The density profile and Gibbs adsorption of a near-critical fluid confinedbetween two identical planar walls is studied by means of Monte Carlo simulation and by density functional theory for a Lennard-Jonesfluid. By reducing the strength of wall-fluid interactions relative tofluid-fluid interactions we observe a crossover from behaviour characteristicof the normal surface universality class, strong critical adsorption, tobehaviour characteristic of a 'neutral' wall. The crossover is reminiscent ofthat which occurs near the ordinary surface transition in Ising films subjectto vanishing surface fields. For the 'neutral' wall the density profile, awayfrom the walls, is almost constant throughout the slit capillary and gives riseto an adsorption that is constant along the critical isochore. The same'neutral' wall yields a line of capillary coexistence that is almost identicalto the bulk coexistence line. In the crossover regime we observe features inthe density profile similar to those found in the magnetisation profile of thecritical Ising film subject to weak surface fields, namely two smooth maxima,located away from the walls, which merge into a single maximum at midpoint asthe strength of the wall-fluid interaction is reduced or as the distancebetween walls is decreased. We discuss similarities and differences between thesurface critical behaviour of fluids and of Ising magnets.
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