Effect of Gravity and Confinement on 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 ∝ L-(βδ/ν + 1). For opposing walls (with h1 = - hL) we confirm that the bulk two-phase coexistence is suppressed, for g = 0, to below the wetting temperature Tw. However, for small g the 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 (∞) ∝ L-1/ν, conform with finite-size scaling.