Foam volume structure

Rinkes [52] has proposed a way to determine the average bubble size which involves moving the foam through a narrow tube with a cross-section comparable to bubble size. Thus, the foam structure is transformed , so that individual foam films separated by gas volume pass through the tube. By determining the number of such films Nf in a unit foam volume, the average (by volume) bubble radius can be calculated... 

A rapid change in the foam structure, related to the film reorientation in the process of internal foam collapse has been reported in [11,12], The method employed was of a multiple light scattering. The rate of structural changes in a unit foam volume is proportional to f2 (after the first 20 min). The foam studied was a shaving cream (Gillette Foamy). 

However, the VL F value for a spherical foam is substantial and amounts to 48% of the total foam volume for a simple cubic packing and to 26% for a hexagonal close packing. For the latter the foam expansion ratio varies from 2.01 to 3.85 which may introduce large errors into the calculation of the //IIlln value. In a polyhedral foam the liquid volume can be neglected with respect to the foam volume but for the determination of //min( ) more detailed information on the structure of the foam is needed. 

A variety of foams can be produced from various types of polyethylenes and cross-linked systems having a very wide range of physical properties, and foams can be tailor-made to a specific application. Polypropylene has a higher thermostability than polyethylene. The production volume of polyolefin foams is not as high as that of polystyrene, polyurethane, or PVC foams. This is due to the higher cost of production and some technical difficulties in the production of polyolefin foams. The structural foam injection molding process, described previously for polystyrene, is also used for polyethylene and polypropylene structural foams (see Figure 2.61). 

Surfactants play an important role in the formation and stability of foams. Investigators have determined foam stability by measuring the half-life (e.g. t 2) the foam. Half-life is the time required to reduce foam voLume to half of its initial value. It has been demonstrated that the foam stability (i.e.half-life) decreased with increasing temperature, whereas the foaminess of the surfactant solution increased with temperature. It is likely that these properties of foam depend on the molecular structure and concentration of the surfactant at the gas/liquid interface. Comparison of the results of static foam stability with that of the dynamic behavior of foam in porous media revealed that the foam stability is not required for efficient fluid displacement or a decrease in the effective air mc >ility in a porous medium. Moreover, the ability of the surfactants to produce in-situ foam was one of the important factors in the displacement of the fluid in a porous medium. 

Choosing the right surfactant or surfactant combination that can deliver a desired foam profile is often done on a trial-and-error basis. As mentioned previously, often the surfactant structure that improves foam volume is not the same as that which contributes to foam stability. Some general guidelines about surfactant structure and optimum foam performance are listed as follows ... 

An example of the bulk volume structure of foam-dried particles (e.g., maltodextrin/sodium caseinate powder) is shown in Fig. 6.3 (Schoonman et al, 2001). Here, the solid matrix, voids, open and closed pores and bubbles, micropores and cracks create a complex structure that affects both heat and mass transfer during drying. 

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