Capillary wave spectrum at adsorbed liquid films

Capillary wave fluctuations at the edges of liquid wetting layers are analyzed in Monte Carlo simulations of a Lennard-Jones fluid adsorbed on a planar wall substrate. The analysis is based on the Fourier modes of the liquid surface, constructed via the intrinsic sampling method. For films thinner than four molecular layers we can quantify the damping of the capillary waves due to the wall potential. Our results are presented in the theoretical framework of the effective surface Hamiltonians, to establish a quantitative link between the molecular structure and the mesoscopic descriptions used for renormalization-group analysis. We observe the predicted exponential decay of the wall damping, with the correlation length of the liquid bulk. However, associated with the molecular layering of the density profiles, we observe a strong oscillatory dependence with respect to the mean film thickness, not included in any theoretical prediction. We discuss the possibility of finite-size effects in this respect. Three different definitions of the intrinsic surface at molecular level are tested, to show that the square gradient terms of the Hamiltonian are robust, while the surface bending (or nonlocal) terms are tied to each specific definitionWe acknowledge the support of the Spanish Minister of Science and Innovation (Grant No. FIS2010-22047-C05) and the Comunidad Autónoma de Madrid under program MODELICO (Grant No. S2009/ESP-1691