Mechanical Agitation Under Positive Pressure... 

Mast, in a pioneering 1972 paper, reported visual observations of foam flow in etched glass micromodels (37 ) His observations showed that some of the conflicting claims about the properties of foam flow in porous media were probably due simply to the dominance of different mechanisms under the various conditions employed by the separate researchers (37). Mast observed most of the various mechanisms of dispersion formation, flow, and breakdown that are now believed to control the sweep control properties of surfactant-based mobility control (37,39-41). 

The next section introduces recent progress in the area of dispersion formation and flow. 

Dispersion Formation, Subdivision, and Coalescence. The ability to create and control dispersions at distances far from the injection well will be critical to the field-use of dispersion-based mobility control. The early studies of Bernard and Holm, followed by more recent work by Hirasaki, Falls, and co-workers, and others showed that the flow properties of surfactant-induced dispersions depend on the presence and composition of oil, volume ratio of the dispersed and continuous phases, capillary pressure, and capillary number (35,37,39-41,52-54,68). However, it is the size of the droplets or bubbles that dominates dispersion flow (39,68). Moreover, early debates on the ratio of droplet (or bubble) size to pore size have been resolved by ample evidence showing that, under commonly employed conditions, droplets are larger than pores (39). Only for very large capillary numbers (i.e., for interfacial tensions of ca. 

Films of wetting fluid that extend across pores and may cause dispersion formation are called lamellae. (See Figures 2 and 3.) Several mechanisms have been identified that collectively determine the number of lamallae and the distribution of droplet sizes of a dispersion in a porous medium. For noncondensible fluids they... 

A key factor in the commercialization of surfactant-based mobility control will be the ability to create and control dispersions at distances far from the injection well (TJ ). Capillary snap-off is often considered to be the most important mechanism for dispersion formation, because it is the only mechanism that can form dispersions directly when none are present (39,40). The only alternative to snap-off is either leave-behind, or else injection of a dispersion, followed by adequate rates of thread breakup and division to maintain the injected lamellae. 

There are three important ways by which capillary pressure affects the dynamics of dispersion formation and disappearance the lower and upper limits on the range of capillary numbers over which capillary snap-off can occur in homogenous media, and an upper limit on the capillary number above which lamellae are unstable and droplets quickly coalesce. 

It should be noted that the field tests were made with only one type of surfactant, and without benefit of many recent research advances in such areas as high-pressure phase behavior and surfactant design, mechanisms of dispersion formation and disappearance, and mechanisms of dispersion flow through porous media. Furthermore, the design and successful performance of field tests pose many technological challenges in addition to those encountered in the prerequisite experimental and theoretical research. 

More field tests will be needed, especially to incorporate research advances in such areas as materials design phase behavior and dispersion morphology mechanisms of dispersion formation, flow, and breakdown and simulation of dispersion-based sweep control. 

The enhancement in dissolution rate as a result of solid dispersion formation, relative to pure drug, varies from as high as 400-fold to less than twofold. Corrigan reviewed the current understanding of the mechanism of release from solid dispersions. The increase in dissolution rate for solid dispersions can be attributed to a number of factors. It is very difficult to show experimentally that any one particular factor is more important than another. The main reasons postulated for the observed improvements in dissolution of these systems are as follows ... 

Table 1 shows a list of materials used as carriers for solid dispersion formation. An excellent review of many of these carriers is included in an article by Ford. Enteric polymers are useful in the formation of solid dispersions of acid labile drugs. In some cases, a combination of carriers is more useful. 

Figure 1. Plot of the W/0/W-type dispersion formation against the weight ratio of Span 80 to Tween 20 in the final form of the samples. "Reproduced with permission from Ref. 1. Copyright 1976, Academic Press. "...

Food dispersion Formation, stability, and mechanical properties... 

Polydispersity coefficient ko (1.22) is one more important magnitude characterizing resulted emulsions. For monodisperse systems - kn = 1 and for polydisperse kn < 1, and the lesser the ko parameter value, the bigger the dispersal of particles by their sizes. The ratio Ls/dd practically doesn t influence on resulted emulsions polydispersity as in the case of dispersions formation with surface-volumetric diameter d32 (Fig. 3.33). Particles dispersal is increased when dd/dc ratio is raised and sufficiently homogeneous emulsions are formed in divergent-convergent canal of tubular apparatus with dd / d = 1,6. In particular for Ls / dd = 2-3 the value of ko at dd / dc = 1,6 is 0,72-0,75, where as at da / d = 2 and 3 kn is decreased down to 0,63 and 0,41 accordingly. 

The generation ofliquid-liquid dispersions, commonly used as emulsions, has many commonalities with the generation of bubble suspensions, but differences also exist, due to the different ranges of interfacial tensions, viscosities and densities between the two systems. Surfactants are often introduced to facilitate dispersion formation and reduce coalescence. 

Deviations of real molecules from the reference system may occur, e.g., due to attractive interactions (dispersion), formation of hydrogen bonds (association), or the nonspherical shape of the molecules (which can be understood as the formation of chains from spherical segments). These contributions are usually assumed to be independent of each other and are accounted for by different perturbation terms. Depending on the kind of considered perturbation and on the expression used for its description, different models have been developed. One of the first models derived from that idea was the Statistical-Associating-Fluid Theory (SAFT) (Chapman et al. [12, 13] Huang and Radosz [14, 15]). 

Section 12-6 provides a discussion of drop suspension, dispersion formation, and the interrelationships between dispersion, coalescence, and suspension. Additional... 

Rod, V., and T. Misek (1982). Stochastic modeling of dispersion formation in agitated liqnid-liqnid systons, Trans. Inst. Chem. Eng., 60(1), 48-53. 

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