Motion of Crystalline Surfaces

Most theories of structural superlubricity are based on the Prandtl-Tomlinson model or the more advanced Frenkel-Kontorova model [1043, 1044], in which the single atom/tip is replaced by a chain of atoms coupled by springs. However, Friedel and de Gennes [1045] noted recently that correct description of relative sliding of crystalline surfaces should include the motion and interaction of dislocations at the surfaces. This concept was taken up by Merkle and Marks [1045] and generalized using the well-established coincident site lattice theory and dislocation drag from solid-state physics. 

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

Under conditions of step flow, the ability to grow good crystalline material is related to the mobility of the adatoms on the surface. These must be able to diffuse freely and find the proper crystal lattice sites for growth, wherever these are available. In this section, we discuss our calculations of the diffusion barriers on the Si (100) surface and the single-height steps. We shall restrict our discussion to the motion of adatoms even though there is considerable evidence that mass transport via dimer diffusion plays a role at high temperatures as well. ... 

Large numbers of functionalized LB films have been prepared. Highly ordered LB films have been formed by the inclusion of surface-active cobaltous phthalocyanine [168] amphiphilic TCNQ was assembled to function as conducting LB films [169] liquid-crystalline LB films, potentially capable of undergoing thermotropic or lyotropic phase transitions [170, 171], have also been generated. Spacer groups introduced into polymeric surfactants (23) helped to stabilize the LB films which they formed by decoupling the motion of pendant polymers (see Fig. 13) [172]. 

As we have seen in Section 6.6.1 such confined liquids may behave quite differently from the bulk lubricant. Near the surfaces, the formation of layered structures can lead to an oscillatory density profile (see Fig. 6.12). When these layered structures start to overlap, the confined liquid may undergo a phase transition to a crystalline or glassy state, as observed in surface force apparatus experiments [471,497-500], This is correlated with a strong increase in viscosity. Shearing of such solidified films, may lead to stick-slip motions. When a critical shear strength is exceeded, the film liquefies. The system relaxes by relative movement of the surfaces and the lubricant solidifies again. 

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