2005/05/11 by Klaus Mecke, S. Dietrich, Siegfried Dietrich
Engineering · Physics and Astronomy · #Fluid Dynamics and Thin Films #Phase Equilibria and Thermodynamics #Spectroscopy and Quantum Chemical Studies #cond-mat.soft #cond-mat.stat-mech
paper · pdf · doi:10.1063/1.2121507
29 pages, 12 figures
arxiv created 2005/05/11 · openalex publication_date 2005/11/22 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Thermal fluctuations cause the local normal vectors of fluid interfaces to deviate from the vertical direction defined by the flat mean interface position. This leads to a nonzero mean value of the corresponding polar tilt angle which renders a characterization of the thermal state of an interface. Based on the concept of an effective interface Hamiltonian we determine the variances of the local interface position and of its lateral derivatives. This leads to the probability distribution functions for the metric of the interface and for the tilt angle which allows us to calculate its mean value and its mean-square deviation. We compare the temperature dependences of these quantities as predicted by the simple capillary-wave model, by an improved phenomenological model, and by the microscopic effective interface Hamiltonian derived from density-functional theory. The mean tilt angle discriminates clearly between these theoretical approaches and emphasizes the importance of the variation of the surface tension at small wavelengths. Also the tilt angle two-point correlation function is determined which renders an additional structural characterization of interfacial fluctuations. Various experimental accesses to measure the local orientational fluctuations are discussed.