| We consider a simple fluid confined between two parallel walls that are either (i) identical or (ii) exert different external fields so that one wall is attractive and is wet by liquid (solvophilic) while the other is repulsive and may be wet by the gas phase (solvophobic).Case (i) is well-studied: below the bulk critical temperature Tcb capillary condensation gives rise to a discontinuous jump in the solvation force (the excess pressure arising from fluid confinement) as a function of wall separation L or chemical potential. In case (ii) a 'delocalized interface phase' may form in the range Tcb >T> Tw, the wetting transition temperature. This phase exhibits a liquid-gas interface near L/2 with pronounced thermal fluctuations. We investigate the properties of this unusual phase using an effective interfacial Hamiltonian approach and a fully microscopic classical density functional theory. For the physically relevant case of a Lennard-Jones fluid, with r -6 interatomic attraction, we determine the scaling functions of the solvation force fs, the adsorption and the correlation length of interfacial fluctuations for temperatures removed from Tcb. We argue that these results remain valid beyond our mean-field treatment. In this temperature range, and for large separations, fs is repulsive and decays as L-3. We discuss the rich phase behaviour that occurs for smaller separations L. |
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