I. Introduction
It is now known that a number of quite diverse interactions occur between two surfaces
depending on whether the interaction occurs in vacuum, in vapour or in a liquid. In practice,
it is also important to distinguish between static (i.e., equilibrium) forces and dynamic (e.g.,
frictional and other energy-dissipating) forces.
In cacuum there are the long-range van der Waals and electrostatic (Coulombic) forces,
while at smaller surface separations - corresponding to molecular contacts (D ~ 0.1-0.2 nm)
- there are additional forces such as covalent, hydrogen bonding and metallic bonding forces.
All these forces determine the adhesion between bodies of different geometries, the surface
and interracial energies of planar surfaces, and the strengths of materials, grain boundaries,
cracks, and other adhesive junctions. These adhesive forces are often strong enough to
elastically or plastically deform the shapes of two bodies or particles when they come into
contact.
When exposed to lJapours (e.g., atmospheric air) two solid surfaces in or close to contact
can now have a surface layer of chemisorbed or physisorbed molecules, or a capillary
condensed liquid bridge, between them. Each of these effects can drastically modify their
adhesion. The adhesion usually falls, but in the case of capillary condensation the additional
Laplace pressure, or attractive "capillary" force, between the surfaces may make the adhesion
stronger than in inert gas or vacuum.
When totally immersed in a liquid the force between two surfaces is once again completely
modified from that in vacuum or air (vapour). The van der Waals attraction is generally
reduced, but other forces now come into play which can qualitatively change both the range
and even the sign of the interaction. Again, the attractive forces can be either stronger or
weaker than in the absence of the intervening liquid medium, e.g., stronger in the case of two
hydrophobic surfaces, but weaker for two hydrophilic surfaces interacting in water. Furthermore,
the force may no longer be purely attractive. It can be repulsive, or the force can
change sign at some finite surface separation, D, and the potential energy minimum may now
occur not at molecular contact but at some small distance farther out.
Until only a few years ago it was believed that only two forces operated between two
surfaces in a liquid such as water - the attractive van der Waals force and the repulsive
electrostatic "double-layer" force. These two forces together form the basis of the well known
Derjaguin-Landau-Verwey-Overbeek, or DLVO theory [1], and are depicted schematically
in fig. 1.
More recent experiments have revealed that other types of attractive (and repulsive)
short-range forces can also arise in liquids, especially at short-range, i.e., at surface separations
below a few nanometers or a few molecular diameters. These forces can be extremely
varied and complex, much more so than was imagined only a few years ago. This realization is
partly due to the ever increasing complexity of the systems being studied; for example, the
liquids are no longer simple one-component liquids but can consist of a polydisperse mixture
of anisotropic polar, amphiphilic or polymeric molecules. In addition, the two surfaces
themselves can be amorphous or crystalline, crystallographically matched or not, rough or
smooth, rigid or fluid-like (soft), hydrophilic or hydrophobic. All these factors are now
recognized as being critically important in determining the strength of the adhesion in
different systems, depending both on the physical and chemical properties of the surfaces as
well as on the nature of the bathing liquid or condensable vapours in the atmosphere.
In particular, both experiments and theory have shown that conventional continuum or
"jellium" theories are generally inadequate for describing the short-range interactions be-