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As it is more practical to measure the forces acting between two contacting surfaces then the energies of surfaces, this chapter covers those surface forces that are derived from surface energies. The starting point is Derjaguin's approximation, which relates the energy between two flat surfaces to other geometries: sphere/flat, sphere/sphere, and crossed cylinders. Next is a discussion of the surface forces in dry contacts with no liquid menisci around the contact points. This discussion covers the cases where adhesion causes deformation (JKR theory) and where deformation is insignificant (DMT theory). The second half of this chapter deals with how liquid menisci around contacts contribute to adhesion forces, both for sphere on flat geometries and for contacting rough surfaces.

This chapter focuses on the two experimental techniques — the surface force apparatus (SFA) and the atomic force microscope (AFM) — that are commonly used for measuring molecular level forces that act between two surfaces at small separation distances. The first part of this chapter covers the fundamental principles of SFA and AFM design. The second half of this chapter illustrates the application of AFM to measuring surface forces with examples the measurement of van der Waals forces, meniscus forces from liquid films and from capillary condensation, and electrostatic double-layer forces.

Capillarity reflects the action of interfacial tension and has been central to understanding intermolecular forces. When a liquid meets a solid surface (with contact angle θ‎) it forms a meniscus which is associated with the rise/depression of liquid in a capillary tube, hence the term capillarity. Interfacial tensions also determine how a liquid wets and adheres to a solid or another liquid. Liquid menisci are curved, and Young, Laplace, and Kelvin have all thrown light upon the properties of curved liquid surfaces. The Young–Laplace equation relates the pressure difference across a curved liquid interface to both the interfacial tension and curvature of the interface. Interfacial tension also gives rise to a dependence of the vapour pressure (and solubility) of a liquid on the curvature of its surface (e.g. drop radius), as expressed in the Kelvin equation. Common methods for measurement of interfacial tensions are described in an Appendix.

This chapter describes how W. H. Wollaston’s expertise in linear optics led him to design and patent in 1804 meniscus lenses for improved eye glasses which he called periscopic spectacles. He also suggested improvements to the camera lucida and microscopes that took advantage of curved lenses. He designed and patented in 1806 a novel and compact drawing aid he named a camera lucida, which was used by the sculptor Francis Chantrey to make preliminary drawings of his subjects. The device remained in wide use for sketching until well into the 20^th century. The chapter also discusses Wollaston’s1805 paper on the forces of moving bodies in which he placed emphasis on the work potential of a body supplying energy to another over a finite distance. Wollaston’s move into new social and scientific networks in London, such as the Chemistry Club and the Geological Society, is described.

As it more practical to measure the forces acting between two contacting surfaces then the energies of surfaces, this chapter covers those surface forces that are derived from surface energies. The starting point is Derjaguin’s approximation, which relates the energy between two flat surfaces to the force in other geometries: sphere/flat, sphere/sphere, and crossed cylinders. Next is a discussion of the surface forces in dry contacts with no liquid menisci around the contact points. This discussion covers the cases where adhesion causes significant deformation (JKR theory), where deformation is insignificant (DMT theory), and the cases in between. How surface roughness impacts adhesion is also discussed. The second half of this chapter deals with how liquid menisci around contacts contribute to adhesion forces, both for the sphere-on-flat geometry and for contacting rough surfaces.

This chapter focuses on the two experimental techniques—the surface force apparatus (SFA) and the atomic force microscope (AFM)—that are commonly used for measuring molecular level forces that act between two surfaces at small separation distances. The first part of this chapter covers the fundamental principles of SFA and AFM design. The second half of this chapter illustrates the application of AFM to measuring surface forces with examples the measurement of van der Waals forces, atomic level repulsive forces, frictional forces, electrostatic double-layer forces, and meniscus forces from liquid films and from capillary condensation.