Experiments with the surface forces apparatus

The radii of the curved surfaces in either setup are macroscopic as we mentioned so that they may be taken as approximately parallel on a molecular length scale around the point of minimum distance h between the opposite bodies (i.e., the two cylinders or the sphere and the plane). In addition, they are locally planar, because mica can be prepared with atomic smoothness over molecularly large areas. 

This setup is then immersed in a bulk reservoir of the same fluid of which the confined film consists. Thus, at thermodjmamic equilibrium, T and are the same in both subsystems (i.e., bulk reservoir and confined fluid). By applying an external force in the direction normal to both substrate surfaces, the thickness of the film can be altered by either expelling molecules from it or imbibing them from the reservoir until thermodynamic equilibrium is reestablished, that is, until the force exerted by the film on the surfaces equals the applied external normal force at the same T and /x. Plotting this force per radius R, F/R, as a function of h yields a damped oscillatory curve in most cases. This is illustrated by plots in Fig. 5.1 where typical curves axe shown for several fluids consisting of branched and unbranched hydrocarbons [146]. As one can see, both the period and the amplitude of oscillations depend on 

The main purpose of the SFA is to measure the forces exerted by a thin fluid film on a solid substrate with nearly molecular precision (143). In the SFA, a thin film is confined between the surfaces of two macroscopic cylinders arranged sucli that their axes are at a right angle (143). In an alternative setup, the fluid is confined between the surface of a macroscopic sphere and a planar substrate [144]. However, crosscd-cylinder and sphere-plane configurations can be mapped onto one another by differential-geometrical arguments [145]. The surface of each macroscopic object is covered by a thin mica sheet with a silver backing, which permits one to measure the separation h between the surfaces by optical interferometry [143]. 

Figime 5.1 Force-distance F/R curves measured in the SFA for various hydrocarbon fluids (from Ref. 146). In this plot, D corresponds to h in Fig. 5.2. 

To make contact with the SFA experiment, one has to realize that the confining surfaces are only locally parallel. Because of the macroscopic, eurvatiue of the substrate surfaces, the stress exerted by the fluid on these curved substrates becomes a local quantity varying with the vertical distance 5 (,t, y) between the substrate surfaces (see Fig. 5.2). As the sphere-plane arrangement (see Section 5.3.1) is immersed in bulk fluid at some pressure F, (T, ft), the total force exerted on the sphere by the film in the z-direction can be expressed as [152] 

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Further support for this model comes from experiments with the surface forces apparatus. Here, two functionalized, curved mica sheets (in a geometry of crossed cylinders) are approaching each other, and the mutual force is measured as a function of the surface separation down into the Angstrom-range. Three basic architectures were studied as schematically sketched in Fig. 8 (a) is the biotin-biotin case which... 

Equation 4.176 is often used in connection to the experiments with the surface force apparatus (SEA) [36,383], in which the interacting surfaces are two crossed cylindrical mica sheets. The divergence in Equation 4.176 for co = 0 reflects the fact that the axes of the two infinitely long cylinders are parallel for co = 0 and thus the area of the interaction zone becomes infinite. 

First experiments with the surface force apparatus, however, showed a different result Experiments with poly(dimethyl siloxane) [1358, 1453], perfluorinated polyether [1454-1456], polybutadiene (PB) [186], and poly(phenylmethyl siloxane) (PPMS) [1453, 1457, 1458] indicated that an immobilized, rubber-like layer of a thickness of typically one to three times the radius of gyration Rg is present on the mica surfaces. Beyond this layer the two interacting surfaces experience a longer ranged repulsion across PPMS [1457], PB ]186], and perfluorinated polyethers [1454, 1456], typically reaching to 1-3 Rg. With PDMS an oscillatory, exponentially decaying interaction was observed ]1358]. The oscillation period of 0.7nm was approximately equal to the diameter of a PDMS chain. 

Some interesting light has been thrown on the nature and roughness of surface layers in contact by experiments of Israelachvili and co-workers with the surface force apparatus [55,79,83,84]. This apparatus enables the surface energy to be evaluated both when the surfaces are advancing into closer contact, yx, and when they are receding further apart, )/r. These two values would be expected to be the same, as indeed they sometimes are. In many cases, however, there is hysteresis, with j/r > yx- Israelachvili and colleagues have studied this phenomenon in some detail. 

It is important to note that the lamellar phase is thus stabilized by the balance of a negative interfacial tension (of the free oil/water interface covered by an amphiphilic monolayer), which tends to increase the internal area, and a repulsive interaction between interfaces. The result, Eq. (48), indicates that the scattering intensity in a lamellar phase, with wave vector q parallel to the membranes, should have a peak at nonzero q for d > d due to the negative coefficient of the q term in the spectrum of Eq. (40). just as in the microemulsion phase. This effect should be very small for strongly swollen lamellar phases (in coexistence with excess oil and excess water), as both very small [96]. Very similar behavior has been observed in smectic liquid crystals (Helfrich-Hurault effect) [122]. Experimentally, the lamellar phase under an external tension can be studied with the surface-force apparatus [123,124] simultaneous scattering experiments have to be performed to detect the undulation modes. 

In previous studies, we have investigated the properties of PLL-g-PEG films with the surface forces apparatus (SFA) under compression. The molecides formbrushlike homogeneous films on the surface, exhibiting predominantly repulsive, nearly elastic interaction forces upon compression. The equilibrium film thickness is dependent on the polymer architecture, adsorption conditions, and temperature. A comparison of brush-brush and brush—hard wall experiments revealed a significant overlap of the two opposing films. 

To measure the force between lipid layers, a lipid is chosen, which at a certain concentration and temperature range forms an L phase. The L phase is a regularly spaced stack of lamellar fluid bilayers separated by water. From a symmetry point of view, the L phase can be considered a smectic-A (SmA) liquid crystal. The mean repeat distance between lipid bilayers in the L phase is measured by X-ray diffraction versus an applied osmotic pressure. Direct experiments have been carried out with the surface force apparatus. Therefore, the bilayers are formed either by spontaneous vesicle fusion [1254-1256] or by depositing two subsequent monolayers with the Langmuir-Blodgett technique [1255, 1257]. Atomic force microscope experiments, which have been carried out between two bilayers formed by spontaneous vesicle fusion, confirmed earlier results [1258]. 

There have also been a number of simulations of more realistic models of polymers at surfaces [65-77], The behavior of these more realistic models of polymers is similar to that of the model systems discussed above with no real surprises. Of course, the use of realistic models allows a direct comparison with experiment. For example, surface forces apparatus measurements [78] show that in some branched alkanes the force is a monotonic rather than oscillatory function of the separation. This is a surprising result because these branched alkanes pack quite efficiently (in fact they crystallize under some conditions), and this would imply that the surface forces should be oscillatory. Several... 

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. 

The potassium bromide and sodium chloride, with > 99.5% purity, were obtained from Merck and used as received. The water used in the experiments was first pre-treated with a Milli-RO 10 Plus system and further purified with a Milli-Q PLUS 185 system. When used in the surface force apparatus it was deaerated for at least one hour using a water jet pump. All glassware was cleaned by Hellmanex... 

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... 

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. 

There is a whole body of experiments on polymer mediated interactions between solid surfaces performed using the surface forces apparatus (mostly by the group of J. Klein [38], but also by other techniques. The theoretical pictrue presented here is in good agreement with most of the results. 

The attractive forces between smooth mica surfaces immersed in liquids have been much measured over the past 30 years by Israelachvili and his colleagues. Israelachvili joined the staff at the Australian National University in Canberra during 1973 and started to do the unthinkable by carrying out experiments in the Mathematics Department. He began to work with Adams in modifying the surface force apparatus previously built in Cambridge. The idea was to fill it full of water and other liquids. A schematic of the equipment is shown in Fig. 6.10. 

The surface forces apparatus measures the forces between surfaces of macroscopic dimensions. The requirement that large areas be brought to very small separations imposes demanding requirements on the flatness of the surfaces used and by far the majority of experiments with the SFA have used freshly cleaved mica as a surface that can be relied upon to be atomically smooth over macroscopic areas. The technique of scanning force microscopy allows the force between a tip of small radius of eurvature and virtually any kind of surface to be measured. In addition, the tip can be scanned over the surface to provide an image, which can be of very high resolution, of the force as a function of position. The drawbacks of the technique are that it is difficult to... 

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. 

Can the observed behaviour on the macroscopic and microstructural scales be reconciled with what we know about frictional sliding under these conditions To answer this question, we turn to other reported work with mica in which the real and apparent areas of contact coincide. Johnson reports that experiments with an atomic friction microscope (AFM), on mica in which the contact dimension is 2 to 10 nm, indicate a frictional shear stress of 1 GPa. Other measurements performed with a surface force apparatus (SFA), in which the contact dimension is in the order... 

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