SOLVATION FORCES IN THIN-FILMS CONFINED BETWEEN MACROSCOPICALLY CURVED SUBSTRATES

The microscopic structure of molecularly thin fluid films confined bet ween solid substrates with macroscopically curved surfaces is investig ated by means of grand canonical ensemble Monte Carlo (GCEMC) simulati ons, in which the thermodynamic state of the film is determined by its chemical potential mu and temperature T. This situation is akin to ex periments involving the surface forces apparatus (SFA) in which the fi lm is confined between two crossed cylinders of macroscopic radius R. The key quantity measured directly in SFA experiments is the ''force p er radius R,'' F(h)/R, exerted by the film on the curved surfaces. Thi s ''solvation force'' can be related to the local stress T-zz(h) norma l to the locally planar surfaces, where h is the shortest:distance bet ween them. Because T-zz(h) and the microscopic structure of the confin ed film can be computed by GCEMC, the relation between T-zz and the ma croscopically defined quantity F/R can be employed to interpret the de pendence of the latter in terms of variations of the film's local micr oscopic structure with h. For a liquid-crystalline film it is shown th at reorientational effects are distinctly manifest in T-zz(h) but are reflected only weakly in F(h)/R and are therefore likely to be missed if the interpretation of SFA experiments is based solely upon the latt er. Pseudo-experimental F(h)/R curves generated by GCEMC for a nematic liquid-crystalline film are in qualitative agreement with recent SFA data [M. Ruths, S. Steinberg, and J. N. Israelachvili, Langmuir 12, 66 37 (1996)], which suggests that one may gain deeper insight into the m icroscopic structure of confined films through a decomposition of expe rimentally determined solvation forces F(h)/R into T-zz(h) by invertin g the integral relation linking the two. (C) 1998 American Institute o f Physics. [S0021-9606(98)50625-3].