Surface force methods Thin film balance

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] ... 

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]. 

Laminar Free Convection. When a stagnant vapor condenses on a vertical plate, the motion of the condensate will be governed by body forces, and it will be laminar over the upper part of the plate where the condensate film is very thin. In this region, the heat transfer coefficient can be readily derived following the classical approximate method of Nusselt [12], Consider the situation depicted in Fig. 14.4 where the vapor is at a saturation temperature Ts and the plate surface temperature is T . Neglecting momentum effects in the condensate film, a force balance in the z-direction on a differential element in the film yields... 

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