Foam breaking bubble formation

The basic step in the formation of a liquid foam is the generation of bubbles. We can accomplish this by forcing gas through a nozzle. We can also nucleate bubbles from supersaturated solutions, such as in beer, or from a superheated liquid (boiling). Another way to form bubbles is by mechanical agitation such as in a washing machine or in breaking waves on the sea. 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

A look at Figure 10.4 shows that 1% ethanol added to water (i.e., about 0.22 molar) causes a significant decrease in surface tension of about 5mN m 1. Half a percent of protein is sufficient for protein-covered foam bubbles to form, but—as discussed in Chapter 11—a small-molecule surfactant is more efficient than protein in the formation of foam and the breaking up of large bubbles into smaller ones. This is because the protein has a low molar concentration and needs to unfold upon adsorption to cause considerable lowering of surface tension, which takes a relatively long time. Hence the ethanol should indeed promote foam formation. 

Defoamers, which, when added to a foaming liquid, break down or inhibit the formation of bubbles in the foam, are of course old in the art. Some of these defoaming compositions are a mixture of compounds such as organosiloxane polymers, including silicone oil and/or normally hydrophilic materials that have their surfaces rendered hydrophobic, such as silica. 

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