posted on 2021-07-12, 19:34authored byYixin Zhang, Duncan A. Lockerby, James E. Sprittles
The
relaxation dynamics of thermal capillary waves for nanoscale
liquid films on anisotropic-slip substrates are investigated using
both molecular dynamics (MD) simulations and a Langevin model. The
anisotropy of slip on substrates is achieved using a specific lattice
plane of a face-centered cubic lattice. This surface’s anisotropy
breaks the simple scalar proportionality between slip velocity and
wall shear stress and requires the introduction of a slip-coefficient
tensor. The Langevin equation can describe both the growth of capillary
wave spectra and the relaxation of capillary wave correlations, with
the former providing a time scale for the surface to reach thermal
equilibrium. Temporal correlations of interfacial Fourier modes, measured
at thermal equilibrium in MD, demonstrate that (i) larger slip lengths
lead to a faster decay in wave correlations and (ii) unlike isotropic-slip
substrates, the time correlations of waves on anisotropic-slip substrates
are wave-direction-dependent. These findings emerge naturally from
the proposed Langevin equation, which becomes wave-direction-dependent,
agrees well with MD results, and allows us to produce experimentally
verifiable predictions.