Universal Fast Mode and Potential-dependent Regimes in Wetting Kinetics

We present simulation results from a comprehensive molecular dynamics (MD) study of surface-directed spinodal decomposition (SDSD) in unstable symmetric binary mixtures at wetting surfaces. We consider long-ranged and short-ranged surface fields to investigate the early-stage wetting kinetics. The attractive part of the long-ranged potential is of the form $V(z) \sim z^{-n}$, where $z$ is the distance from the surface and $n$ is the power-law exponent. We find that the wetting-layer thickness $R_1(t)$ at very early times exhibits a power-law growth with an exponent $\alpha = 1/(n+2)$. It then crosses over to a universal fast-mode regime with $\alpha=3/2$. In contrast, for the short-ranged surface potential, a logarithmic behavior in $R_1(t)$ is observed at initial times. Remarkably, similar rapid growth is seen in this case too. We provide phenomenological arguments to understand these growth laws. Our MD results firmly establish the existence of universal fast-mode kinetics and settle the related controversy.