Biconcave drop

Foam films of different size, shape and spatial orientation are obtained at the approach of individual bubbles or the surfaces of a biconcave drop, or at bubble contact with the solution/air interface, or at withdrawing a frame from a solution, etc. Individual foam bubbles are usually used in the study of foam properties. They prove to be most useful in many cases, for example, in the determination of foam film elasticity, the estimation of gas diffusion from the bubble through the film, the detection of the rupture of the foam bubble films [e.g. 1], Beginning with the remarkable bubbles of Boys [2] and reaching to present day studies, single foam bubbles have since long attracted a considerable interest (see, for instance, the monograph of Dukhin, Kretzschmar and Miller [3]). 

The measuring cell of Scheludko and Exerowa [e.g. 15-20] has proven to be a suitable and reliable tool for formation of microscopic horizontal foam films. It is presented in Fig. 2.2, variants A, B and C. The foam film c is formed in the middle of a biconcave drop b, situated in a glass tube a of radius R, by withdrawing liquid from it (variants A and B) and in the hole of porous plate g (variant C). Photographs of formation of black foam film via black spots taken under a microscope are presented in Fig. 2.3. 

A new way to form a microscopic foam film in the middle of a biconcave drop (Fig. 2.2) has marked the further improvement of the microinterferometric technique. The increase in accuracy and reliability of the photometric and registering devices contributed to this improvement. The experimental details and the metrological characteristics of the microinterferometric technique for determination of foam film thickness has been the object of numerous studies [e.g. 16,23,39-43,58],... 

A formula that accounts for the change in the meniscus radius in the middle of the biconcave drop during film formation is given in [64]... 

A more precise formula that accounts for the degree of wetting of the glass tube carrying the biconcave drop, by the liquid is presented in [65]... 

The direct experimental measurement of pB can be accomplished with a number of techniques [18,66]. One of them, possessing several advantages, represents the classical method for direct measurement of the liquid rise in a tube identical with the glass tube holding the biconcave drop in the measuring cell (see Fig. 2.2 A). 

The microinterferometric method employed in the study of kinetics of foam film thinning allows to establish experimentally the liquids that form or do not form foam films. If a liquid possesses even small affinity to produce a foam, a circular film with clearly pronounced Newton rings is formed when it is drawn out of the biconcave drop. Films from aqueous surfactant solution can be obtained even at very small decrease in the surface tension (Act < 10 4 N m 1). It is sufficient to ensure a tension gradient between the film center and periphery. 

Compared to aqueous solutions, foam formation by organic liquids is impeded. When the surfaces of the biconcave drop from a pure organic liquid (benzene, nitrobenzene, toluene,... 

The question of the ( -potential value at the electrolyte solution/air interface in the absence of a surfactant in the solution is very important. It can be considered a priori that it is not possible to obtain a foam film without a surfactant. In the consideration of the kinetics of thinning of microscopic horizontal foam films (Section 3.2) a necessary condition, according to Reynolds relation, is the adsorption of a surfactant at both film surfaces. A unique experiment has been performed [186] in which an equilibrium microscopic horizontal foam film (r = 100 pm) was obtained under very special conditions. A quartz measuring cell was employed. The solutions were prepared in quartz vessels which were purified from surface impurities by a specially developed technique. The strong effect of the surfactant on the rate of thinning and the initial film thickness permitted to control the solution purity with respect to surfactant traces. Hence, an equilibrium thick film with initial thickness of about 120 nm was produced (in the ideal case such a film should be obtained right away). Due to the small film size it was possible to produce thick (100 - 80 nm) equilibrium films without a surfactant. In many cases it ruptured when both surfaces of the biconcave drop contacted. Only very precise procedure led to formation of an equilibrium film. 

Films were formed by approaching the surfaces of a biconcave drop in a porous plate. The change in film thickness was achieved by gradually increasing the capillary pressure (reversibly and isothermally). 

Fig. 8.2. Cell used for studying microscopic foam films (A) in a glass tube (B) with a reservoir of surfactant solution d (C) in a porous plate, a, glass tube film holder b, biconcave drop c, microscopic foam film d, glass capilla e, surfactant solution f, optically flat glass g, porous plate.

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... 

A schematic diagram of the cylindrical cell is shown in Figure 2.10. Foam films may be prepared by first introducing a biconcave drop into the cell followed... 

The measuring cell, in which the microscopic thin liquid films are formed and studied, is the basic part of micro interferometric apparatus. Figure 6.1 presents the main details of three measuring cells. In the Scheludko-Exerowa cell (Figure 6.1a) the film is formed in the middle of a biconcave drop at constant capillary pressure. This is a horizontal round film of radius r of about 50-100 pm. A small portion of the liquid is sucked out of the drop through the capillary using a micro-metrically driven... 

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