Three-phase foam structure

Macroscopic Observations of Three Phase Foam Structure. In Figures 2 and 3 are shown foam drainage results in the presence of Salem crude oil with the surfactants C. OS and C..AOS at 1 wt% NaCl. obtained from the transmitted light microscope. Surfactant chain length is clearly seen to be a significant factor (compare Figure 2 with Figure 3). 

Microscopic Observations of Three Phase Foam Structure. We will now discuss observations of the microscopic oil droplet-foam interactions, and observations regarding the attachment of oil droplets to the air-water surface. 

Figure 12. Three phase foam structure as a function of time.

The interactions between an oil phase and foam lamellae are extremely complex. Foam destabilization in the presence of oil may not be a simple matter of oil droplets spreading upon foam film surfaces but may often involve the migration of emulsified oil droplets from the foam film lamellae into the Plateau borders where critical factors, such as the magnitude of the Marangoni effect in the pseudoemulsion film, the pseudoemulsion film tension, the droplet size and number of droplets may all contribute to destabilizing or stabilizing the three phase foam structure. 

Finally we should comment that it is necessary to employ in the calculation of the spreading coefficient (which is often used as a stability criterion) accurately measured values of the various tensions operative in the pseudoemulsion film to determine whether oil is spreading or nonspreading in the three phase foam structure. 

There appear then three primary mechanisms for stabilizing (or destabilizing) a three phase foam. The first derives from the micelle structuring in the film and depends directly upon surfactant concentration and electrolyte concentration. The second is a surface tension gradient (Marangoni) mechanism which relates to the short range intermolecular interactions and the rate of surface expansion. And the third is an oil droplet size effect which depends upon the magnitude of the dynamic interfacial tension. 

Fig. 7. Schematic structures of different syntactic foams (a) two-phase foam, (b) three-phase foam, and (c) four-phase foam.

Ice cream is simultaneously both an emulsion and a partially solidified foam, so it comprises three phases at once. The ice cream would be too solid to eat without the air, and too cold to eat without discomfort. The air helps impart a smooth, creamy consistency. The solid structure is held together with a network of globules of emulsified fat and small ice crystals (where small in this context means about 50 xm diameter). 

Fig. 6a-c. Graphic representation of syntactic foam structures 85) a Random dispersion of spheres, two-phase composite b Hexagonal closed-packed structure of uniform-sized spheres, two-phase composite c Three-phase composite containing packed microspheres, dispersed voids, and binding resin... 

Figure 6 is a graphic representation of foam structures in which the microspheres are dispersed randomly (a) and uniformly in close packing (b). In both structures, the two phases fill completely the whole volume (no dispersed air voids) and the density of the product is thus calculated from the relative proportions of the two. Measured density values often differ from the calculated ones, due to the existence of some isolated or interconnected, irregularly shaped voids as shown in Fig. 6c. The voids are usually an incidental part of the composite, as it is not easy to avoid their formation. Nevertheless, voids are often introduced intentionally to reduce the density below the minimum possible in a close-packed two-phase structure. In such three-phase systems the resin matrix is mainly a binding material, holding the structure of the microspheres together.

For practical calculations of durability and for the determination of application conditions it should be taken into account that these materials are composed of a three-phase structure (gas - solid - liquid). A certain amount of a liquid phase, due to the condensation of water vapor in the air, is practically always present inside plastic foams. The presence of the liquid phase plays a decisive role in mass, gas and heat transfer and sharply reduces the heat and electrical insulation properties of polymeric foams (see Chap. 6). 

Schematic diagrams of the structure of syntactic foams are shown in Figure 50. Two-phase syntactic foam consists of microspheres and a matrix resin. Three-phase syntactic foam consists of microspheres, matrix resin and air voids.

In contrast with two-phase bubble-containing fluids, aerosols, and emulsions, foam has a least three phases. Along with gas and the free continuous liquid phase, foam contains the so-called skeleton phase, which includes adsorption layers of surfactants and the liquid between these layers inside the capsule envelope. The volume fraction of the skeleton phase is extremely small even compared with the volume fraction of the free liquid. Nevertheless, this phase determines the foam individuality and its structure and rheological properties. It is the frame of reference with respect to which the diffusion motion of gas and the hydrodynamic motion of the free liquid can occur under the action of external forces and internal inhomogeneities. At the same time, the elements of the skeleton phase themselves can undergo strain and relative displacements as well as mass exchange with the other phases (solvent evaporation and condensation and surfactant adsorption and desorption). 

Flotation has been used for more than 100 years to separate sulphides, oxides and other salts from ores, as well as to obtain phosphates, barite, chromite and other materials. Up to 90% of copper, lead, nickel, zinc are extracted using flotation in the USA [152 - 153]. In Russia, flotation is widely used to additionally obtain apatite, barite and phosphates. Flotation of iron oxides is not used in practise yet, but the number of experiments carried out in this direction is rather large. The main physicochemical principles of flotation have been discussed above [59 -74]. Here, only some practical problems will be discussed. In [153], requirements are pointed out which apply to three-phase flotation foams, and the main components of the process are defined, i.e. surfactant - collector surfactant - frother activator, depressants, colligend, gangue. The peculiarities of flotation and foam separation in batch and continuous modes are outlined as well as the structure and properties of the main types of flotation agents described. As surfaces of the majority of mineral particles are hydrophilic in nature, hydrophobisation of particles is necessary for a selective separation. 

The toughening mechanisms summarized earlier on provide good insight into possible mechanisms responsible for toughness improvement through second phase infusion. The hypotheses can be applied to the ternary system of foam structures where the cell struts show similar behavior as bulk composite materials. It is hard to identify the fracture and toughening mechanism for the complex three-dimensional... 

Fig. 29.8 Porosity dependence of a and b Y of porous Au foams with different pore sizes of a K = 600 sphere [71]. Inset illustrates the K and the L and the three-phase structure (pore, skin, and matrix) in the porous sphere (reprinted with permission from [9])...

To be semisolid, a system must have a three-dimensional structure that is sufficient to impart solidlike character to the undistributed system that is easily broken down and realigned under an applied force. The semisolid systems used pharmaceutically include ointments and solidified w/o emulsion variants thereof, pastes, o/w creams with solidified internal phases, o/w creams with fluid internal phases, gels, and rigid foams. The natures of the underlying structures differ remarkably across all these systems, but all share the property that their structures are easily broken down, rearranged, and reformed. Only to the extent that one understands the structural sources of these systems does one understand them at all. 

The expansion process has been the subject of extensive investigation because it is the foundation of foamed plastics [13-21]. In general, the expansion process consists of three steps creation of small discontinuities or cells in a fluid or plastic phase, growth of these cells to a desired volume, and stabilization of the resultant cellular structure by physical or chemical means. 

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