Measurements foam film

Firstly, the stability of the foam formed by P-casein and whole casein appears very different, the former being more stable. In order to further investigate this issne, we evalnate several surface properties of these two proteins. The surface tension and surface rheology do not seem to be accurate enough to account for this large difference in foam stability, since they show very similar values. However, the thickness of the foam films stabilized by the two proteins respectively seems to determine the ultimate behavior of the foam. Hence, the thicker foam film measured for P-casein probably prevents coalesce of air bubbles resulting in more stable foam formed by this protein as compared to whole casein. 

V. Bergeron and C.J. Radke Equilibrium Measurements of Oscillatory Disjoining Pressures in Aqueous Foam Films. Langmuir 8, 3020 (1992). 

LB. Ivanov, A.S. Dimitrov, A.D. Nikolov, N.D. Denkov, and P.A. Kralchevsky Contact Angle Film and Line Tension of Foam Films. II. Film and Line Tension Measurements. J. Colloid Interface Sci. 151, 446 (1992). 

Measurements have been carried out on the excess tensions, equilibrium thicknesses, and compositions of aqueous foam films stabilized by either n-decyl methyl sulfoxide or n-decyl trimethyl ammonium-decyl sulfate, and containing inorganic electrolytes. 

The interaction between the two gas-liquid interfaces across a foam film can directly be measured by a thin-film balance (TFB) [571-573], A single thin foam film is formed in a hole drilled through a porous glass plate (Fig. 12.19). The plate, and therefore the liquid in the foam, is connected to a reservoir with a constant reference pressure Pr by a capillary tube. The film holder is placed into a closed cell. A constant pressure Pg is applied to the gas in the... 

The direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

The techniques involving microscopic foam films can be applied to the study of other types of thin films, such as emulsion films [e.g. 4-9], thin films on a substrate [e.g. 10-12], as long as a suitable measuring cell is utilised. 

Measuring cells for formation and study of microscopic foam films... 

Fig. 2.2. Scheme of the measuring cell for the study of microscopic foam films A in a glass tube B ... 

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. 

The measuring cells for the study of microscopic horizontal foam films (especially variant A, Fig. 2.2 and variant D, Fig. 2.4) have been widely used in [e.g. 26-47]. 

Thickness is one of the main parameters of a foam film. The most widely employed technique for its determination is interferometry. It is based on the comparison between the intensities of the light falling on the thin film and that reflected from it. This technique permits to evaluate the thickness of equilibrium as well as thinning films. It has been used by Perrin [48] and Wells [49] with soap films. The intensity of the reflected light was measured with an interferometer comparing the two parts of the visual field (in a microscope). Thus the film thickness was determined with an accuracy of about 0.5 nm. 

The accuracy of thickness measurements with this microinterferometric technique is 0.2 nm. For thinner foam films (< 30 nm) it is necessary to account also for the film structure. The three-layer film model with an aqueous core of thickness /12 and refractive index n2 and two homogenous layers of adsorbed surfactant of thickness h each and refractive index i is... 

One of the most important thermodynamic characteristics of foam films is the contact angle appearing at the contact of the film with the bulk phase (solution) from which it is formed. The measurement of the contact angle and the film tension related to it is an... 

Fig. 2.10. Scheme of the measuring cell for study of black foam film at a-particle irradiation 1 - film... 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

A block-scheme of the apparatus for the study of foam films under applied pressure is shown in Fig. 2.11. The films are formed in the porous plate of the measuring cell (Fig. 2.4, variant D and E). The hydrodynamic resistance in the porous plate is sufficiently small and the maximum capillary pressure which can be applied to the film is determined by the pore material. The porous plate measuring cell (Fig. 2.4, variants D and E) permits to increase the capillary pressure up to 105 Pa, depending on the pore size and the surface tension of the solution. When the maximum pore size is 0.5 pm, the capillary pressure is 310s Pa (at cr = 70 mN/m). The cell is placed in a thermostating device, mounted on the microscopic table. 

When a vertical foam film is illuminated, part of the light is scattered by thermal fluctuational microwaves in the film surface. Measuring the intensity of the scattered light makes possible the calculation of the film tension and the energy of molecular interactions in the film [89,90]. 

Elasticity can be measured either with macroscopic vertical or horizontal foam films [94-97] or with individual foam bubbles [98]. 

Bianko and Marmur [99] have developed a new technique for the measurement of Gibbs elasticity of foam films. In order to exclude the effect of the mass transfer of the surfactant, the stretching of an isolated soap bubble is used. The surface tension needed for the calculation of the elasticity modulus is determined by the pressure in the bubble and the radius of curvature. The modulus obtained are considerably lower than those derived by the technique of Prins et al. [95]. 

The conductometrical technique for the study of foam films allows to measure directly their lateral electrical conductivity. It is widely used for indirect determination of film thickness study of kinetics of film thinning and rupture as well as study of other processes. Fig. 2.16 presents a scheme of one of the devices for measurement of the lateral electrical conductivity of foam films [16]. 

The measuring cell given in Fig. 2.17 is used by Yamanaka et al. [112] to study the lateral electrical conductivity of black foam films formed by lysophospholipids. 

Optical measurements of foam films are very complicate. In order to have a clear and precise interpretation of the results, especially for the thinnest black films, it is necessary to make models. The simplest and the most widely used one is the three-layer model (see Section 2.1.3). 

Corkill et al. [56] have used for the first time the infrared spectroscopy for foam films. The measurement of the adsorption of the infrared light provides information about the water content in the foam films which is of major significance for the black foam films. These studies involved the use of dispersion type instruments. In order to obtain measurable values of adsorption, the infrared light is passed through a series of vertical films (up to 10) formed in a cylindrical tube acting as a frame. Additional information about the film structure the authors derived from the correlation between the optical infrared transmission data and the film reflectance measurements. Here a three-layer model of the film structure consisting of an aqueous core sandwiched between two adsorption layers is assumed (see Section 2.1.3). 

This method gained a significant improvement with the introduction of the contemporary infrared technique with a Fourier transformer (FT-IR), permitting to obtain measurable values of adsorption of the infrared light even from single black foam films. The thickness of the aqueous core is derived from the adsorption at 3400 cm 1 which is related to the OH stretching vibration of the water molecules. Umemura et al. [114] have employed the polarised Fourier transformed infrared spectra for the study of the water content of NaDoS black films. The cell used to form films of ca. 2 cm2 area is illustrated in Fig. 2.19. By fitting the calculated curved of polarised FT-IR spectra to the respective experimentally obtained... 

The same principle of formation of a spherical foam film is used in the device (Fig. 2.23) for direct measurement of the film tension y in static [126] and dynamic [127] conditions. The lower part of the capillary is placed in a closed space filled up with a gas, whose pressure can be measured precisely and by means of a special pump can be varied at different rates. The manometer registers the difference with the atmospheric pressure Ap, which is equal to the capillary pressure pa of the spherical foam film... 

Measurement of the coefficient of gas permeability trough foam films The coefficient of gas permeability K is defined by [132]... 

The device, presented in Fig. 2.25 (without electrodes 1 and 6) has been used in [136] for the determination of K by the so-called stationary bubble method . A foam film forms on the porous plate acquiring the shape of a hemisphere. The radius of curvature R is practically equal to the radius of the perimeter at the base of the hemispherical bubble. Because of the gas passing from the bubble through the foam film into the atmosphere, R decreases with time t. The values of r are measured and K is calculated from... 

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