Foam texture

Barring direct measurement of foam texture, we adopt the following reasoning. Because of the generation of foam bubbles by the snap-off and division mechanisms (4), bubble sizes are expected to be approximately that of pore bodies. Thus, the linear bubble density should scale roughly as n 6/Dwhere... 

Many food products (salad dressings, whipped toppings, ice cream etc.) are dispersed colloid systems, such as emulsions, suspensions or foams. Texture, structure and stability of these dispersions have fundamental importance for the food manufacturer. Our chapter presents new methods, most of them developed in our laboratory, and mechanisms which can be very helpful for the food researcher or developer. 

Foam characteristics in several commercial beers were evaluated using one-dimensional NMR image. Foam texture differences and regions of cling, head, and liquid beer can be clearly distinguished. Significant differences in rates of foam collapse were measured [22]. 

Any foam for which the length scale of the confining space is greater than the length scale of the foam bubbles. The converse case categorizes some foams in porous media, distinguished by the term lamellar foam . See also Foam, Foam Texture. 

One of the first applications of biodegradable materials is based on the cooked, extruded, and expanded starch known from the food and chemical sectors (Fig. 14.23). Starch is cooked with water in the extruder and chemically modified as necessary or mixed with plasticizers, then expanded to a starch foam and dried. The extrudate is ground so that the functional properties thus created can be used in the food/chemicals sector. The foamed, cut, and dried extrudate is the end product for loose-fill packaging applications. The degree of expansion is a measure of the foam texture. It increases strongly with product temperature at the die, helped by a higher specific mechanical energy input. However, both measures increase the water-solubility of the product. 

Foam mobility has been proven to be strongly dependent also on bubble size and bubble distribution by size (foam texture) [162,163]. The latter is affected by the dispersion technique used, solution concentration, etc. (see Chapter 1). 

Gas mobility in the presence of a foam is dominated by foam texture (bubble size) [171]. The strong fall in permeability in the presence of a foam is a result of foam trapping established not only in the macroscopic studies but by the direct observations of transparent micromodels [153,158] as well. Foam trapping is a batch process the immobile foam can become mobile with time and vice versa [158]. 

Extensive mobility control applications of foams are limited by inadequate knowledge of foam displacement in porous media, plus uncertainties in the control of foam injection. Because of the importance of in situ foam texture (bubble size, bubble size distribution, bubble train length, etc.), conventional fractional flow approaches where the phase mobilities are represented in terms of phase saturations are not sufficient. As yet, an adequate description of foam displacement mechanisms and behavior is lacking, as well as a basis for understanding the various, often contradictory, macroscopic core flood observations. 

In the present paper, pore level descriptions of bubble and bubble train displacement in simple constricted geometries are used in developing mobility expressions for foam flow in porous media. Such expressions provide a basis for understanding many of the previous core flood observations and for evaluating the importance of foam texture and interfacial mobility. Inclusion of the effects of pore constrictions represents an extension of the earlier efforts of Hirasaki and Lawson (1). 

In comparison. Equation 17 indicates that the total contact length Lf of the films is the important parameter for immobile interface systems. Foaming systems of this type would show no dependence on foam texture (bubble size and bubble size distribution), but would exhibit very large apparent viscosities in porous media if the bubble trains were sufficiently long. 

Equation 53 has the same general form as Equations 44 and 49. At this point, K3 and n must be considered as empirical parameters which will be dependent on the in situ foam texture, gas saturation, and the porous medium structure. 

Although the current permeability model properly reflects many of the important features of foam displacement, the authors acknowledge its limitations in several respects. First, the open pore, constricted tube, network model is an oversimplification of true 3-D porous structures. Even though communication was allowed between adjacent pore channels, the dissipation associated with transverse motions was not considered. Further, the actual local displacement events are highly transient with the bubble trains moving in channels considerably more complex than those used here. Also, the foam texture has been taken as fixed the important effects of gas and liquid rates, displacement history, pore structure, and foam stability on in situ foam texture were not considered. Finally, the use of the permeability model for quantitative predictions would require the apriori specification of fc, the fraction of Da channels containing flowing foam, which at present is not possible. Obviously, such limitations and factors must be addressed in future studies if a more complete description of foam flow and displacement is to be realized. 

Numerous studies (2-12) have shown that the behavior of foams flowing in porous media depends on a host of variables, e.g., capillary pressure, capillary number, foam quality, presence of oil, and composition of oil, but is dominated by foam texture (5,6)... 

Foam (5) is a collection of gas bubbles with sizes ranging from microscopic to infinite for a continuous gas path. These bubbles are dispersed in a connected liquid phase and separated either by lamellae, thin liquid films, or by liquid slugs. The average bubble density, related to foam texture, most strongly influences gas mobility. Bubbles can be created or divided in pore necks by capillary snap-off, and they can also divide upon entering pore branchings (5). Moreover, the bubbles can coalesce due to instability of lamellae or change size because of diffusion, evaporation, or condensation (5,8). Often, only a fraction of foam flows as some gas flow is blocked by stationary lamellae (4). 

Mechanistic prediction of foam flow in porous media seems to be impossible without a transport equation governing foam texture, i.e., foam bubble size. 

The continuum form of the bubble population balance, applicable to flow of foams in porous media, can be obtained by volume averaging. Bubble generation, coalescence, mobilization, trapping, condensation, and evaporation are accounted for in the volume averaged transport equations of the flowing and stationary foam texture. 

Caramel with a foam texture can be obtained by adding ready-made stable foam (Frapp6) to the mixture. Alternatively the so-called pulling operation is applied. 

In this chapter, we discuss much of the work accomplished since Fried, but without attempting a complete review. Useful synopses are available in the articles and reports of Hirasaki (2, 3), Marsden (4), Heller and Kuntamukkula (5), Baghidikian and Handy (6), and Rossen (7). Our goals are to present a unified perspective of foam flow in porous media to delineate important pore-level foam generation, coalescence, and transport mechanisms and to propose a readily applicable one-dimensional mechanistic model for transient foam displacement based upon gas-bubble size evolution [i.e., bubble or lamella population-balance (8, 9)]. Because foam microstructure or texture (i.e., the size of individual foam bubbles) has important effects on flow phenomena in porous media, it is mandatory that foam texture be accounted for in understanding foam transport. 

In equation 4, the subscripts f and t refer to flowing and trapped foam, respectively, and ni is the foam texture or bubble number density. Thus, nf and t are, respectively, the number of foam bubbles per unit volume of flowing and stationary gas. The total gas saturation is given by Sg = 1 — Sw = S + St, and Qb is a source—sink term for foam bubbles in units of number per unit volume per unit time. The first term of the time derivative is the rate at which flowing foam texture becomes finer or coarser per unit rock volume, and the second is the net rate at which foam bubbles trap. The spatial term tracks the convection of foam bubbles. The usefulness of a foam bubble population-balance, in large part, revolves around the convection of gas and aqueous phases. 

Falls et al. (9) and Friedmann et al. (39) pointed out that if a lamella arrives at a germination site prior to the total elapsed time for snap-off, then snap-off is precluded. An upper limit is then placed on the evolution of foam texture. We find in our systems that strong coalescence forces come into play before this upper limit is attained. 

It is readily argued that the intersection of coalescence and generation rates in Figure 11 leads to a stable steady state. If the system is perturbed away from this point, it naturally returns. Consider a small, positive perturbation in the local liquid saturation. The coalescence rate then declines, and the foam texture becomes finer. This change causes an increased flow resistance that returns the liquid saturation back to the stable operating point. The converse negative saturation perturbation is similarly argued to be stable. 

Figure 14 reports the calculated transient foam bubble density, np as a function of dimensionless distance. At all time levels, foam bubbles are coarsely textured near the inlet, but within the first fifth of the core, texture becomes much finer. Beyond the first fifth of the core, the limiting capillary-pressure regime develops foam texture in this region is nearly constant as is the liquid saturation in Figure 12. Foam texture also increases rapidly with respect to time. At 0.23 PV, foam bubble density im-...

Texture. Foam texture is an important parameter that affects the rheology of the foam fluid. Texture of a foam is a means of classifying a foam according to its bubble size, shape, and distribution within the foam matrix. Texture is a description of the manner in which the gas bubbles are distributed throughout the liquid phase of the foam. This property not only influences the foam s rheology but also its fluid loss, proppant transport, and cleanup properties. The texture of a foam is a qualitative rather than a quantitative value, and therefore a number cannot be used to describe it a physical description will be used. Factors that effect the texture of foams are quality, pressure, foam generating technique, and chemical composition. 

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