Adhesion and short-range forces between surfaces. Part II: Effects of surface lattice mismatch

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 KCl solution where oscillatory structural forces are present. In air, the adhesion is relatively independent of the twist angle θ in the range −10° < θ < +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 θ = 0°, corresponding to maximum alignment of the surface lattices. As little as ±1° away from this peak the energy decreases by 50%. In aqueous KCl 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 θ = 0° (again decreasing by 50% at ±1° away from θ = 0°). The repulsive maxima were also affected near θ = 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.