Gravity, confinement, and phase equilibria

Abstract

We study a fluid or Ising lattice gas confined between two horizontal walls with separation L, in a gravitational field of strength g. For identical walls (with equal surface fields h1 = hL) the familiar capillary condensation and capillary criticality extend to non-zero g. Finite-size scaling predicts that the locus of criticality shrinks to g = 0 for L→∞, in the manner g α L−(βδ/ν+1). For opposing walls (with h1 = –hL) we confirm that bulk two-phase coexistence is suppressed, for g = 0, to below the wetting temperature Tw. However, for small g (and h1 of appropriate sign) bulk two-phase coexistence extends to higher temperatures that increase extremely rapidly with g, up to a maximum temperature that is shifted from the L = ∞ bulk critical point in the manner Tc,max(L)–Tc(∞)αL−1/ν, conform with finite-size scaling.