Capillary waves at the nanoscale
Intrinsic surface and nanoscale capillary waves
Using molecular dynamics simulations of simple liquid slabs, we analized liquid-vapor interfases: their structure, dynamics and also the kinetics of adsorbed atoms with the objective of a better understanding of the liquid-vapour interfase at molecular scale. An important problem is to elucidate the very existence of the intrinsic surface: a concept which has been, and continues to be polemic, because of the difficult interpretation of an instantaneous surface location at the (fuzzy) molecular scale.
In order to reconstruct plausible solutions of the instantaneous liquid-vapor interfase we use a percolative procedure called Intrinsic Sampling Method [Phys. Rev. Lett. 91, 166103]. The interfases so constructed only depends of a free parameter determining the number of atoms per unit area (occupation number). From these surface reconstruction it is possible to provide (via the equipartition energy of the surface Fourier modes) a structural measure surface tension and its dependece with the wavenumber. In Phys. Rev. Lett. 101, 106102 (2008) we follow a different, dynamic, route to the surface tension. Nanoscopic capillary waves (CW) are ussually overdamped by viscosity and the damping rate can be obtained from the exponential decorreation of the surface modes in time. Using the theoretical hydrodynamic dispersion relation, the damping rate leads to a dynamic measure of the surface tension. The dynamic prediction is found to be more robust that the structural one (i.e. independent on the occupation number). For a certain (optimum) value of the occupation number both (structural and dynamic) measures of surface tension coincides. Interestingly enough the surface atom kinetics indicate that surfaces with this optimum occupation number have the largest deadsortion times (surface atoms are more stable meaning that the surface is better defined). This result strongly supports the existence of the intrinsic surface, which provides consistent structural, dynamic and kinetics. Finally, the hydrodynamic picture remains valid up to surprisingly small wavelengths, of about four molecular diameters. At shorter scales, surface tension driven CWs cease to exist and we find a transition to a molecular diffusion regime.
Co-workers:
Pedro Tarazona (UAM) ,
Enrique Chacon (CSIC)