STIFFNESS INSTABILITY IN SHORT-RANGE CRITICAL WETTING

Recent theoretical work has shown that an interface separating two fluid phases suffers changes in its (bare) effective stiffness, SIGMA(l) = SIGMA(infinity) + DELTASIGMA(l), when located at a distance l from a planar wall: terms varying as l(k)e(-jkappal) appear in DELTASIGMA (where 0 less-than-or-equal-to k less-than-or-equal-to j = 1, 2.... and kappa is the inverse bulk correlation length in the fluid wetting the wall). This may induce first-order wetting transitions when critical wetting had been expected. This general behavior of DELTASIGMA(l) is confirmed using an integral/adsorption constraint to determine 1, in place of the original crossing constraint. The exact linearized functional renormalization-group technique is used to analyze the full wetting-phase diagram as a function of T, of omega = k(B)T(cW)kappa2/4piSIGMA(T(cW)), and of q, the amplitude of the -le-2kappal term in DELTASIGMA. For dimensions d > 3, any positive q (as generally expected) yields first-order wetting. The same is true for d = 3 provided omega < 1/2; but when omega > 1/2 nonclassical critical behavior is still found for small q < q(t)(omega) > 0. Detailed expressions are obtained for [l], xi(parallel-to), etc., in the various critical and first-order regions. Numerical estimates show that previous Ising-model simulations probably encountered weakly first-order wetting transitions which might explain discrepancies with earlier renormalization-group predictions.