Thin pressure balance technique

Thin Liquid Films - Pressure Balance Technique... 

The Thin Liquid Film - Pressure Balance Technique has been used by a number of researchers that introduced several technical improvements [e.g. 51,80,86], For example, values of n less than 100 Pa prove much more difficult to measure, so there should be an entire conformity with the equation giving the balance of pressures acting in the film and the... 

Microscopic foam films from amphiphilic ABA triblock copolymers have been used to assess steric interactions. Most of the work on copolymers [128,129] has been carried out with the Thin Liquid Film-Pressure Balance Technique (see Chapter 2, Section 2.1.8). Nevertheless, some intriguing results have been obtained with the dynamic method for surface force measurement [127]. 

The most detailed information about the interaction of two interfaces can be obtained from the disjoining pressure vs. thickness isotherm. Disjoining pressure isotherms were obtained for foam films from 0.7-1.410 5 mol dm 3 F108 aqueous solutions. A disjoining pressure range encompassing 4 orders of magnitude (1 -104 Pa) has been monitored by two complementary techniques the dynamic method and the Thin Liquid Film-Pressure Balance Technique [128,129] (see Section 2.1.8). 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

Thus, the experimental TT(/j) isotherms of NaDoS films plotted with the Thin Liquid Film-Pressure Balance Technique represent an excellent example of interpretation of long-and short-range interactions and the transition between them. 

The CBF/NBF transition has already been considered in Section 3.4.1 with respect to the experimental n(/i) isotherms of disjoining pressure obtained with the Thin Liquid Film-Pressure Balance Technique. Theoretical concepts and comparison with the DLVO- and contemporary theories describing surface forces acting in this range of film thicknesses have also been discussed. 

The CBF/NBF transition at pH discussed above was performed at constant ionic strength and capillary pressure. Obviously, such a transition can also be realised when the capillary pressure is altered, for instance, with the Thin Liquid Film-Pressure Balance Technique (see Section 2.1.8). Thus, it is possible to conduct the experiments at lower ionic strength which proves to be important when ri(/i) isotherms of Ci0(EO)4 and NP20 [285], and non-ionic sugar-based surfactants [260] are plotted with respect to pH (see Section 3.4.1). 

In order to investigate the influence of external pressure (disjoining pressure) experiments with single foam films have been carried out using the Thin Liquid Film -Pressure Balance Technique, described in Section 2.1.8 [e.g. 47,48], The radius of the microscopic foam films was close to the initial film radius in a real polyhedral foam (about 0.2 to 0.3 mm). Fig. 7.7 presents a histogram of the distribution by size (diameter) of films in the foam. The most probable film size (under these conditions) has permitted us to choose a suitable radius for the single foam films for these experiments. 

Due to the flexibility of fluid interfaces, the mechanical techniques used in the surface-force instruments cannot generally be applied. Instead, the forces in a thin-liquid film can be balanced by an applied capillary pressure, which is at the origin of the technique referred to as the Thin-Film Balance (TFB). The general concept was first realized by Derjaguin and Obuchov when they... 

In the gravimetric method, the adsorbent (usually in the form of powder) is placed into a bulb, which is mounted on a sensitive balance and the bulb is then evacuated. Next, the weight increase of the adsorbent solid as a function of the absorptive gas pressure is monitored at constant temperature. More recently, the quartz crystal microbalance (QCM) technique has been applied this is very sensitive to mass increases. Quartz is a piezoelectric material and the thin crystal can be excited to oscillate in a traverse shear mode at its resonance frequency when a.c. voltage is applied across the metal (usually gold) electrodes, which are layered on two faces of the crystal. When the mass on the crystal increases upon adsorption, its resonance frequency decreases. The increase in the mass is calculated from the reduction in resonance frequency. On the other hand, adsorption on single flat surfaces can also be measured by ellipsometry, which measures the film thickness of transparent films optically using the difference between light reflection from bare and adsorbed surfaces. 

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