Direct measurement of forces between surfaces

4 Direct measurement of forces between surfaces 3.4.1 Introduction 

When two surfaces covered with polymer and smrounded by a solvent for the polymer approach each other a repulsive force will be generated when the outer segments of the adsorbed polymers begin to overlap. This is caused by a 

Forces between polymer-covered surfaces are a subset of the general subject of intermolecular forces, which are more fully discussed elsewhere (Maitland etal. 1981, Israelachvili 1991). Before the SFA and the results obtained from it are discussed, a brief consideration of intersurface forces is needed. If the intermolecular potential energy function between two individual molecules is generated solely by van der Waals forces it is purely attractive and of the form 

If a large sphere of radius R made up of these molecules approaches the flat surface, the interaction potential is obtained by integration of equation (3.4.3) over the sphere s diameter we obtain 

If both surfaces are flat and the two bodies have infinite thickness, then on close approach the nett interaction energy per unit surface area for = 6 is 

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J. N. Israelachvili, Techniques for direct measurements of forces between surfaces in liquids at the atomic scale, Chemtracts Anal. Phys. Chem. 1 1 (1989). 

For measurements between crossed mica cylinders coated with phospholipid bilayers in water, see J. Marra andj. Israelachvili, "Direct measurements of forces between phosphatidylcholine and phosphatidylethanolamine bilayers in aqueous electrolyte solutions," Biochemistry, 24, 4608-18 (1985). Interpretation in terms of expressions for layered structures and the connection to direct measurements between bilayers in water is given in V. A. Parsegian, "Reconciliation of van der Waals force measurements between phosphatidylcholine bilayers in water and between bilayer-coated mica surfaces," Langmuir, 9, 3625-8 (1993). The bilayer-bilayer interactions are reported in E. A. Evans and M. Metcalfe, "Free energy potential for aggregation of giant, neutral lipid bilayer vesicles by van der Waals attraction," Biophys. J., 46, 423-6 (1984). 

The phenomenon of cavitation was also observed by Christenson and Claesson [82], around 20 years ago, who used DODAB LB films and the equilibrium method for forces measurements. The direct measurement of forces between hydrophobhic surfaces in water were rather recently thoroughly reviewed by the above authors [94] and the reader who is interested in this intriguing topic is referred to this review and many references therein. 

In this part we will present the power of atomic force microscopy (AFM) for the nanoscale investigations of the solid-isotropic liquid crystal interfaces. The work described here is focused on direct measurements of forces between solid objects, immersed in an isotropic liquid ciystal that is close to the phase transition to an ordered liquid crystalline phase. Solid surfaces induce some liquid crystalline order in the surrounding liquid crystal and this influences the force between the objects. Consequently, it is possible to extract important information about the behaviour of the liquid crystalhne order in the few nanometer thick interfacial layer by studying the forces. 

At the times when DLVO theory was developed, the direct measurement of forces between colloidal particles and surfaces in solution was not possible, and the macroscopic observation of colloidal stability was the only experimental reference data. With increasing technological advancement, setups have been developed for the direct observation of such forces The surface force apparatus (SFA) allows for the measurements of forces between surfaces in solution [6], and with an atomic force microscope (AFM), forces on a colloidal particle can be detected [7]. It is a major success that DLVO theory predicts forces that agree nicely with the measured forces for large particle separations (more than 3-10 nm), but at the same time, it is obvious that in the regime of short particle separations, not aU effects are captured by DLVO. When the barrier for coagulation occurs at such low separations, the DLVO prediction for colloidal stability is not accurate (Fig. 2). 

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. 

Hydration and Hydrophobic Forces, As surfaces approach each other to distances less than 10 nm, a force exists that is not accounted for in conventional DLVO theory. This force can be repulsive or attractive in nature and can be of magnitude greater than either the double-layer or van der Waals interactions. This force was discovered by Israelachivili and coworkers (18-20) using a unique apparatus that they developed that directly measured the force between two mica surfaces. For solid mica surfaces immersed into water, the force was repulsive and oscillatory and exponentially decayed as a function of distance from the surface (see Figure 10). 

The silicon nitride tip is used mostly for C-AFM. Measurements can be done in ambient air, controlled atmospheres, or in non-aggressive liquids. AFM also allows surface forces, and even molecular forces, to be directly quantified [23]. For example, the interaction forces between a silicon tip and microfiltration and ultrafiltration membranes in an electrolyte solution can be measured [24]. The geometry of the cantilever is not simple, and in some cases not even known, so comparison with theory is difficult. However, attaching a sphere to the cantilever instead of a tip enables the measurement of interaction between surfaces of known geometry [25]. This technique has been used to measure interactions between different materials in air... 

Historically the measurement of forces between solid and fluid interfaces began at approximately the same time. However, with the advent of the Surface Force Apparatus (SFA) and Atomic Force Microscope (AFM) over the last few decades, studies concerning the direct measurement of interactions between solid interfaces have received more attention than those concentrated on fluid interfaces (i.e. thin-liquid films - films which have at least one fluid-fluid interface such as foam, emulsion... 

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]. 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

Horn R G, Clarke D R and Clarkson M T 1988 Direct measurement of surface forces between sapphire crystals in aqueous solutions J. Mater. Res. 3 413-6... 

Pashley R M, McGuiggan P M, Ninham B W, Brady J and Evans D F 1986 Direct measurements of surface forces between bilayers of double-chained quaternary ammonium acetate and bromide surfactants J. Phys. Chem. 90 1637-42... 

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. 

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

The LB monolayers of dimethyldioctyadecylammonium ions on molecularly smooth muscovite mica surfaces were investigated. Direct measurements of the interaction between such surfaces were carried out using the surface force apparatus. A long-range attractive force considerably stronger than the expected van der Waals force was measured. Studies on the electrolyte dependence of this force indicate that it does not have an electrostatic origin but that the water molecules were involved in this. 

FIG. 10.7 Direct measurements of van der Waals dispersion forces. The measurements correspond to the force between two flat (mica) surfaces separated by a distance d. The line shown is the theoretical expression for unretarded van der Waals force. The figure shows that the unretarded expression describes the measurements sufficiently accurately for d about 6.5 nm or less. (Redrawn with permission of J. N. Israelachvili and G. E. Adams, J. Chem. Soc., Faraday Trans. 1, 78, 975 (1978).)... 

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.)... 

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

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