Sound and shear wave attenuation in confined fluids*

Lubricated friction is an inherent multi-scale problem involving two surfaces under relative motion, separated by a viscous fluid. These surfaces typically have complex topography and chemistry, and together with the confined fluid, determine energy dissipation at the interface. We investigate sound and shear wave attenuation in confined fluids by means of both Molecular Dynamics (MD) and continuum simulations. For the latter, we show that the reduction of dimensionality in the lubrication equations leads to wavelength-independent attenuation coefficients. Instead, wave attenuation is determined by the geometry of the gap and fluid-wall interactions. This can lead to an underestimation of energy dissipation, when transferring continuum models to the molecular scale. On the other hand, MD simulations of confined fluids reveal a transition in their spectral attenuation behavior at characteristic wavelengths that are in the order of the gap height. In this talk, we will discuss the implications of this behavior for multi-scale simulations of friction.