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 ∼ t1/3.5. 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.