Dynamics of capillary evaporation. II. Free energy barriers

We investigate the free energy barrier of vapor tube formation in a metastable liquid confined between hydrophobic walls. The model we use is a lattice gas model with nearest neighbor interactions whose evaporation dynamics has been reported in the preceding paper (paper I). We apply transition state theory and a constrained umbrella sampling technique, taking as our transition state a vapor pocket in the middle of the liquid layer. The calculated transmission coefficients show that the size of a vapor pocket is indeed a reasonable order parameter to describe capillary evaporation. The umbrella sampling method gives estimates of free energy barrier for vapor tube formation that are within an order of magnitude agreement with direct Monte Carlo simulation runs. In all the cases studied, the estimated free energy barriers are much smaller than those predicted by a previous mean-field approach. (C) 2000 American Institute of Physics. [S0021-9606(00)50838-1].