Molecular dynamics study of capillary wave damping by monolayer films at the air-water interface*

Capillary waves (CW, wavelength ≤ 1 cm) at the air-water interface are governed by the restoring force of surface tension with damping by viscous dissipation. Long-chain amphiphiles can form a monolayer film via self-assembly at the water surface. The viscoelastic behaviour of monolayers can affect the CW frequency and damping coefficient in a dynamic manner. Previous studies which relied on light scattering techniques and hydrodynamic theories were able to study the CW damping by monolayers down to the wavelength of several microns. However, at the nanoscale, lacking suitable experimental techniques, the validity of the hydrodynamic description on CW damping by monolayers remains unknown. In this study, the CW damping effect of monolayers are investigated using molecular dynamics (MD) simulations. The time correlation of interfacial profiles are collected under the influence of palmitic acid (C₁₆H₃₂O₂) monolayer. The monolayer phase transition and viscoelastic properties are examined, and the validity of hydrodynamic theory at the nanoscale is discussed.