Adhesion and short-range forces between surfaces. Part II: Effects of surface lattice mismatch
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The adhesion forces and interaction potentials between two mica surfaces as a function of the orientation (twist angle) of their surface lattices are reported. The forces were measured in air, in water, and in an aqueous KC1 solution where oscillatory structural forces are present. In air, the adhesion is relatively independent of the twist angle 6 in the range -10° < 8 < +10° due to a 0.4 nm thick amorphous layer at the interface. In water, apart from a relatively angle-independent baseline adhesion, a sharp adhesion peak (energy minimum) occurs at 6 = 0°, corresponding to maximum alignment of the surface lattices. As little as ±1° away from this peak the energy decreases by 50%. In aqueous KC1 solution, due to potassium ion adsorption the water between the surfaces becomes ordered, resulting in an oscillatory structural force where the adhesive minima occur at discrete separations corresponding to an integral number of water layers. The adhesion energies corresponding to the first three potential minima were angle dependent near 0 = 0° (again decreasing by 50% at ±1° away from 6 = 0°). The repulsive maxima were also affected near 6 = 0°. The results show that the whole interaction potential between two surfaces in liquids depends on the orientation of the surface lattices, and that these effects can extend at least four molecular layers. We discuss the consequences of these findings for material properties such as grain boundary energies, cracks, and friction.
I. INTRODUCTION
The force between two molecularly smooth surfaces in liquids is known to depend on the molecular structure of the liquid molecules. This is especially true at short range, below a surface separation of about ten molecular solvent diameters, where the force is usually an oscillatory function of the distance.1'2 These oscillatory or structural forces arise from the molecular geometry and local structure of the liquid medium and reflect the ordering of the liquid molecules into discrete layers when constrained between two surfaces. In addition to the forces being dictated by the intervening medium, the properties of the surfaces also play a crucial role. These properties include the chemical nature, crystal structure, and/or roughness of the surfaces.3"6 For example, if—due to surface roughness—the real contact area is only a small fraction of the apparent (macroscopic) contact area, the adhesion will be much less than the maximum theoretical value. Although the crystal structure is known to affect the short-range forces between two solid surfaces across a medium, this has never been directly measured nor explored theoretically. On the other hand, many studies have been conducted between contacting surfaces in vacuum. For example, the surface energy of a crystalline solid in a vacuum is known to depend on which plane is exposed.5 Some insights have also been 2232
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 10, Oct 1990
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obtained from friction experiments. For example, the frictional forces of sapphire
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