Thin-film balance method

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...

First, we note that a force versus distance curve may be obtained in two fundamentally different ways. The force can be controlled and the resulting separation measured. This is the principle used in the thin film balance. Alternatively, the distance can be controlled and the resulting force determined, and this is the method utilized in the SFA, AFM and MASIF approaches. The methodology used in these latter techniques does, however, differ in several important respects. Perhaps the most fundamental difference is how the force and the surface separation are determined. 

In addition to measuring the equilibrium thickness of thin films, the method is widely used to analyze film stability and drainage [810, 811, 813]. In many practical applications, a system is far away from equilibrium and highly dynamic. One example is a flotation cell in which particles and bubbles are mixed. The attachment of a particle to a bubble is limited by the hydrodynamic interaction rather than equilibrium surface forces [695]. When a particle and a bubble approach each other, the liquid in between needs to have time to flow out of the closing gap [728]. This process of film drainage is also studied with the thin film balance. 

Direct quantitative measurements of steric repulsion were made with the surface forces apparatus [1353-1360] and the atomic force microscope [1361-1364]. Although we focused on the interaction between solid surfaces, steric forces also act between fluid interfaces. The first force versus distance curves of steric repulsion were recorded across a liquid foam lamellae with a thin film balance by Lyklema and van Vhet [1365]. Another example is the force measurement between vesicles using the osmotic stress method by Kenworthy et cd. [1366]. Experimentally, the Milner, Witten, and Cates and the de Gennes model both fit force curves measured between polymer bmshes in good solvents reasonably well. 

In this chapter we review some data on the interactions between two soUd-liquid or two air-liquid interfaces obtained with a range of surface force techniques. It is beyond the purpose of this chapter to describe the merits and drawbacks of the various methods and the interested reader is referred to the original articles describing the surface force apparatus (SFA) [10], the atomic force microscope (AFM) colloidal probe [11], the thin film balance (TFB) [12] and total internal reflection microscopy (TIRM) [13] as well as a more recent review [14]. It is, however, important to be aware that the different techniques use different interaction geometries, and the results can be compared only by using the Derjaguin approximation [15,16] ... 

A different analyser method based upon the same effect (deviating thermal conductivities of gaseous components) is shown in Fig. 6.127. The gas to be analysed diffuses into the measuring cell. Here, a thermal conductivity sensor made of three superimposed silicon chips shows a balanced (zero) output of two thin film resistors fitted on a membrane on the chip in the middle of this stack. One of these thin film resistors is exposed to the gas to be measured. Due to its thermal conductivity, the pair of thin film resistors show an unbalanced signal output. 

Other authors have also used approximate methods to solve the radiation problem. Li Puma and Yue (2003) used a thin film slurry model which does not include scattering effects. More recently, Li Puma et al. (2004), Brucato et al. (2006), and Li Puma and Brucato (2007) have used six flux models for different geometries. Salaices et al. (2001, 2002) used a model which allows for an adequate evaluation of the absorbed radiation in terms of macroscopic balances, based on radiometric measurements. They measured separately total transmitted radiation and nonscattered transmitted radiation, modeling the decay of both radiative fluxes with concentration by exponential fimctions. 

A major advance in technique was made by Curtis, Scheibli, and Bradley (49) in 1950 when they employed a jolly balance device (the Shell Thin Film Evaporometer) to follow weight loss of solvent when applied to a filter paper cone. The New York Paint and Varnish Production Club made a detailed study of the Evaporometer (50), made some modifications in the method, and concluded that the instrument gave results that were repeatable within one laboratory, reproducible at different locations, and in general provided better results than alternative methods investigated. The instrument has been improved over the years and now is produced in a highly automated model (51). 

The methods of steam distillation have been summarized by Bernhauer [13] and Thormann [14]. A detailed discussion of practical and theoretical aspects of steam distillation as illustrated by the distillation of essential oils is given by von Weber [15], Rigamonti [16] developed a nomogram which can be used to calculate the steam requirements for various enrichments. Prenosil [16a] compared theoretical and experimental steam distillation data for multicomponent mixtures. He modified the calculating method by introducing a value for evaporation efficiency. Steam distillation can also be carried out in thin-film apparatus. Berkes etal. [16 b] give a description of the material transfer conditions of a steam distillation performed in such apparatus in terms of the balance equations. 

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