Surface force apparatus lateral forces measurement

Later, in 1973, Tabor and Israelachvili developed a surface force apparatus, SFA, to measure the interaction force in a vacuum at the 1.5 nm level for the first time. In this equipment, the interaction forces between two crossed cylinders coated with freshly cleaved mica sheets having atomically smooth surfaces were measured. One of the cylinders is mounted on a piezoelectric transducer, and the other cylinder is mounted on a spring of known and adjustable spring constant with a force resolution down to 1CT8N. SFA has been further developed for performing measurements in liquids and vapors. In these developed SFA versions, the separation distance between the cylinders can be measured interfero-... 

Chen et al. [301] used a surface forces apparatus (SFA) to measure the force between two hard, solid-like polybutadiene (PB = 10 Da) layers, about 110 nm thick, immersed in liquid PDMS. The measured force was found to be independent of the approach velocity in the range V = 0.01-50 nm s . Four stages were identified (i) an initial smooth approach, (ii) a jump when the two layers were about 250 nm apart, (iii) an approximately 4-nm thick layer of PDMS was trapped between the two PBD surfaces (1 or 2 molecular layers), and (iv) coalescence. Large normal and lateral deformations were consistent with the calculated van der Waals forces. The results imply that liquid droplets (or biological cells) can sense each other at relatively large distances. 

Whereas the first experiments on structmral forces in liquid crystals were performed using a Surface Force Apparatus (SFA, discussed later on in part 3 of this chapter), a temperature controlled AFM was used later [4] to measure the forces between the AFM probe and the surface, mediated by a liquid crystal in between. Compared to other methods, using an AFM for measuring surface forces has some advantages and also some drawbacks. The most important advantage is, that the temperature of the sample can be controlled in a simple way to better than 0.01 K. Furthermore, the sample is not in direct contact with large mechanical parts like in the case of a SFA, and only a small amount of liquid crystal is needed for the experiment. The drawback of the AFM is, that the separation between surfaces is not measured directly... 

The first direct measurements of intermolecular forces between surfaces in liquid at the nanoscale were taken using a surfaces forces apparatus by Tabor and Winterton in 1968 and later by Israelachvili et ah in the 1970s. 

Fig. 1 Unit for shear force measurements attached to the surface force apparatus. The sectioned piezo tube (Morgan Matroc) bends depending on the amplitude and polarity of the applied input voltage 17in. The symmetrical leaf springs transfer this bending into lateral oscillations of the liquid crystal film which is sandwiched between two mica sheets glued to the crossed quartz lenses the upper lens attached to the piezo tube and the lower lens mounted on the cantilever spring. The magnitude of these oscillations is detected as the output voltage Uom of the capacitance probe (Japan ADE). In this configuration our shear force unit is a modification of the one proposed first by Klein et al. [8]...

An important part of designing a friction measurement apparatus is choosing the probe size, shape, and material. Because friction is an interaction between two surfaces, the probe geometry and material will affect the values calculated for the friction coefficient of the other surface. Also, results will be more accurate when the probe s normal force is maintained at a constant value or continuously monitored previous methods used to maintain the normal force include spring mechanisms or static weights to weigh down the probe. These parameters are revisited critically later in this article. 

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