Spectral analysis of the velocity relaxation in nanoscale channels

We investigate the fluid velocity relaxation in a nanoscale channel using molecular dynamics simulations and hydrodynamic modeling. The relaxation dynamics are described in terms of the spatially resolved Fourier modes, and we establish from simulations that only modes with a wave vector equal to or smaller than the external driving wave vector are excited. The relaxation features a predominantly viscous behavior for small wave vectors and becomes more viscoelastic as the wave vector increases. From the Fourier mode analysis, we can reconstruct the spatially averaged relaxation using particular modes, and we conclude that two modes are governing the dynamics: the system's largest wavelength mode and the external driving mode. By comparing model output based on the Maxwell–Wiechert model and the modified Maxwell model to simulation data, we see that the latter model performs best. Moreover, from this model, we conclude that the system features a wave vector-independent reduced viscous response and a wave vector-dependent enhanced elastic response for the wavelengths studied here.