Surface force apparatus applications

Experimental techniques based on the application of mechanical forces to single molecules in small assemblies have been applied to study the binding properties of biomolecules and their response to external mechanical manipulations. Among such techniques are atomic force microscopy (AFM), optical tweezers, biomembrane force probe, and surface force apparatus experiments (Binning et al., 1986 Block and Svoboda, 1994 Evans et ah, 1995 Israelachvili, 1992). These techniques have inspired us and others (see also the chapters by Eichinger et al. and by Hermans et al. in this volume) to adopt a similar approach for the study of biomolecules by means of computer simulations. 

In this article, we report an investigation of the tribological properties of PLL-g-PEG films carried out with a surface forces apparatus. Although the conditions applied in the experiments (moderate surface pressures, low velocities) are far removed from those of many practical macrotribological applications, they help to elucidate the underlying mechanisms of film structure, lubrication, and repair. 

In recent years it has been demonstrated that also adhesion (or adhesion hysteresis) plays an important role in friction. Israelachvili and coworkers could show that friction and adhesion hystereses are, in general, directly correlated if certain assumptions are fulfilled. These authors have proposed models based on data obtained by surface forces apparatus (SFA) experiments, e. g. the cobblestone model of interfacial friction (4). In addition, several groups described the application of continuum contact mechanics (e.g. Johnson-Kendall-Roberts (JKR) theory (5)) to describe friction data measured between flat surfaces and nanometer sized contacts (d). 

In order to facilitate the calculation of capillary forces, several approximations on the meniscus shape have been proposed. They are mainly applied for experimental conditions where the radius of curvature of the meniscus interface is much smaller than the radius of curvature of the solid surfaces. This is relevant for the surface force apparatus where the surface has centimetric radius, while the meniscus is typically tens of hundreds of nanometers. The most used approximation is the toroidal approximation assuming the liquid interface has a circular profile. Obviously, such a meniscus does not exhibit a constant curvature. Nevertheless, this approximation gave good results, in particular for small contact angles, and is therefore widespread (see Ref. 15 for its application in various geometries and section 9.3.1.1 for an example of its application in atomic force microscopy [AFM]). In the case of capillary condensation between a plane and a sphere with a large radius of curvature R, in contact, the tension term of the capillary force is negligible and the Laplace term leads to the simple formula F = AnRy cos 9 A parabolic profile is also sometimes used to eliminate some numerical difficulties inherent in circle approximation. 

The first instrument to directly measure the van der Waals forces between molecularly smooth cylindrical mica surfaces was the Surface Force Apparatus (SFA) developed by Tabor and Winterton (1969) and Israelachvili and Tabor (1972, 1973). The results from SFA were the first to confirm the predictions of Lifshitz theory of van der Waals forces down to surface separations as small as 1.5 nm. The SFA technique has since then been modified and has found applications in many different areas, both biological and non-biological to provide information on the different forces acting between the surfaces and molecules. However, a limitation ofthis technique is that it measures interactions between membranes, proteins or various metal oxide layers or films, which have to be deposited/adsorbed onto the molecularly smooth mica surfaces. 

Experimentally, rupture forces in biological systems can be measured with atomic force microscopy, surface force apparatus, optical tweezers, or the biomembrane force probe technique. Each of these methods operates on different time.scales and gives additional insights into the dynamics strength of biological bonds (see Ref. 75). Since none of these methods yields details on the atomic level, this is a nice application for molecular dynamics. 

The statics and dynamics of microstructures are governed by the forces that create or maintain them. Rarely can the forces be measured directly. But forces between special surfaces immersed in fluid can now be accurately gauged at separations down to 0.1 nm with the direct force measurement apparatus, an ingenious combination of a differential spring, a piezoelectric crystal, an interferometer, and crossed cyhndrical surfaces covered by atomically smooth layers of cleaved mica (Figure 9.4). This recent development is finding more and more applications in research on liquid and semiliquid microstructures, thin films, and adsorbed layers. 

A force Is applied to the gel s surface by a syringe piston powered by compressed air. Measurements can be Influenced by a skin on the jelly s surface or uneven application of pressure (3 ). Swenson et al. (48) modified the Tarr-Baker apparatus to produce a balance-plunger type Instrument. Although they found a linear stress-strain region within the elastic limits of the gel they also found elasticity was somewhat dependent on the rate of loading. 

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