Forces surface

The measurement of the surface tensions of liquids is a starting point for enumerating adhesion due to physical adsorption. They can be measured to within 0.1 mN m i by measuring the force needed to remove a metal ring from the surface of a liquid, with a torsion balance. 

A correction factor has to be applied to the measured surface tension according to a method of Harkins and Jordan [44], which has been justified theoretically by Freud and Freud [45]. Table 5 [45] shows the details of measurements for four liquids Tl is the surface tension directly obtained from the force and /l is the corrected value. The correction factor (F) is a function of R V and RJr where R is the radius 

The surface tension of a liquid is numerically the same as its surface free energy, but the two parameters have different dimensions. The usual SI units for surface tension are millinewtons per metre (mN m ) and those for surface energy are millijoules per square metre (mJ m ). 

The interfacial tension can be obtained by measuring the force needed to remove the ring from the interface between two immiscible liquids. Because the densities of the two liquids are not greatly different, a large volume of the lower liquid is raised above the interface and quite a deep layer of the upper liquid is needed to contain it the Harkins-Jordan correction factor is now larger. 

The measurement of contact angles is relatively straightforward, their interpretation is a much more difficult matter. Contact angles for liquids on flat solid surfaces (0) can be measured by the following methods. 

In general, the force between two particles (F) may be either attractive or repulsive. The force depends upon the surface to surface separation distance (D) between the particles and the potential energy (VO at that separation distance. The relationship between the force and potential energy is that the force is the negative of the gradient of the potential energy with respect to distance. 

The relationships between force and distance as well as the underlying physical and chemical mechanisms responsible for those forces are described below for several of the forces with the most technological significance. 

A whole range of phenomena in interface science revolve around the effect of surface forces. Many practical applications in colloid science come down to the problem of controlling the force between colloidal particles, between particles and surfaces, and between two surfaces. For this reason scientists have devoted considerable effort to understanding surface forces and being able to influence them. 

The introduction to this topic begins with Sec. 1.5E. There we discussed how surface forces arise. Such forces are present in any system in which there is a two-phase interface, i.e., solid-liquid, solid-gas, liquid-gas, or liquid-liquid. Thus, they are present in all the examples treated so far in this text, but in those examples they are generally small and can be neglected without measurable error. But they must be taken into account in very small systems or in systems in which other forces are small or zero. To see why surface forces are important in small systems, consider the pressure difference due to surface tension in a spherical drop or bubble. 

As long as D is large, this pressure difference is negligible. For example, if this is a drop of water at 68 F with a 1-in diameter, then 

We may appreciate why surface forces become important in small systems by observing that the surface forces are proportiorial to the diameter of the body, whereas the pressure forces are proportional to the projected area (i.e., proportional to the diameter squared) and the gravity and inertia forces are proportional to the volume (i.e., the diameter cubed). For other shapes some other dimension replaces the diameter, but the ideal is the same. Hence if we hold the shape of a system constant and increase all jits dimensions, the inertia and gravity forces grow most quickly, the pressure force at an intermediate rate, and the surface forces most slowly. j 

Surface tension forces are also important in pi oblems in which the other forces are negligible. For example, in an ordinary liquid storage tank on earth, the shape and position of the liquid are determined by gravity and pressure forces, as discussed in Chap. 2. However, in rockets and earth satellites, which have zero gravity, the position and shape of a liquid in a tank are largely determined by surface forces [1,2]. 

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Small drops or bubbles will tend to be spherical because surface forces depend on the area, which decreases as the square of the linear dimension, whereas distortions due to gravitational effects depend on the volume, which decreases as the cube of the linear dimension. Likewise, too, a drop of liquid in a second liquid of equal density will be spherical. However, when gravitational and surface tensional effects are comparable, then one can determine in principle the surface tension from measurements of the shape of the drop or bubble. The variations situations to which Eq. 11-16 applies are shown in Fig. 11-16. 

J. N. Israelachvili, Intermolecular and Surface Forces, Academic, Orlando, FL, 1985. G. Kortiim, Treatise on Electrochemistry, Elsevier, New York, 1965. 

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... 

B. V. Deijaguin, N. V. Churaev, and V. M. Muller, Surface Forces, translation from Russian by V. I. Kisin, J. A. Kitchener, Eds., Consultants Bureau, New York, 1987. 

R. J. Hunter, Foundations of Colloid Science, Vol. 1, Clarendon Press, Oxford, 1987. J. Israelachvili, Intermolecular and Surface Forces, 2nd ed., Academic, San Diego, CA,... 

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

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-... 

These authors doubt that such interactions can be estimated other than empirically without fairly accurate knowledge of the structure in the interfacial region. Sophisticated scattering, surface force, and force microscopy measurements are contributing to this knowledge however, a complete understanding is still a long way off. Even submonolayer amounts of adsorbed species can affect adhesion as found in metals and oxides [74]. 

The flotation of mica has been correlated to the adhesion force measured from surface force (SFA—see Section VI-4) experiments although, to these authors, it is clear that dynamic effects prevent an absolute comparison [69, 70],... 

K. Kurihara, Direct Measurement of Surface Forces of Supramolecular Systems Structures and Interactions, in Microchemistry, H. Masuhara et al., Elsevier Science, 1994. 

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]. 

Craig V S J 1997 An historical review of surface force measurement techniques Colloids Surf. A Physicochem. Eng. Aspects 129-30 75... 

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

The technological importance of thin films in snch areas as semicondnctor devices and sensors has led to a demand for mechanical property infonnation for these systems. Measuring the elastic modnlns for thin films is mnch harder than the corresponding measurement for bnlk samples, since the results obtained by traditional indentation methods are strongly perturbed by the properties of the substrate material. Additionally, the behaviour of the film under conditions of low load, which is necessary for the measnrement of thin-film properties, is strongly inflnenced by surface forces [75]. Since the force microscope is both sensitive to surface forces and has extremely high depth resolntion, it shows considerable promise as a teclnhqne for the mechanical characterization of thin films. 

Finally, Berger et al [192] have developed a teclmique whereby an array of force curves is obtained over the sample surface ( force-curve mapping ), enabling a map of the tip-sample adliesion to be obtained. The autiiors have used this approach to image differently oriented phase domains of Langimiir-Blodgett-deposited lipid films. 

Burnham N A and Colton R J 1989 Measuring the nanomechanical properties and surface forces of materials using atomic force microscope J. Vac. Sc/. Technol. A 7 2906... 

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. 

The measurement of surface forces calls for a rigid apparatus that exhibits a high force sensitivity as well as distance measurement and control on a subnanometre scale [38]. Most SFAs make use of an optical interference teclmique to measure distances and hence forces between surfaces. Alternative distance measurements have been developed in recent years—predominantly capacitive techniques, which allow for faster and simpler acquisition of an averaged distance [H, 39, 40] or even allow for simultaneous dielectric loss measurements at a confined interface. 

The accurate and absolute measurement of the distance, D, between the surfaces is central to the SFA teclmique. In a typical experiment, the SFA controls the base position, z, of the spring and simultaneously measures D, while the spring constant, k, is a known quantity. Ideally, the simple relationship A F(D) = IcA (D-z ) applies. Since surface forces are of limited range, one can set F(D = go) = 0 to obtain an absolute scale for the force. Furthennore, SF(D = cc)/8D 0 so that one can readily obtain a calibration of the distance control at large distances relying on an accurate measurement of D. Therefore, D and F are obtained at high accuracy to yield F(D), the so-called force versus distance cur >e. 

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. 

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... 

Figure Bl.20.8. DLVO-type forces measured between two silica glass surfaces in aqueous solutions of NaCl at various concentrations. The inset shows the same data in the short-range regime up to D = 10 mn. The repulsive deviation at short range (<2 nm) is due to a monotonic solvation force, which seems not to depend on the salt concentration. Oscillatory surface forces are not observed. With pemiission from [73].

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