Films Containing Antifoam Drops

The so-called Scheludko cell has found wide application in studies of foam films. This cell was first proposed by Scheludko and Exerowa [35, 36] more than half a century ago. It consists of a (usually) glass cylinder containing a biconcave drop of surfactant solution. A tube inserted into the side of the cylinder permits control of 

FIGURE 2.8 Typical setup for observation of antifoam drops in foam film using a Scheludko cell. (From Scheludko, A., Exerowa, D., Commun. Dept. Chem. Bulg. Acad. Sci., 7, 123, 1959 Scheludko, A.,Adv. Colloid Interface Sci., 1, 391, 1967.) 

This arrangement has been found to be particularly useful in studying the behavior of antifoam drops in draining foam films. It permits observations of both the movement of the drops as they interact with draining films and also the actual events of foam film rupture induced by those drops [24, 37]. 

Antifoam drop sizes are usually 1 microu. Therefore, films containing such drops must be of at least the same order. The thickness profiles of such films can therefore be readily estimated from the interference fringes observed in monochromatic reflected light, which such thickness imply (provided the order of at least one reference fringe is known). 

If the syringe controlling the film size is in the same plane as the film, then drainage of that film after the movanent of the syringe barrel has ceased is entirely driven 

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In this Section instability of asymmetric films is explained by decrease in the surfactant adsorption. Another reason for this instability can be the presence of solid particles at the water-oil interface. Such a heterogeneous defoaming is created when a foam is broken down by the antifoam drops that contain solid hydrophobic particles. The mechanism of action of such types of antifoams will be discussed in Section 9.4. 

According to Koczo et al. [86] antifoam drops containing solid hydrophobic particles flow out from the foam films into Plateau borders and are trapped there. In the borders these drops form unstable asymmetric films, whose rupture leads to formation of lenses. During the process of diminishing of the border size a bridge is formed through which the hydrophobic particle destabilises the film between the lens and the opposite surface of the border. 

Figure 37. Suggested antifoaming mechanisms for mixed-type antifoams. Key a, the oil drops containing solid particles collect in the Plateau borders b, they get trapped in the thinning Plateau border c, the pseudoemulsion film breaks, a drop enters and forms a solid phis oil lens d, the lens gets trapped in a later stage of thinning and e, the lens bridges the film at the Plateau border and the bridge ruptures.

We therefore have a mechanism that could, at least in principle, exist in drying paint films containing non-spreading antifoam drops. It would clearly mainly concern aqueous paint films on hydrophobic surfaces. However, it would not be confined to oils of low viscosity—the phenomenon occurs in aqueous films even with oils of viscosities as high as 10 mPa s [39], although the effect of the high viscosities of the aqueous phase likely to prevail in drying paint films is not apparently known. 

Figure 8.11. The suggested antifoaming mechanism for a mixed-type antifoamer (a) oil drops (containing solid particles) collect in the Plateau border (b) the drops become trapped in the thinning border (c) the pseudo-emulsion film breaks and a drop enters and forms a solid plus oil lens (d) the lens becomes trapped during thinning (e) the lens bridges the film at the plateau border and the bridge (from ref (12)), reproduced by permission of Academic Press...

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