Techniques microinterferometric

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],... 

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... 

A block-scheme of this apparatus is presented in Fig. 2.12, A and B. Compared to that given in Fig. 2.11, an adequate improvement is the use of fibre optic cable which conducts the light from Hg-Xe arc lamp to the reflected light microscope. The thickness is determined by the microinterferometric technique of Scheludko-Exerowa (see Section 2.1). 

More precise verification of the theory was achieved with films studied by the microinterferometric technique. Though performed a long time ago (1960), these experiments deserve attention, since they represent the first quantitative proof of the DLVO-theory conducted on a model system (foam film) which still hold true. Independent studies were performed of the X ei(k) and Vlvw(h) as well as of their joint action at various electrolyte concentrations. At very low Cei equilibrium films of large thickness formed in which the electrostatic interaction was prevailing and their behaviour could be described completely with this interaction. At such film thickness Y vw was still very low so that the equilibrium film state was reached at equal electrostatic disjoining and capillary pressure (n, = p ). Fig. 3.15 depicts the equilibrium thickness dependence on electrolyte concentration for saponin microscopic foam films. 

Calculation of Eq. (3.78) indicated that the equilibrium thickness depended weakly on temperature, for instance when the temperature is changed with 10°C, the equilibrium thickness changed only with about 1%. The microinterferometric technique allowed to establish that the equilibrium film thickness did not depend on film diameter, i.e. there was no diameter effect [158-160]. 

The method of equilibrium foam film allows to study the ( -potential at various aspects by means of the microinterferometric technique (see Chapter 2). For instance, to determine cpo at electrolyte solution/air interface (no surfactant) which is very hard to realise experimentally to find the origin of the surface charge in this case [186,187] to find the isoelectric points at the solution/air interface [173,188] to study the effect of the concentration of various kinds of surfactants [95,100,189,190] ionic effects influence of Na+... 

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... 

IR adsorption at 3400 cm 1 following the law of Lambert-Beer with molar extinction coefficient e = 60. The microinterferometric technique h2 is evaluated by correcting the equivalent water thickness with 3.6 nm. Here, the hydrophilic heads are incorporated in the aqueous core. Fig. 3.53 presents the compared h2(Cei) and d2(Cei) dependences. It is seen that within the whole electrolyte concentration range studied h2 is higher than d2. 

MEASUREMENT OF BINARY DIFFUSION COEFFICIENTS OF FIVE SYSTEMS OF LIQUID HYDROCARBONS BY A MICROINTERFEROMETRIC TECHNIQUE AT 25 C. 

A MODIFIED MICROINTERFEROMETRIC TECHNIQUE FOR MEASUREMENT OF DIFFUSION COEFFICIENTS OF LIQUIDS. 

Various experimental techniques can be used to exploit equation (21.2) and determine the film thickness. However, the one most commonly implemented in thin-liquid film work, i.e. the microinterferometric technique, is due to Scheludko. In order to obtain film thicknesses with the standard interference equations, a measurement... 

The techniques employed in the microinterferometric determination of film thickness have been further developed and used for studying not only microscopic [e.g. 13,14,16-20,23,50-54] but macroscopic films as well [8,55-57]. 

Since, however, each model involves some assumptions, the calculation of h2 always renders certain inaccuracy. The most important problem in the three-layer model concerns the position of the plane that divides the hydrophobic and hydrophilic parts of the adsorbed surfactant molecule. In some cases it seems reasonable to have this plane passing through the middle of the hydrophilic head of the molecule, in others the head does not enter into the aqueous core. That is why it is worth comparing film thicknesses determined by the interferometric technique using the three-layer model, to those estimated by other methods. An attempt for such a comparison is presented in [63]. Discussed are phospholipid foam films the thickness of which was determined by two optical techniques the microinterferometric and FT-IR (see Section 2.2.5). The comparison could be proceeded with the results from the X-ray Reflectivity technique that deals not only with the foam film itself but also with the lamellar structures in the solution bulk, the latter being much better studied. Undoubtedly, this would contribute to a more detailed understanding of the foam film structure. 

The experimental determination of the surface diffusion in foam films with FRAP technique is combined most successfully with the microinterferometric method of Scheludko-Exerowa (see Section 3.5) [39,40]. A systematic study showed the dependences of D on some film parameter such as film composition, type and film thickness, influence of proteins and lipid phase state, molecular length and charge, etc. (see Section 3.5.3). 

During the process of three phase foam thinning, three distinct films may occur foam films (water film between air bubbles), emulsion films (water between oil droplets) and pseudoemulsion films (water film between air and oil droplets) (Figure 1). To study the behavior of these films and particularly the oil droplet-droplet, oil droplet-air bubble and oil droplet-foam frame interactions it is necessary to utilize numerous microscopic techniques, including transmitted light, microinterferometric, differential interferometric and cinemicrographic microscopy. 

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