Scheludko-Exerowa cell

The film drainage has been carried out in a Scheludko-Exerowa cell [13,155,161,189,233] and the film thickness has been measured with the microinterferometric technique (see Section 2.1.1). Experimental details for the case considered can be found in [127], The capillary pressure pa is measured in a separate experiment with the technique described in Section 2.1.4. The capillary pressure is a monotonously decreasing function of... 

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... 

Fig. 7.15 shows foam films formed in the measuring cell of Scheludko-Exerowa (variant A), in a porous plate cell (variant C) (see Section 2.1.2) and a foam. If the radii of the foam films in the measuring cells and the dispersity of the foam (respectively, the radii of the films in the foam) are properly chosen, as well as the applied capillary pressure in the films (variant B) and in the foam liquid phase, the experimental conditions with single foam films and foam films in the foam can be very close. 

Figure 2.16. The thin-film balance method used for evaluating the stability and drainage of foam films (a) schematic representation of the geometry of films in a foam, which occurs in the measuring cell of the Scheludko-Exerowa system (b) schematic of the set-up used for studying microscopic thin aqueous films (c) a typical interferogram of photocurrent versus time of drainage for the thinning process (adapted from ref. (6)), with permission from Elsevier Science...

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. 

In the method developed by Exerowa, Cohen and Nikolova [144] the insoluble (or slightly soluble) monolayers are obtained by adsorption from the gas phase. A special device (Fig. 2.28) was constructed for the purpose a ring a in the measuring cell of Scheludko and Exerowa for formation of microscopic foam films at constant capillary pressure (see Section 2.1.2.). The insoluble (or slightly soluble) substance from reversoir b is placed in this ring. Conditions for the adsorption of the surfactant on either surface of the bi-concave drop are created in the closed space of the measuring cell. The surfactant used was n-decanol which at temperatures lower than 10°C forms a condensed monolayer. Thus, it is possible to obtain common thin as well as black foam films. The results from these studies can be seen in Section 3.4.3.3. 

In the device presented in Fig. 3.120,a the asymmetric film forms when a water droplet approaches the surface of an organic liquid. The thickness of the film obtained is controlled by either lowering the level of the organic liquid or by rising the water level, with the aid of microscrews. In the cell shown in Fig. 3.120,b the film forms in a capillary contacting a porous material. The principle of action of this device is similar to that employed in the study of foam films formed in the porous plate measuring cell of Exerowa-Scheludko (see Chapter 2, Fig. 2.2C). 

For experimental observation of the drainage and stability of liquid films the capillary cell illustrated in Fig. 16 is widely used (Scheludko and Exerowa 1959). First, the cylindrical glass cell is filled with the working liquid (say, water solution) next, a portion of the liquid is sucked out from the cell through the orifice in the glass wall. Thus, in the central part of the cell a liquid film is formed, which is encircled by a Plateau border. By adjustment of the capillary pressure the film radius, / , is controlled. The arrow (see Fig. 16) denotes the direction of illumination and... 

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.)... 

The disjoining pressures in thin foam films can be as high as 30 kPa (0.3 bar). Such pressures cannot be applied using devices like that shown in Figure 2.8. They can, however, be applied directly to a foam film contained in a cylindrical cell prepared from a porous frit where the menisci in the fine holes in the latter determine the maximum capillary pressure in the Plateau border. In this approach, pressure on the film is directly applied by increasing the surrounding gas pressure. In turn, this produces an enhanced capillary pressure in the liquid trapped in the pores of the frit, presumably as the liquid moves to smaller pore radii. This technique was first used by Mysels and Jones [40] and later refined by Exerowa and Scheludko [41],... 

The direct measurement of the disjoining pressure as a function of film thickness can be achieved by the thin film balance introduced by Derjaguin et al. [91] and Scheludko and Exerowa [768]. The principle of this so-called Scheludko cell is shown in Figure 7.6. The film is formed within a ring with 2—4 mm diameter. A small hole at the side of the ring allows to draw Uquid from the film and thus adjust and measure the capillary pressure applied. The film thickness is deduced via optical interferometry. Almost exclusively aqueous solutions are studied. Originally, the thin film balance was constructed to measure the force across foam films. Modified versions are also used to measure forces across emulsion films [732, 734]. 

---