Lamellar foam

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. 

Foam A dispersion of gas bubbles in a liquid or solid in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae). Sometimes distinctions are drawn between concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral (Dry Foam and Polyederschaum), and low concentration foams in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical (Gas Emulsions, Gas Dispersions, Wet Foams, and Kugelschaum). Bulk foams may also be distinguished from lamellar foams. See also Aerated Emulsion, Foam Texture, Froth. 

Fig. 8.3 SEM images of hexadecyl-functionalized magnesium phyllosilicate showing (A) intact spheroids (scale bar = 20pm) and (B) fractured spheroid with foam like interior (scale bar = 20pm). (C) TEM image of a wall fragment showing lattice fringes corresponding to a periodic lamellar structure (scale bar = 50 nm).

What is most amazing of all in this picture is the degree of microscopic order present in a solution that appears quite unexceptional to the imaided eye. Usually, we associate T>eauty and aesthetic appeal with symmetry and regular shapes, just as in the examples of the ordered lamellar phase and lamellar focal conics. However, sometimes also asymmetric shapes have that special quaUty about them that conveys what we call beauty. Figure 4 shows a water-rich foam composed of dish soap with coconut oil. It consists of tightly-packed bubbles of very different sizes that create an asymmetric pattern of astounding beauty [3]. 

Since, however, each model involves some assumptions, the calculation of h2 always renders certain inaccuracy. The most important problem in the three-layer model concerns the position of the plane that divides the hydrophobic and hydrophilic parts of the adsorbed surfactant molecule. In some cases it seems reasonable to have this plane passing through the middle of the hydrophilic head of the molecule, in others the head does not enter into the aqueous core. That is why it is worth comparing film thicknesses determined by the interferometric technique using the three-layer model, to those estimated by other methods. An attempt for such a comparison is presented in [63]. Discussed are phospholipid foam films the thickness of which was determined by two optical techniques the microinterferometric and FT-IR (see Section 2.2.5). The comparison could be proceeded with the results from the X-ray Reflectivity technique that deals not only with the foam film itself but also with the lamellar structures in the solution bulk, the latter being much better studied. Undoubtedly, this would contribute to a more detailed understanding of the foam film structure. 

It is also interesting to study the role of surface forces in formation of multilayer structures from amphiphiles in the foam film as well as the comparison with the lamellar mesomorphic phases, produced in aqueous dispersions of amphiphile molecules. 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

Analysis of the results and comparison with the lipid phase transition observed iq the bulk lipid/water systems allows to conclude that the lowest temperature during heating at which measurable diffusion occurred correlates with the onset of formation of the lamellar Ln liquid crystalline phase of the given phospholipid. Therefore, the data support a correlation between the surface and bulk phase transitions. This was confirmed in recent studies where the lipid surface phase transition was successfully measured for the first time in foam film by independent means involving the detailed investigations of the temperature dependences of the W(C) curve for the foam film and its thickness. 

So, the basic feature of the systems studied, determining the behaviour of foam films and foams, is the formation of micellar structures (probably of lamellar type) with high oil content, which results in formation of stratified films of various thicknesses. 

It is interesting to note, however, that a polymer foam of sufficient durability could be produced even at high surfactant concentrations if, for example, a formulation of polymerising surfactant such as sodium co-acrylamidoundecanate or oleic alcohol is used [134]. This can be realised only in the region of the phase diagram corresponding to the existence of a lamellar liquid-crystalline structure. 

The perceived crispiness of chips and many cereal-based products is the key determinant characterising product quality. Most crispy products are characterized by a cellular, lamellar or puffed structure often described as solid foam. The crispiness of a product is closely related to its structure and the extent of porosity. However, sensory evaluation of crispiness is difficult to correlate with instrumental parameters mainly because crispiness is not a clearly defined attribute (Roudaut et al. 2002). 

DepoCyt (Pacira Pharmaceuticals, San Diego, CA) is a slow release liposome-encapsulated cytarabine formulation, recently approved for intrathecal administration in the treatment of neoplastic meningitis and lymphomatous meningitis (30-32). The Depo-Foam platform used in DepoCyt , is essentially a spherical 20-pm multi-lamellar matrix comprised of phospholipids/ lipid mixture, similar to normal human cell membranes (phospholipids, triglycerides and cholesterol) (33). 

This is particularly the case with nonionic surfactants that produce lamellar hquid crystaUine structures in the fihn between the bubbles [24, 25]. These hquid crystals reduce fihn drainage as a result of the increase in viscosity of the fihn. In addition, the hquid crystals act as a reservoir of surfactant of the optimal composition to stabihse the foam. 

In this book a wide range of authors expertise and experiences have been brought together to yield the first book on foams that focuses on the uses and occurrences of foams in the petroleum industry1. This broad range has allowed for a variety of foams and applications to be highlighted, foams that are bulk or lamellar, aqueous or nonaqueous, and flowing or static. It also covers the occurrences of foams in a wide variety of situations in porous media, well-bores, flotation vessels, and process plants. 

Once the oil is present at the interface, and it bridges the lamellar liquid and the gas phase, it would be predicted (from thermodynamics) to spread spontaneously over a foam if its spreading coefficient , 5, is positive (55). For an oil—foam system S is given, for unit surface areas, by... 

Type C Foams. These foams showed a severe interaction effect. Upon contact with foam, the oil emulsified into very small droplets that were drawn up into the foam as described. However, in this case, the smaller oil droplets filled even the thinner lamellar regions, and lamellae were observed to rupture frequently as... 

---