Light reflection techniques

The thickness of horizontal filns of n-decane sandwiched between two water (or aqueous electrolyte) droplets has been determined by a light reflectance technique. The films were stabilised by three surfactants an xyx block copolymer of poly(ethyleneoxide) and poly(12-hydroxystearic) acid soya bean lecithin Arlacel 83 (sorbitan sesquioleate). Results obtained for two and three component mixtures of the surfactants were compared with those for the single surfactants. The results showed that, provided sufficient polymer is present in the film, the thickness is determined by the longest oleophilic chain, namely the poly(12-hydroxystearic) acid. 

The film thickness obtained by the light reflectance technique is compared with that from low angle X-ray diffraction and a theoretical value in Table IV. The larger value from X-ray... 

Spectroscopic methods, such as fluorescence recovery and quenching, Fourier-transform infrared spectroscopy (FT-IR), and light reflection technique have been used for studies of adsorbed proteins (for example Burghardt Axelrod 1981, Thompson et al. 1981, van Wagenen et al. 1982), and surfactant adsorption layers (for example Ldsche et al. 1983, L6sche Mohwald 1989, Daillant et al. 1991, Henon Meunier 1992, Mohwald 1993). Considerable progress has been made in recent years with respect to the sensitivity of detectors and the efficiency of computers, so that the power of these methods has increased remarkably. 

A quite different means for the experimental determination of surface excess quantities is ellipsometry. The technique is discussed in Section IV-3D, and it is sufficient to note here that the method allows the calculation of the thickness of an adsorbed film from the ellipticity produced in light reflected from the film covered surface. If this thickness, t, is known, F may be calculated from the relationship F = t/V, where V is the molecular volume. This last may be estimated either from molecular models or from the bulk liquid density. 

The advent of lasers allowed optical interferometry to become a useful and accurate technique to determine surface motion in shocked materials. The two most commonly used interferometric systems are the VISAR (Barker and Hollenbach, 1972) and the Fabry-Perot velocity interferometer (Johnson and Burgess, 1968 Durand et al., 1977). Both systems produce interference fringe shifts which are proportional to the Doppler shift of the laser light reflected from the moving specimen surface. Both can accommodate a speci-... 

PIV has become the most popular technique to measure velocity and turbulent properties (Figure 15.1). The movement of seed particles in a millimeter-thick laser sheet is measured by correlating two photos taken a few milliseconds apart. With two cameras, it is also possible to obtain a 3D vector of the velocity in that plane. The method gives, in general, very good resolution of the flow, but it requires optical access. Also, measurement close to walls can be problematic due to light reflections that disturb the measurements. One extension of PIV is the micro-PIV that uses fluorescent tracer particles, which allows all direct light, for example, reflections at the walls, to be filtered out [1]. 

The main technique that has been used for the measurement of opacity has been to prepare a standard disc of AB cement 1-0 mm thick and aged for 24 hours at 37 °C. This disc, contained in a small trough of water to prevent desiccation, is placed in a reflectometer on a black background. It is then illuminated with diffuse light and the amount of light reflected from it, is measured. The disc is then placed on a white background of 70% reflectivity, and the new amount of reflected light, measured. The contrast ratio 7 o/ o is defined as the Cp., opacity (Crisp et al., 1979). 

Freely suspended films provide a perfect homeotropic alignment of smectic LCs since the layers always orient parallel to the LC/air interface.33 The director structure in such films can then be determined by analyzing the optical properties of plane-polarized light reflected from the surface of the films at a slightly oblique angle.34 The technique gains additional power when electrodes are added to the setup, allowing observation of the behavior of the films in the presence of an electric field parallel to the plane of the film.35... 

Light scattering technique was used in determining the oil solubilization rate. Debye s equation ( ) was used in the interpretation. The basic principle involves the measurement of the surfactant aggregate size during the solubilization. As the oil goes into the surfactant micelle, the increased size will be reflected by the turbidity of the solution. 

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

In the diffuse reflectance technique (Fig. 14.2d), light scattered by a thick layer of particles is directed by the integrating sphere to a detector. Absorption... 

The film is observed by a microscope using reflected light The film holder and the objective are immersed in air in the case of foam (i.e., air/liquid/air) film and in the oil phase, in the case of an O/W/O emulsion film, respectively. The film thickness can be determined by measuring the intensity of the light reflected from the film surfaces [9]. Further details of the technique will be discussed in Chapter 2. 

Organic photochemical reactions in monolayer organi-zates are strongly influenced by the restricted molecular mobility in these systems. Reactions at the air-water interface where molecular relaxation is possible, can be followed by measuring the enhanced light reflection in the spectral range of the absorption band of the involved species. In monolayer systems, photoinduced electron transfer processes have been studied by fluorescence techniques. 

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