Early-time wetting kinetics in surface-directed spinodal decomposition for off-critical quenches: A molecular dynamics study

We present results from the molecular dynamics (MD) simulation of surface-directed spinodal decomposition (SDSD) in binary fluid mixtures ($A+B$) with off-critical compositions. The aim is to elucidate the role of composition ratio in the early-time wetting kinetics under the influence of long-range surface potential. In our simulations, the attractive part of surface potential varies as V(z)= -εₐ/zⁿ, with εₐ being the surface-potential strength. The surface prefers `A' species to form the wetting layer. Its thickness [R₁(t)] for the majority wetting (number of A-type particles [NA] > number of B-type particles [NB]), grows as a power-law with an exponent $1/(n+2)$. This is consistent with the early-time kinetics in the form of potential-dependent growth present in the Puri-Binder model. However, for minority wetting (NA < NB), the growth exponent in R₁(t) is less than $1/(n+2)$. Furthermore, on decreasing the field strength εₐ, we recover $1/(n+2)$ for a minority wetting case. We provide phenomenological arguments to explain the early-time wetting kinetics for both cases.