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

Abstract

In this paper we investigate the phase behaviour of a `simple' fluid confined to a slit of nanoscopic width sz by chemically decorated, plane-parallel substrates consisting of slabs of weakly and strongly adsorbing solid which alternate in the x-direction with period sx. In the y-direction the substrates, occupying the half-spaces - z - sz/2 and sz/2 z , are translationally invariant. On account of the interplay between confinement (i.e., sz) 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 sx (0 ½) 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 sx for a mean-field 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.