Multiple Beam Interferometry

The surface separation between the mica surfaces is measured using multibeam interferometry. The multilayer structure formed by the silver layers, mica sheets, and 

nmia and n are the refractive indices of mica and the medium in the gap, respectively. Note that the positions of odd fringes depend only on the gap distance D whereas positions of even fringes will also depend on the refractive index. This is why 

Historically, Bailey and Courtney-Pratt [176] were the first to combine crossed cylinder mica sheets as molecularly smooth surfaces and FECO for distance measurement in 1955. They used this approach to map out the contact area between mica surfaces at different loads and shear forces. The first SFA was built by Tabor and Winterton in 1968 [30] and subsequently by Tabor and IsraelachviU in 1972 [56]. In 1976, the newly designed SFA Mk 1 by Israelachvili and Adams [55, 177] was introduced that allowed measurements in liquids or vapors. With the SFA Mk 2, an adjustable double-cantilever spring extended the range of measurable forces and an optional friction device [644] allowed to measure shear forces of molecular thin films at defined loads. Between 1985 and 1989 Israelachvili and McGuiggan developed the SFA Mk 31 to overcome several limitations of the SFA Mk 2 [179]. The new system had a more compact design for higher attainable stiffness and lower thermal drift, an improved control system for mechanical movement of surfaces, and was more easy to clean. 

Developments by other groups included the construction of an SFA with a glass chamber by iQein [180] for measurements in liquids and the SFA Mk 4 by Parker et al. [181]. The latter was based on the S FA Mk 2 but was more easy to assemble and use. The so-called extended SFA or eSFA was developed by Heuberger et al. [175,182] on the basis of the SFA Mk 3. To significantly reduce instrumental drift, a thorough analysis of sources of errors [162] and thermal drift [163] was carried out and used to optimize construction and temperature control [182]. The second improvement is the use of fast spectral correlation [175] to make use of the full FECO pattern to numerically obtain separation distance D with a precision of w25 pm as well as the refractive index of the gap medium at the rate of several hertz. Alternatively, distance D can be monitored with higher time resolution (1 kHz) by detecting the shift of a 

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The absolute measurement of the distance, D, between the surfaces is central to the SFA teclmique. In interferometric SFAs, it is realized tluough an optical method called multiple beam interferometry (MBI), which has been described by Tolansky [47]. 

Toiansky S 1948 Multiple Beam Interferometry of Surfaces and Films (Oxford Oxford University Press)... 

The force, FC(D), is determined by virtue of multiple beam interferometry. The piezoelectric crystal is expanded or contracted by a known amount AD and subsequently the actual distance by which the surfaces move is determined interferometrically. Any difference between the expected distance of motion and the actual distance that the surfaces moved with respect to each other, AD0, when multiplied by the spring constant, k, gives, according to Hooke s law, the difference between the forces at the initial and the final separations, AFc ... 

Tolansky, Multiple beam Interferometry (Clarendon Press, Oxford, 1948), p. 24. 

Israelachvili JN. Thin film studies using multiple-beam interferometry. J Colloid Interface Sci 1973 44 259-272. 

Figure 31. The five-fold growth spiral on the surface of the Mutsure-jima phlogopite-lM, as revealed by multiple-beam interferometry (courtesy of I.

S. Tolansky, Multiple Beam Interferometry, Clarendon, Oxford, 1948. 

In our laboratory, two techniques have been extensively used for studying protein behavior at various interfaces. The first technique censists of in situ measurement of protein adsorption with labeled proteins the second technique based on multiple-beam interferometry measures surface forces between two mica sheets with adsorbed proteins (Tabor-Israelachvili technique). While the in situ measurements enable quantitation of protein adsorption, force-distance measurements provide direct experimental data on the extension of adsorbed protein layers towards the solution and on their conformation. 

Fig. 13.15), so as to resemble the surface of a latex particle. The two rubber cylinders, A and B, were mounted vertically, one above the other. The upper cylinder was held fixed during the experiment whereas the position of the lower cylinder could be moved up and down by a micrometer drive unit (D). Moving the cylinder A up towards B, after polymer had been adsorbed onto the poly(methyl methacrylate) surfaces, distorted the soft, elastic surfaces in the centi region, resulting in flat discs in the zone of interaction. The distance of separation between the surfaces was measured by multiple beam interferometry. The applied pressure caused an equilibrium contact area to be formed, the magnitude of which could also be determined interferometrically. The relationship between contact area and the equilibrium pressure was established by direct calibration. In this fashion, the equilibrium pressure could be measured for a given distance of separation. 

As Newton recognized, this short range of action of molecular forces has made the force measurement extremely difficult. Only in the past few years or so has it been possible to make the necessary measurements at the nanometer level to prove these ideas. Two problems have been overcome. The first one was the measurement technique. Optical methods, such as multiple beam interferometry and laser optical levers, have been developed to measure atomic distances. Piezoelectric actuators were invented to control nanomovements. The second issue was surface smoothness. Smooth surfaces of oxides such as mica, silica ot alumina have been found. Also tiny smooth probes have been made on a large scale by electronic wafer fabrication routes. Tbese new techniques have allowed a new and proper definition of molecular adhesion. 

The deposition rate of the polymer films was monitored by quartz crystal oscillating technique and controlled by multiple beam interferometry according to Tolansl s... 

Tolansky, S. "Multiple Beam Interferometry of Surfaces and Films" Clarendon Press Oxford (1948)... 

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