Shear-induced phase transitions in fluids confined between chemically decorated substrates

In this paper we investigate the phase behaviour of a 'simple' fluid confined to a slit of nanoscopic width s(z) by chemically decorated, plane-parallel substrates consisting of slabs of weakly and strongly adsorbing solid which alternate in the x-direction with period s(x). In the y-direction the substrates, occupying the half-spaces -infinity less than or equal to z less than or equal to -s(z)/2 and s(z)/2 less than or equal to z less than or equal to infinity, are translationally invariant. On account of the interplay between confinement (i.e., s(z)) and chemical decoration, three fluid phases are thermodynamically permissible, namely (inhomogeneous) gaslike and liquidlike phases and 'bridge phases' consisting of high(er)-density fluid over the 'strong' part which alternates in the x-direction with low(er)-density fluid over the 'weak' part of the substrate. In the x-y plane the two are separated by an interface. Because of their lateral inhomogeneity, bridge phases can be exposed to a shear strain alpha s(x) (0 less than or equal to alpha less than or equal to 1/2) by misaligning the substrates in the x-direction. Depending on the thermodynamic state of the confined fluid and details of the chemical decoration, shear-induced first-order phase transitions are feasible during which a bridge phase may be transformed into either a gaslike (evaporation) or a liquidlike phase (condensation). These phase transitions are studied by computing phase diagrams as functions of ols, for a meanfield lattice-gas model. The lattice-gas calculations are amended by grand canonical ensemble Monte Carlo simulations of a fluid confined between chemically decorated substrate surfaces. The combination of the two sets of data reveals that the lattice-gas model captures correctly key characteristics of shear-induced first-order phase transitions in this rather complex system despite its mean field character.