Effect of Solid-Liquid Interactions on Substrate Wettability and Dynamic Spreading of Nanodroplets: A Molecular Dynamics Study

Solid-liquid interaction plays a key role in substrate wettability and spreading dynamics of liquid droplets. Yet, how the solid-liquid interaction controls wettability and the spreading process is still not fully understood. Here, we employ molecular dynamics simulations to study water nanodroplet spreading on a flat substrate under a wide range of solid-liquid interaction strengths by varying the collision diameter and depth of the potential well in 12-6 Leonard-Jones potentials between water molecules and substrate atoms. We find that the substrate transitions from hydrophobic to hydrophilic with increasing solid-liquid interaction strength. We show that the cosine of the equilibrium contact angle increases linearly with the interaction strength below a critical value, beyond which the liquid spreads completely with a precursor film (PF) and a spreading radius (including both the bulk liquid and the PF) growing roughly as R similar to t(1/3.5). We also observe that an overlarge solid-liquid interaction strength hinders the growth of spreading radius R after obvious PFs with a thickness of one or two water molecules form on the substrate. We demonstrate that spreading phenomena are associated with the landscape of potential fields controlled by the solid-liquid interaction parameters. These new findings provide an improved understanding of spreading processes at the molecular scale.