ADHESION FORCES BETWEEN SURFACES IN LIQUIDS AND CONDENSABLE VAPORS

When a liquid is confined within a highly restricted space its properties become quantitatively and qualitatively different from the bulk (continuum) values: for example, when confined between two surfaces the molecules of a liquid may structure into quasi-discrete layers whose properties become "quantized" with the number of layers, and such ultra-thin films often behave more like a solid or a liquid crystal than a normal liquid, for example, withstanding finite compressive and shear stresses. The forces between surfaces or particles in liquids can also be very complex once the surfaces approach closer than 5-10 molecular solvent diameters. This is the regime where continuum and mean field theories (e.g., of monotonically attractive van der Waals forces) break down and where the interactions become sensitive to such fine details as the molecular structure of the liquid molecules and surfaces. These short-range forces determine the adhesion and friction between surfaces and the properties of colloidal dispersions. During the last ten years experiments using force balance techniques such as the surface forces apparatus, and theories employing computer simulations, have totally changed our conception of the short-range forces in liquids. It is now known that the force-laws are rarely monotonically attractive or repulsive: the force can change sign at some small but finite separation or it can be an oscillatory function of separation. Some of these forces are now well understood, but the more important ones are not. These include the repulsive "hydration" and attractive "hydrophobic" forces between hydrophilic or hydrophobic surfaces in water, the forces between metallic surfaces, and the role of static and dynamic roughness on adhesion. An understanding of these interactions is both of fundamental interest as well as having immediate practical applications in controlling the properties of materials and complex macromolecular (e.g., colloidal) systems. Here we shall review the current state of our knowledge in this area, where to a large extent, experiment is ahead of theory.