Adhesion surface force apparatus

Yamada S and Israelachvili J N 1998 Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus J. Rhys. Chem. B 102 234-44... 

The surface forces apparatus (Section 2.3) enables the estimation of a surface energy term, Fq (Eq. 9), providing sufficiently smooth surfaces can be produced. In recent years Chaudhury, Pocius and colleagues have made a valuable contribution to the field of adhesion by developing the technique to study energies of adhesion and of surface energies of polymers [81-85]. These SFA results provide alternatives to values based on traditional destructive tests or contact angle measurements. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Chapter 1 is a view of the potential of surface forces apparatus (SFA) measurements of two-dimensional organized ensembles at solid-liquid interfaces. At this level, information is acquired that is not available at the scale of single molecules. Chapter 2 describes the measurement of surface interactions that occur between and within nanosized surface structures—interfacial forces responsible for adhesion, friction, and recognition. 

Israelachvili, J. N., Intermodular and Surface Forces, 2d ed., Academic Press, New York, 1991. (Graduate and undergraduate levels. An excellent source for the relation between molecular-level van der Waals interactions and macroscopic properties and phenomena such as surface tension, cohesive energies of materials, adhesion, and wetting. Also discusses direct measurement of van der Waals forces using the surface force apparatus.)... 

Fig. 2. Photographic image of the surface forces apparatus Mark IV and images of FECO fringes when the mica surfaces are close to contact (1), and when they are in adhesive contact (2). Figure adapted from http //www.rsphysse.anu.edu.au/SFA/, with permission. From the Australian National University, Tim Wetherell.

Closely related to cleavage experiments are adhesion measurements. Adhesion measurements have been done with mica in the so-called surface force apparatus (SFA) [62-64], In the surface force apparatus two crossed mica cylinders of... 

For surfaces that deform plastically, the contact area A is proportional to the applied load. A single elastic contact deforms as as load increases, and would not be expected to follow this rule. However, when considering an exponential surface height distribution, which leads to a multiplicity of elastic asperity contacts, a linearity between load and contact area is recovered. Using instrumentation developed in the last 15yr, notably the atomic force microscope (AFM) and surface forces apparatus (SFA), researchers have explored the universality of friction-load proportionality over a much wider range of dimensions and surface characteristics. Indeed, SFA experiments have shown friction-load proportionality between atomically smooth mica surfaces in dry air over square micrometers of contact area. A contact mechanics expression for elastic contacts that incorporates the effects of adhesion was used. Similarly, AFM experiments of... 

The overall conclusion was that adhesion energetics in liquids could be readily undCTStood by this technique. However, the adhesion forces varying with separation were not easy to measure by this method, because of the large deformations across the contact. Force measurement required the surface force apparatus of Israelachvili, to maintain the surfaces in rigid cylindrical form and to keep a known gap between the surfaces. 

Several properties of surfaces have origins that are very sensitive to the surface, but are essentially macroscopic in character. Examples include adhesion, surface tension and the contact angle. Techniques to measure these properties are not discussed in this chapter, but rather are mentioned in connection with discussions of the property involved elsewhere in this book. However, surface properties can also be probed at a microscopic level for example the forces between surfaces at microscopic separations may be probed using the surface forces apparatus (SFA). The scanning force microscope... 

The JKR equation is usually employed to interpret the results of the Surface forces apparatus and of Atomic force microscopy, which may be employed to study adhesion between two surfaces. 

The study of fundamental adhesion has been hampered because standard Tests of adhesion provide a result that is a complicated combination of fundamental adhesion, the physical properties of the adherend and the viscoelastic/plastic character of the adhesive (see Adhesion - fundamental and practical, Peel tests). Our understanding of adhesion has been significantly improved with the advent of mechanical devices that are able to probe the forces of adhesion under conditions that minimize all of the confounding effects of adherend, viscoelasticity, and so on. The Surface Forces Apparatus (SFA) as developed by Israelachvili and Tabor is a mechanical device that has allowed adhesion scientists to directly measure the forces of adhesion under very low rate, light loading, almost equilibrium conditions. Attention is also drawn to Atomic force microscopy. 

Surface forces apparatus A V POCIUS Adhesion measured via contact mechanics... 

The imderstanding of the ftindamentals of interfacial friction has heen emerging in the last decades, and is in part based on measurements with the surface forces apparatus (109,110). In particular, friction was shown to be related to adhesion hysteresis rather than adhesion itself (110). 

The importance of acidic interfacial pH during plaque formation was anticipated by the adhesion of individual purified mfps, as measured by a surface forces apparatus. Asymmetric deposition of monomolecular mfp films on mica resulted in adhesion energies that showed a strong pH-dependence for all mfps. For example, for the interfacial proteins mfp-3f and mfp-5, the greatest adhesion always occurred at pH 2.5-3, and decreased... 

Figure 9.7 The pH dependent adhesion of mussel adhesive proteins. (A) In the asymmetric configuration (Inset, upper right) of mfp-3 and mfp-5 on mica in the surface forces apparatus, both the adhesion force and energy decrease exponentially with the increasing pH. (B) The same trend for mfp-5 (inset, lower right), but plotting only the energy corresponding to the force minima against the pH.

Schmitt, F. J., Yoshizawa, H., Schmidt, A., Duda, G., Knoll, W., Wegner, G. and Israelachvili, J., Adhesion energy hysteresis and friction between ultrathin polyglutamate films measured with the surface forces apparatus. Macromolecules, 28, 3401-3410 (1995). 

It has recently become common to use the JKR theory (Johnson, Kendall Roberts, 1971) to extract the surface and inteifacial energies of polymeric materials from adhesion tests with micro-probe instruments such as the Surface Force Apparatus and the Atomic Force Microscope. However the JKR theory strictly applies only to perfectly elastic solids. The paper will review progress in extending the JKR theory to the contact mechanics and adhesion of linear viscoelastic spheres. The observed effects of adhesion hysteresis and rate-dependent adhesion are predicted by the extended eory. 

Some adhesion measuring equipment, notably the Surface Force Apparatus, comprises thin films of polymer mounted on an elastic substrate. The equivalent interface fracture problem to that considered above (Figure 6) is shown in Figure 10, where each elastic solid is covered by a viscoelastic layer of thickness h. The case of the opening crack has been examined by Huntley (75). He shows that the effective work of adhesion, given for viscoelastic solids by equation (18), is modified by the thickness of the layer approximately to ... 

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). 

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