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Abstract
The present state of experimental knowledge concerning the forces between surfaces in liquids is reviewed, with particular attention given to results obtained from direct measurements using molecularly smooth surfaces. It is concluded that results on van der Waals and electric double-layer forces are in excellent agreement with current theories. Evidence is given for the existence of ‘structural forces’ those arising from a surface-induced enhancement or modification of liquid structure, though the controversial question of their existence in aqueous solutions remains unresolved. The forces in polymer solutions appear to be complex, exhibiting hysteresis and time-dependent effects, and may well be determined by non-equilibrium relaxation mechanisms. Finally, adhesion forces are considered: the Johnson and Derjaguin theories are compared, and results are presented which show that the contribution of surface tension (meniscus) forces to the adhesion of curved surfaces in contact is accurately described by equations derived from bulk thermodynamics clown to meniscus radii as small as 1 nm.
