Mica surface

The structurally similar molybdenum disulfide also has a low coefficient of friction, but now not increased in vacuum [2,30]. The interlayer forces are, however, much weaker than for graphite, and the mechanism of friction may be different. With molecularly smooth mica surfaces, the coefficient of friction is very dependent on load and may rise to extremely high values at small loads [4] at normal loads and in the presence of air, n drops to a near normal level. 

In accordance with equation (Bl.20.1). one can plot the so-called surface force parameter, P = F(D) / 2 i R, versus D. This allows comparison of different direct force measurements in temis of intemiolecular potentials fV(D), i.e. independent of a particular contact geometry. Figure B 1.20.2 shows an example of the attractive van der Waals force measured between two curved mica surfaces of radius i 10 nun. 

Figure Bl.20.2. Attractive van der Waals potential between two curved mica surfaces measured with the SFA. (Reproduced with pemiission from [4], figure 11.6.)...

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

The silica discs tliat now hold the back-silvered mica samples are finally mounted into the SFA so that die cylinder axes are crossed and the clean mica surfaces are facing each other. 

Nonetheless, the syimnetric interferometer remains very useful, because there, the wavelengdis of fringes with even cliromatic order, N, strongly depend on the refractive index, n, of the central layer, whereas fringes with odd cliromatic order are almost insensitive to This lucky combhiation allows one to measure the thickness as well as the refractive index of a layer between the mica surfaces independently and siniultaneously [49]. 

Figure Bl.20.6. Short-range adliesion of a mica-mica contact as a fiinction of the relative crystallographic orientation of the mica surfaces, measured in a dry nitrogen atmosphere. With penuission from [94].

The measurement of surface forces out-of-plane (nonual to the surfaces) represents a central field of use of the SFA teclmique. Besides the ubiquitous van der Waals dispersion interaction between two (mica) surfaces... 

The well defined contact geometry and the ionic structure of the mica surface favours observation of structural and solvation forces. Besides a monotonic entropic repulsion one may observe superimposed periodic force modulations. It is commonly believed that these modulations are due to a metastable layering at surface separations below some 3-10 molecular diameters. These diflftise layers are very difficult to observe with other teclmiques [92]. The periodicity of these oscillatory forces is regularly found to correspond to the characteristic molecular diameter. Figure Bl.20.7 shows a typical measurement of solvation forces in the case of ethanol between mica. 

Figure Bl.20.7. The solvation force of ethanol between mica surface. The inset shows the fiill scale of the experimental data. With pennission from [75].

Figure Bl.20.9. Schematic representation of DLVO-type forces measured between two mica surfaces in aqueous solutions of KNO3 or KCl at various concentrations. The inset reveals the existence of oscillatory and monotonic structural forces, of which the latter clearly depend on the salt concentration. Reproduced with pennission from [94].

Frantz P and Saimeron M 1998 Preparation of mica surfaces for enhanced resoiution and cieaniiness in the surface forces apparatus Tribal. Lett. 5 151-3... 

Dedinaite ket ai 998 Interactions between modified mica surfaces in triglyceride media Langmuir 14 5546-54... 

Kekicheff P, Christenson FI K and Ninham B W 1989 Adsorption of cetyltrimethylammonium bromide to mica surface below the critical micellar concentration Colloid Surf. 40 31-41... 

Advances have been made in directly measuring the forces between two surfaces using freshly cleaved mica surfaces mounted on supports (15), and silica spheres in place of the sharp tip of an atomic force microscopy probe (16). These measurements can be directly related to theoretical models of surface forces. 

The SFA, originally developed by Tabor and Winterton [56], and later modified by Israelachvili and coworkers [57,58], is ideally suited for measuring molecular level adhesion and deformations. The SFA, shown schematically in Fig. 8i,ii, has been used extensively to measure forces between a variety of surfaces. The SFA combines a Hookian mechanism for measuring force with an interferometer to measure the distance between surfaces. The experimental surfaces are in the form of thin transparent films, and are mounted on cylindrical glass lenses in the SFA using an appropriate adhesive. SFA has been traditionally employed to measure forces between modified mica surfaces. (For a summary of these measurements, see refs. [59,60].) In recent years, several researchers have developed techniques to measure forces between glassy and semicrystalline polymer films, [61-63] silica [64], and silver surfaees [65,66]. The details on the SFA experimental procedure, and the summary of the SFA measurements may be obtained elsewhere (see refs. [57,58], for example.). 

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

SFA measurements on mica. Horn et al. [68] studied the deformation of mica surfaces in contact. In these studies, Horn et al. established the applicability of Hertz theory of contact mechanics to non-adhering layered solids by measuring... 

Fig. 6. Lateral stiffness vs. load data for a silicon nitride tip vs. mica surface in ultra-high vacuum. Solid line is fit of the JKR model to the data. Reprinted with pennission from ref. [67].

Cobalt at a dilution of 1 gg per liter was completely recovered and successfully determined by use of an ion-exchange membrane. Similar results were obtained with other heavy metals. Potassium in amounts from 5 to 150 /xg inJ75 ml.was extracted from aqueous solutions and from mica surfaces, on which it is present as an exchangeable ion.26... 

FIGURE 9.4 The direct force measurement apparatus shown here ean measure the forees between two eurved molecularly smooth surfaces in liquids. Mica surfaces, either raw or eoated, are the primary surfaees used in this apparatus. The separation between the surfaces is measured by optieal teehniques to better than 10 nm. The distance between the two surfaces is controlled by a three-stage meehanism that ineludes a voltage-driven piezoelectric crystal tube supporting the upper mica surface this crystal tube can be displaced less than 10 nm in a controlled fashion. A force-measuring spring is attached to the lower mica surface and its stiffness can be varied by a factor of 1,000 by shifting the position of a movable clamp. Reprinted with permission from Proc. Natl. Acad Sci. USA, 84, July 1987, 4722. 

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. 

Klein, J., Forces Between Mica Surfaces Bearing Adsorbed Macromolecules in Liquid Media, J. Chem. Soc., Faraday Trans., Vol. 179,1983, pp. 99-118. 

Another remarkable feature of thin film rheology to be discussed here is the quantized" property of molecularly thin films. It has been reported [8,24] that measured normal forces between two mica surfaces across molecularly thin films exhibit oscillations between attraction and repulsion with an amplitude in exponential growth and a periodicity approximately equal to the dimension of the confined molecules. Thus, the normal force is quantized, depending on the thickness of the confined films. The quantized property in normal force results from an ordering structure of the confined liquid, known as the layering, that molecules are packed in thin films layer by layer, as revealed by computer simulations (see Fig. 12 in Section 3.4). The quantized property appears also in friction measurements. Friction forces between smooth mica surfaces separated by three layers of the liquid octamethylcyclotetrasiloxane (OMCTS), for example, were measured as a function of time [24]. Results show that friction increased to higher values in a quantized way when the number of layers falls from n = 3 to n = 2 and then to M = 1. 

---