Surface-forces apparatus

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. 

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The most widely used technique in SFA for determining the distance or gap between the sample surfaces is based on the theory of multi-beam interference. A diagram of the optical system for the gap measurement is schematically shown in Fig. 15. 

Before being glued to the glass sample, one face of each mica sheet has to be coated or spread with a thin layer of silver in 50 — 60 nm thick (reflectivity 96 % —98 %) for facilitating the optical interference. As shown in Fig. 16(a), when a beam of white light goes up vertically through the lower sample and reaches the silver film, the beam is partly re- 

The surface force apparatus (SFA) has been used extensively over the past 30 years to measure the force directly as a function of separation between surfaces in liquids and vapors. If the force-measuring spring is replaced with a mechanically more rigid support, the two opposing surfaces become an ideal model pore for the study of confinement effects on phase behavior [16], A detailed review can be found in reference ]. Briefly, the shift of the melting temperature AT can be related to the size h of the condensate measured with SFA according to 

Not until the development of the surface forces apparatus (SFA) in the late 1960s could experiments be undertaken to study the interaction forces resulting form the liquid structure at these interfaces. The development of the scanning probe microscope in the 1980s finally allowed the study of the liquid structure at solid-liquid interfaces with nanometer-scale lateral resolution. 

The first direct measurements of intermolecular forces between surfaces in liquid at the nanoscale were taken using a surfaces forces apparatus by Tabor and Winterton in 1968 and later by Israelachvili et ah in the 1970s. 

In this technique, depicted in Fig. 1.4, forces are measured between two partially silvered atomically smooth mica sheets mounted on silica cylinders, which are positioned with their long axes at 90 degrees to each other. The vertical separation between the cylinders is controlled mechanically for coarse control and for fine control using calibrated piezoelectric transducers. One of the cylinders is fixed, and the other is mounted on a cantilever spring of a calibrated spring constant, capable of measuring forces as low as 10 nN. The separation of the partially silvered layers is determined to a resolution of 0.1 nm using optical interferometry. The difference between the piezoelectric translation and surface separation allow 

Crossed Silica Cylinders mounted with partially silvered mica sheets 

SEA measurements between mica surfaces in an inert silicone liquid, OMCTS, revealed oscillatory solvation forces (section 1.3.3) as the surfaces approached each other, below about 10 nm surface separation. The periodicity of these oscillations corresponded to a molecular diameter of 0.9 nm for the quasi-spherical molecules. These measurements demonstrated that at least five layers of OMCTS molecules were adsorbed to each mica surface (see Fig. 1.5). 

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A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

Fig. VI-4. Illustration of the surface force apparatus with the crossed-cylinder geometry shown as an inset. The surface separations are determined from the interference fringes from white light travelling vertically through the apparatus. At each separation, the force is determined from the deflection in the force measuring spring. For solution studies, the entire chamber is filled with liquid. (From Ref. 29.)...

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

The modification of the surface force apparatus (see Fig. VI-4) to measure viscosities between crossed mica cylinders has alleviated concerns about surface roughness. In dynamic mode, a slow, small-amplitude periodic oscillation was imposed on one of the cylinders such that the separation x varied by approximately 10% or less. In the limit of low shear rates, a simple equation defines the viscosity as a function of separation... 

The surface forces apparatus (Section VI-3C) has revealed many features of surfactant adsorption and its effect on the forces between adsorbent surfaces [180,181]. A recent review of this work has been assembled by Parker [182]. 

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... 

Compared witii other direct force measurement teclmiques, a unique aspect of the surface forces apparatus (SFA) is to allow quantitative measurement of surface forces and intermolecular potentials. This is made possible by essentially tliree measures (i) well defined contact geometry, (ii) high-resolution interferometric distance measurement and (iii) precise mechanics to control the separation between the surfaces. 

Interactions between macromolecules (protems, lipids, DNA,.. . ) or biological structures (e.g. membranes) are considerably more complex than the interactions described m the two preceding paragraphs. The sum of all biological mteractions at the molecular level is the basis of the complex mechanisms of life. In addition to computer simulations, direct force measurements [98], especially the surface forces apparatus, represent an invaluable tool to help understand the molecular interactions in biological systems. 

Luckham P F and Manimaaran S 1997 Investigating adsorbed polymer layer behaviour using dynamic surface forces apparatuses—a review Adv. Coiioid interface Sc/. 73 1 -46... 

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... 

Heuberger M, Luengo G and israeiachviii J N 1997 Topographic information from muitipie beam interferometry in the surface forces apparatus Langmuir 3839-48... 

Muiier C, Machtie P and Heim C A 1994 Enhanced absorption within a cavity. A study of thin dye iayers with the surface forces apparatus J. Rhys. Chem. 98 11 119-25... 

Levins J M and Vanderiick T K 1994 Extended spectrai anaiysis of muitipie beam interferometry a technique to study metaiiic fiims in the surface forces apparatus Langmuir 10 2389-94... 

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

A wide variety of measurements can now be made on single molecules, including electrical (e.g. scanning tunnelling microscopy), magnetic (e.g. spin resonance), force (e.g. atomic force microscopy), optical (e.g. near-field and far-field fluorescence microscopies) and hybrid teclmiques. This contribution addresses only Arose teclmiques tliat are at least partially optical. Single-particle electrical and force measurements are discussed in tire sections on scanning probe microscopies (B1.19) and surface forces apparatus (B1.20). 

Surfaces can be characterized using scaiming probe microscopies (see section B1.19). In addition, by attaching a colloidal particle to tire tip of an atomic force microscope, colloidal interactions can be probed as well [27]. Interactions between surfaces can be studied using tire surface force apparatus (see section B1.20). This also helps one to understand tire interactions between colloidal particles. 

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.

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. 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

Measurements of surface and interfacial energies surface forces apparatus... 

An alternative approach to measuring surface energies is provided by the surface forces apparatus, SFA [21]. The apparatus uses surfaces of defined geometry, such... 

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 development of a host of scanning probe devices such as the atomic force microscope (AFM) [13-17] and the surface forces apparatus (SFA) [18-22], on the other hand, enables experimentalists to study almost routinely the behavior of soft condensed matter confined by such substrates to spaces of molecular dimensions. However, under conditions of severe confinement a direct study of the relation between material properties and the microscopic structure of confined phases still remains an experimental challenge. 

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