Air-bubble films

An air-bubble packing film consists of a pair of adhering films - the base and the coating. The coating film contains molded cylindrical voids in the form of tablets. The air-bubble films are perfect dampers of impact loads, and bubbles filled with corrosion inhibitor vapors are capable of protecting products packed into them. 

Consider the situation illustrated in Fig. VI-9, in which two air bubbles, formed in a liquid, are pressed against each other so that a liquid film is present between them. Relate the disjoining pressure of the film to the Laplace pressure P in the air bubbles. 

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

Continuous deaeration occurs when the viscose is warmed and pumped into thin films over cones in a large vacuum tank. The combination of the thinness of the Hquid film and the dismption caused by the boiling of volatile components allows the air to get out quickly. Loss of water and CS2 lower the gamma value and raise the cellulose concentration of the viscose slightly. Older systems use batch deaeration where the air bubbles have to rise through several feet of viscose before they are Hberated. 

A variation of the preceding process is used to produce oriented vinyUdene chloride copolymer films. The plastic is extmded into tube form and then is supercooled and subsequently biaxiaHy oriented in a continuous bubble process. The supercooled tube is flattened and passed through two sets of pinch roUs, which are arranged so that the second set of roUs travels faster than the first set. Between the two sets, air is injected into the tube to create a bubble that is entrapped by the pinch roUs. The entrapped air bubble remains stationary while the extmded tube is oriented as it passes around the bubble. Orientation is produced in the transverse and the longitudinal directions, creating excellent tensile strength, elongation, and flexibiUty in the film. The commercial procedure has been described (157). 

Small air bubbles also scatter light because the refractive index of air is about 1.0, whereas the refractive index of most polymers is approximately 1.5. Air bubbles in films are sometimes usehil in increasing opacity but the efficiency in scattering light is much less than for mtile Ti02 because the difference in refractive index is much smaller. 

Flotation. The slurry of ground ore leaving the grinding circuit may be separated from part of the water in thickeners or may go directly to the flotation cells. The latter are rectangular tanks into which air is injected or drawn via impellers. Flotation is based on producing a water-repellent chemical film on the exposed sulfide minerals in the ground ore. The sulfide minerals collect on the surface of the air bubbles and rise to the top of the flotation cell, where they can be removed from the froth. The froth overflows the cells in collector troughs called launders. 

A small amount of a particular solvent may be needed to aid application, to enable the release of small air bubbles in sprayed films, or to activate thickeners. 

Very popular is plastic cushioning material used in packaging, usually laminated thermoplastic films that incorporate air bubble pockets. 

The structure of whipped cream is quite complex. A coating of milk protein surrounds small globules of milk fat containing both solid and liquid fats. These globules stack into chains and nets around air bubbles. The air bubbles are also formed from the milk proteins, which create a thin membrane around the air pockets. The three-dimensional network of joined fat globules and protein films stabilizes the foam, keeping the whipped cream stiff. 

Secondly, a stable icing foam requires a low surface tension. Consider the case of egg whites in a beater. With slight whipping, entrapped air bubbles are large and the whites appear foamy, yet transparent and runny. With longer whipping the whites become less transparent, white, and more solid. Thus, because of the low surface tension inherent in the egg protein in solution, more and more air may be incorporated and held in place by the colloidal protein which forms a film around each air cell. 

The foaming capacity of milk is related to the ability to form stable air bubbles. One factor that can stabilize air bubbles is the presence of a film coating on... 

It would therefore be of obvious importance to study the electrical behaviour of a surface liquid-air, since in that case we can determine changes in surface tension directly. Such experiments were already carried out by Quincke, who examined under the microscope the travel of a small air bubble in capillary filled with liquid, in an electric field. The method is therefore substantially cataphoresis, but has the drawback that there is only a minute thickness of liquid between the air surface and the glass wall, and that the portion of liquid film adjacent to the latter also travels in the electric field. Quincke found that air bubbles in water were negatively... 

Spread the mull on one salt plate and cover it with another plate. There should be no air bubbles, just an even film of the solid in the oil. 

Dielectric constants of these materials can be further lowered by known means such as by incorporating air bubbles into the materials or by inhibiting crystallization. A difference of a couple of hundredths in the DE value may be important when one is at the low extremes. Recently Singh et al. calculated the DEs ofpolyimide films from the measured free volume fraction and found that the calculated values, are close to the experimental result.1415 ... 

Voids often look similar to air bubbles. The appearance of voids in filaments or films, however, results for different reasons. Voids can be produced during stretching in the area of necking by a kind of folding mechanism. The formation of voids may also depend on the generation of a radial gradient structure during solidification of the threads. 

Figure 1.1 A standard lipid-containing SPMD with three molecular welds near each end. Note the low interfacial tension causes intimate contact (i.e., the presence of a lipid film on the membrane interior surface) between the triolein and the membrane even where air bubbles exist. Reprinted with permission from the American Petroleum Institute (Huckins et al., 2002a).

Therefore, the stability and lifetime of such thin films will be dependent on these different characteristics. This is evident from the fact that, as an air bubble is blown under the surface of a soap or detergent solution, it will rise up to the surface. It may remain at the surface if the speed is slow, or it may escape into the air as a soap bubble. Experiments show that a soap bubble consists of a very thin liquid him with an iridescent surface. But, as the huid drains away and the thickness decreases, the bubble approaches the equivalent of barely two surfactant molecules plus a few molecules of water. It is worth noting that the limiting thickness is of the order of two or more surfactant molecules. This means that one can see with the naked eye the molecular-size structures of thin liquid hlms (TLFs) (if curved). 

The process begins with an air bubble inside the liquid phase. At the surface, the bubble detaches and moves up under gravity. The detergent molecule forms a bilayer in the bubble film. The water in between is the same as the bulk solution. This may be depicted as follows a surface layer of detergent is applied, a bubble forms with air and a layer of detergent, and the bubble at the surface forms a double layer of detergent with some water in between TLF varying from 10 pm to 100 pm). 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

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