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

Abstract

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 ∼ 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.

Energies Associated with a Meniscus along a Flat Vertical Wall

“…Experimental observations at the nanoscopic scale of contact lines actively advancing across partially wetting surfaces are scare . However, molecular simulations show that liquids can immediately establish a local contact angle on partially wetted surfaces that is similar to the final macroscopic value ( θ c ≈ θ), , displaying the sorts of shapes depicted in Figure .…”

Molecular Origin of Wetting Characteristics on Mineral Surfaces

“…In the later stage, however, the streamline near the contact line becomes more parallel to mineral surfaces with much slower velocity . In fact, the value of α in the initial wetting stage is also related to the initial distance between mineral surfaces and water droplet, the structure of mineral surfaces, and even the size of the droplet. , …”

“…Considering the simulation accuracy and computation efficiency, the SPC/E water model is used to simulate water molecules . The ClayFF force field is utilized to model the interactions between the atoms or ions in montmorillonite, quartz, and pyrophyllite. ,, For the potential parameters of kerogen and calcite, one can refer to previous works. ,, All potential parameters used in our simulations are provided in Tables S1 and S2 in the SI.…”

“…To calculate the long-range Coulombic interactions beyond 1 nm, the particle–particle particle–mesh method is used . For the sake of computational efficiency, the bonded interactions in water molecules are constrained via the SHAKE algorithm. , All MD simulations in the current work are performed on the large-scale atomic/molecular massively parallel simulator (LAMMPS) …”

“…Apart from wettability, other wetting characteristics, such as the formation of precursor films, the advancement mode of the contact line and the precursor film, and the evolution of the solid–liquid friction coefficient, would also significantly influence the wetting process and fate of water on solid surfaces. Simulation results of wetting on a flat surface have shown that stronger wettability tends to accelerate the spreading of the precursor film. , In the absence of the precursor film, the evolution of solid–liquid friction, which is closely related to the advancement mode of the contact line, significantly affects the wetting process as well . The molecular-level structures and chemistries of mineral surfaces are also expected to influence the wetting characteristics and thus make the problem more complex.…”