SURFACE AMPLITUDE RATIOS AND NUCLEATED WETTING NEAR A CRITICAL END-POINT

We examine two recently discovered phenomena which occur at the liquid /vapor surface of a critical binary liquid mixture; namely, (i) the un iversality of the critical adsorption amplitude ratio and (ii) the obs ervation of the nucleation of wetting droplets after a temperature que nch from the one phase into the two phase region. The recent theoretic al work of Fisher and Upton on the universality of the surface tension amplitude ratio suggests that the ratio of the critical adsorption am plitude above and below T(c) should also be universal. This is indeed found to be the case for five critical liquid mixtures where the modif ied critical adsorption amplitude ratio has an experimental value of R (MA) = 1.18 +/- 0.13. This result agrees reasonable well with the RG c alculations of Diehl and Smock. In the two-phase region, for sufficien tly small reduced temperatures t, the critical liquid mixture hexadeca ne + acetone has an acetone-rich wetting layer at the liquid/vapor sur face at equilibrium. We examine the time development of this wetting l ayer after a temperature quench from the one phase into the two phase region. The liquid/vapor surface initially drops into a metastable sur face state which is well described by critical adsorption. The lifetim e ,tau0, of this metastable state is many hours and depends upon the r educed temperature, t. At the nucleation time tau0 acetone-rich drople ts nucleate and grow at the liquid/vapor surface; good agreement is fo und for tau0 between theory and experiment for t greater-than-or-equal -to 8 x 10(-5). An essential ingredient of the theory is the inclusion of a line tension term in the free energy of the nucleated droplet. A t large times tau the droplets coalesce into a metastable wetting film which eventually undergoes a hydrodynamic instability to an equilibri um wetting film.