Friction surface force apparatus

Friction can now be probed at the atomic scale by means of atomic force microscopy (AFM) (see Section VIII-2) and the surface forces apparatus (see Section VI-4) these approaches are leading to new interpretations of friction [1,1 a,lb]. The subject of friction and its related aspects are known as tribology, the study of surfaces in relative motion, from the Greek root tribos meaning mbbing. 

Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57]. 

The surface forces apparatus of crossed mica cylinders (Section VI-4D) has provided a unique measurement of friction on molecular scales. The apparatus is depicted in Fig. VI-3, and the first experiments involved imposing a variation or pulsing in the sepa-... 

The force between two adjacent surfaces can be measured directly with the surface force apparatus (SEA), as described in section BT20 [96]. The SEA can be employed in solution to provide an in situ detennination of the forces. Although this instmment does not directly involve an atomically resolved measurement, it has provided considerable msight mto the microscopic origins of surface friction and the effects of electrolytes and lubricants [97]. 

Kumacheva E 1998 Interfacial friction measurements in surface force apparatus Prog. Surf. Sc/. 58 75... 

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... 

Figure C2.9.3 Schematic diagrams of the interfaces reaiized by (a) tire atomic force microscope, (b) tire surface forces apparatus and (c) tire quartz crystai microbaiance for achieving fundamentai measurements of friction in weii defined systems.

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. 

Alsten, J. V., and Granick, S., "Friction Measured with a Surface Forces Apparatus, Tribol. Trans., Vol. 32,1989,pp. 246-250. 

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. 

In spite of the strong economic importance of friction and wear and the resulting scientific effort, our understanding of the fundamental processes is still rudimentary. This results from the complexity of these topics. In addition, this complexity demands a multi-disciplinary approach to tribology. In recent years the development of new experimental methods such as the surface forces apparatus, the atomic force microscope, and the quartz microbalance made it possible to study friction and lubrication at the molecular scale. However, this new wealth of information does not alter the fact, that there are no fundamental equations to describe wear or calculate friction coefficients. Engineers still have to rely largely on their empirical knowledge and their extensive experience. 

In 1988 a modified surface forces apparatus (SFA) was introduced [470,471] to analyze friction. The principle of operation of the SFA has already been introduced in Section 6.4. The modified version allowed a relative shearing of the two mica surfaces. In the SFA, the substrate has to have an atomically flat, transparent surface. In most cases mica is used to fulfill these requirements. Although there is a strong limitation in the choice of materials, due to the high resolution in the vertical direction, the SFA has become an important tool to study the friction and lubrication properties of molecularly thin films. 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

Conceptually, predecessors of the scanning force microscope are the surface force apparatus (SFA) [73,74] and the stylus profilometer [75,76]. The SFA enables measurement of normal and friction forces between molecularly smooth surfaces as small as 1 nN as a function of distance with a resolution of 0.1 nm. In addition to the local force measurement, the profilometer provides a topographic map of the surface by scanning the surface with a sharp probe. However, the profilometer is not suitable for structure characterisation because of the large radius of the tip (about 1 pm) and the low sensitivity of the force sensor (in the range of 1(T2 to 1(T5 N). 

Frictional Force Microscopy Fourier-transform infrared spectroscopy scanning electron microscope surface force apparatus Secondary ion mass spectroscopy scanning tunneling microscope X-ray photoelectron spectroscopy bovine serum albumin immunoglobulin G... 

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 three major new atomic-scale experimental methods developed in the last decade are the quartz crystal microbalance (QCM) [2 4], atomic and friction force microscopes (AFM/FFM) [5,6], and the surface force apparatus (SEA) [7,7a,8]. These new tools reveal complementary information about tribology at the nanometer scale. The QCM measures dissipation as an adsorbed him of submonolayer to several monolayer thickness slides over a substrate. AFM and FFM explore the interactions between a surface and a tip whose radius of curvature is 10 100 nm [9]. The number of atoms in the contact ranges from a few to a few thousand. Larger radii of curvature and contacts have been examined by gluing spheres to an AFM cantilever [10,11]. SEA experiments measure shear forces in even larger-diameter ( 10 pm) contacts, but with angstrom-scale control of the thickness of lubricating hlms. 

The effect of polymer brushes on the reduction of sliding friction was also observed for solid friction, using surface force apparatus (SEA) measurements [83-86]. For example, Klein et al. reported a massive lubrication between mica surfaces modified by repulsive polyelectrolyte brushes in water [83]. These results show that polymer dangling chains on solid or gel surfaces can dramatically reduce the surface friction if the polymer brush has a repulsive interaction with the sliding substrate. 

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