STICK-SLIP PHASE-TRANSITIONS IN CONFINED SOLID-LIKE FILMS FROM AN EQUILIBRIUM PERSPECTIVE

We investigate the transition from stick to slip conditions in sheared monolayer films confined between two plane parallel solid substrates. Each substrate consists of N-s atoms rigidly fixed in the (100) confi guration of the face-centered cubic lattice. Shearing of the film is e ffected in a quasistatic (reversible) process in view of the low shear rates (on a molecular scale) in corresponding laboratory experiments employing the surface forces apparatus (SFA). To mimic operating condi tions of the SFA as closely as possible we employ the grand isostress ensemble in which the temperature T, the chemical potential mu of the film, the stress T-zz exerted normally on the substrates, and the shea r stress T-z,T-x acting on the film in the x direction are among the t hermodynamic state parameters. We analyze the average transverse align ment of the substrates (i.e., the registry) [alpha(x)l] (where l is th e lattice constant of the substrate) and its fluctuations xi(2): = [(a lpha(x)l - [alpha(x)l])(2)] in corresponding Monte Carlo simulations. Up to the so-called yield point [alpha(x)(yield)l], [alpha(x)l] increa ses with T-z,T-x; in the thermodynamic limit T-z,T-x reaches its maxim um at [alpha(x)(yield)l] and xi(2) --> infinity. For [alpha(x)l] less than or equal to [alpha(x)(yield)l] the substrate ''sticks'' to the fi lm; for [alpha(x)l] > [alpha(x)(yield)l] the substrate ''slips'' acros s the film's surface. States characterized by [alpha(x)l] > [alpha(x)( yield)l] are inaccessible in the grand isostress ensemble because they are thermodynamically unstable. Thus, the stick-slip phase transition occurs at [alpha(x)(yield)l] in the thermodynamic limit. An analysis of the grand isostress potential indicates that stick-slip transitions can be viewed as continuous phase transitions where the yield point i s an analog of the (liquid-gas) critical point. In a finite system the stick-slip phase transition occurs at rupture points [alpha(x)(ruptur e)l] < [alpha(x)(yield)l] because xi(2) may exceed a system-size-depen dent free-energy barrier.