Measurement of surface forces

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

All techniques mentioned so far are mainly used to study the force between solid surfaces. In many applications one is interested in the disjoining pressure between liquid-liquid or liquid-gas interfaces, such as those found in foams and emulsions. One such technique is described in Section 12.5.3. 

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K. Kurihara, Direct Measurement of Surface Forces of Supramolecular Systems Structures and Interactions, in Microchemistry, H. Masuhara et al., Elsevier Science, 1994. 

Compared witii other direct force measurement teclmiques, a unique aspect of the surface forces apparatus (SFA) is to allow quantitative measurement of surface forces and intermolecular potentials. This is made possible by essentially tliree measures (i) well defined contact geometry, (ii) high-resolution interferometric distance measurement and (iii) precise mechanics to control the separation between the surfaces. 

The measurement of surface forces calls for a rigid apparatus that exhibits a high force sensitivity as well as distance measurement and control on a subnanometre scale [38]. Most SFAs make use of an optical interference teclmique to measure distances and hence forces between surfaces. Alternative distance measurements have been developed in recent years—predominantly capacitive techniques, which allow for faster and simpler acquisition of an averaged distance [H, 39, 40] or even allow for simultaneous dielectric loss measurements at a confined interface. 

The measurement of surface forces out-of-plane (nonual to the surfaces) represents a central field of use of the SFA teclmique. Besides the ubiquitous van der Waals dispersion interaction between two (mica) surfaces... 

Horn R G, Clarke D R and Clarkson M T 1988 Direct measurement of surface forces between sapphire crystals in aqueous solutions J. Mater. Res. 3 413-6... 

Pashley R M, McGuiggan P M, Ninham B W, Brady J and Evans D F 1986 Direct measurements of surface forces between bilayers of double-chained quaternary ammonium acetate and bromide surfactants J. Phys. Chem. 90 1637-42... 

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

The precise direct measurement of surface forces is a subject of current interest, since it provides sufficiently reliable distinction of the forces, along with the elucidation of their mutual influence, their dependence on the distance between the interacting surfaces in systems of different composition, temperature, etc. All this enables a more critical application of the theories (old and new) of the known surface forces. On the other hand, the direct measurement of surface forces stimulate theoretical analyses. 

The Surface Force Apparatus (SFA), designed by J. Israelachvili is a commonly used instrument for the measurements of surface forces. The... 

Smith C. P., S. R. Snyder, and H. S. White, Measurement of Surface Forces, in Electrochemical Interfaces Modern Techniques for In-Situ Interface... 

By measuring the surface forces one can therefore obtain important information not only on the structure of a liquid crystal, but also on the influence of confining surfaces on orientational and positional ordering on a molecular level. This Chapter describes experimental techniques that are used for the measurements of surface forces and is focused on the results of the experiments that have been recently performed in the isotropic, nematic and smectic-A phases. 

Colloid Probe Atomic Force Microscopy. Colloid probe atomic force microscopy (AFM) uses a microsphere as a probe for the quantitative measurement of surface forces.The main advantage of using a microsphere instead of a sharp tip is the improved definition of the contact geometry and thus the ability to perform quantitative comparisons with theoretical models of interfacial forces. 

A very important technique is the use of a surface force apparatus (SFA) (88). It is capable of measuring the surface forces directly between two molecularly smooth surfaces, eg mica, with a sensitivity of a few milUdynes (10 uN) and a distance resolution of about 0.1 nm. These flat smooth surfaces of mica can be covered with polymer brushes to obtain tribological properties between different polymer brush materials. Direct measurement of surface forces is also possible by AFM (89). 

Connor, J., Horn, R. and Miklvcic, S., Measurement of surface forces acting between deformable surfaces, UzbeckJ. Phys., 1, 99-112 (1999). 

Claesson, P. M., Horn, R. G. and Pashley, R. M., Measurements of surface forces between mica sheets immersed in aqueous quaternary ammonium ion solutions, J. Colloid Interface Sci., 100, 250-263 (1984). 

Neuman, R., Berg, J. and Claesson, P. M., Direct measurements of surface forces in papermaking systems, Nordic Pulp Paper Res. J., 8, 96-104 (1993). 

Since the advent of sensitive techniques for the measurement of surface forces in liquids in the 1970s, it has been apparent that a liquid medium in close proximity to a solid surface is not a structureless continuum as found in bulk, but rather, the discrete molecular nature of the solid and the liquid at the interface leads... 

The direct measurement of surface forces is challenging due to their short-ranged nature. One has therefore to combine a sensitive detection of forces vith a precise control of distance on the subnanometer scale. A critical quantity in experiments on force versus distance relations is the distance of closest approach that is actually achieved in an experiment. Surface roughness and contaminations are serious issues vhen trying to establish intimate contact between two objects. In contrast to friction forces, which were already studied by Leonardo da Vind, systematic studies of surface forces were not done before the beginning of the twentieth century. For an overview of the history of the development of devices to measure surface forces, see Refs [157, 158]. 

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