Soap film rupture

The rupture process of a soap film is of some interest. In the case of a film spanning a frame, as in Fig. XIV-15, it is known that rupture tends to originate at the margin, as shown in the classic studies of Mysels [207, 211]. Rupture away from a border may occur spontaneously but is usually studied by using a spark [212] as a trigger (a-radia-tion will also initiate rupture [213]). An aureole or ridge of accumulated material may be seen on the rim of the growing hole [212, 214] (see also Refs. 215, 216). Theoretical analysis has been in the form of nucleation [217, 218] or thin-film instability [219]. 

Now we can understand what happens in the case of soap bubbles. As discussed in Chapter 7, the soap molecules spread on both air-water interfaces enclosing a film of water and provide the repulsion necessary to maintain a water film, similar to the case we discuss in Example 11.3. In the absence of soap molecules, the (attractive) air-air van der Waals forces through the water film rupture the film so that the bubbles are unstable. 

Mechanisms of Single-Foam Film Stability. Soap bubbles and soap films have been the focus of scientific interest since the days of Hooke and Newton (2—9). The stability and structure of foams are determined primarily by the relative rate of coalescence of the dispersed gas bubbles (10). The process of coalescence in foams is controlled by the thinning and rupture of the foam films separating the air bubbles. Experimental observations suggest that the lifetime (stability) of foam films is determined primarily by the thinning time rather than by the rupture time. Hence, if the approaching bubbles have equal size, the process of coalescence can be split into three stages ... 

In order to study the interference produced by reflection and transmission from a soap film it is advisable to keep the film in a controlled environment in order to prevent the film rupturing. 

When a soap film finally ruptures it will break up into many small droplets. Most of the energy of the film is converted into kinetic energy of the droplets. This produces droplets with typical velocities of the order of 10 cm per sec. 

A rigid soap film created in an enclosure is prevented from thinning by evaporation. If it is allowed to drain in a saturated, vibrationless, clean, environment the film will not rupture. In such circumstances the drainage will be by viscous flow alone. The fluid will flow between the rigid, or plastic surfaces of the soap film (Fig. 2.7(a)). The profile of the film will be parabolic as indicated in Fig. 2.7(b). The parabolic cross-section is joined at its highest point to avertical black film (Fig. 2.7(b)). The black film will have a constant... 

Though foam and emulsion films might exist for a long time, on some timescale they will collapse. The rupture of foam and emulsion films has been studied by various methods both experimentally [797] and theoretically [798]. It is obvious that the stability of foam films is influenced by surface forces. For example, in 1924 Bartsch reported that electrolytes decrease the life time of certain foams [799], presumably by decreasing electrostatic stabilization. Surface forces alone, however, do not determine the life time of a soap film. 

Berkman and Egloff explain that some additives increase the flexi-bihty or toughness of bubble walls, rather than their viscosity, to render them more durable. They cite as illustrations the addition of small quantities of soap to saponin solutions or of glycerin to soap solution to yield much more stable foam. The increased stability with ionic additives is probably due to elec trostatic repulsion between charged, nearly parallel surfaces of the hquid film, which acts to retard draining and hence rupture. 

Mixtures of ethylene and methylene distearamide with hydrocarbons have also been claimed to be effective particle-oil antifoams for control of detergent foam in front-loading textile washing machines [38]. Such mixtures represent weU-known antifoams for general application (see, e.g.. Table 4. A3). They may be prepared by milling the mixtures or by cooling a melt of the mixtures (since the solubility of distearamides at ambient temperatures is low) to form particulate dispersions in hydrocarbons. The particles of these alkylene distearamides are intrinsically hydro-phobic [39] and have the properties necessary for rupture of the relevant pseudoemulsion films (see Chapter 4). Unlike the soaps and alkyl phosphoric acid esters, there is no requirement for interaction with any ingredient, such as water hardness, present in the wash solution. 

One sees, according to this discussion, that, in the films of the third category, the moment of rupture is hastened by evaporation also our caps of household soap solution ( 248) which, in an atmosphere saturated with water vapor, persisted several hours, lasted only four to five minutes when they were produced in the closed bottle, but without saturation of its atmosphere if one had formed them in the entirely open air, no doubt they would have burst earlier still. 

Mr. Henry had made, shortly before, a first communication concerning approximate measurements of the cohesion of liquids he sought to evaluate this cohesion in soap water by weighing the quantity of water which adheres to a bubble of this substance immediately before the rupture, and by determining the thickness of film by the observation of the color that it has, according to the scale of thin films of Newton. I translate here literally the passage from the report, because it is not clear. Mr. Henry concludes from his experiments that the cohesion of water, far from being as weak as was believed, rises to several hundred pounds per square inch, and is probably equal to that of the ice. 

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