Wetting and contact-line effects for spherical and cylindrical droplets on graphene layers: A comparative molecular-dynamics investigation

TL;DR

This study uses molecular dynamics simulations to refine water-graphene interaction parameters and compares contact angles of spherical and cylindrical droplets, revealing size-dependent effects and limitations of classical models.

Contribution

It provides a new estimate for the oxygen-carbon interaction in water-graphene systems and highlights the limitations of Young's equation at nanoscale droplet sizes.

Findings

Refined the water-graphene interaction parameter to about 0.2 kJ/mol.

Showed that contact angles differ between spherical and cylindrical droplets.

Identified that classical contact angle models do not fully apply at nanoscale.

Abstract

In Molecular Dynamics (MD) simulations, interactions between water molecules and graphitic surfaces are often modeled as a simple Lennard-Jones potential between oxygen and carbon atoms. A possible method for tuning this parameter consists of simulating a water nanodroplet on a flat graphitic surface, measuring the equilibrium contact angle, extrapolating it to the limit of a macroscopic droplet and finally matching this quantity to experimental results. Considering recent evidence demonstrating that the contact angle of water on a graphitic plane is much higher than what was previously reported, we estimate the oxygen-carbon interaction for the recent SPC/Fwwater model. Results indicate a value of about 0.2 kJ/mol, much lower than previous estimations. We then perform simulations of cylindrical water filaments on graphitic surfaces, in order to compare and correlate contact angles resulting from these two different systems. Results suggest that modified Young's equation does not describe the relation between contact angle and drop size in the case of extremely small systems and that contributions different from the one deriving from contact line tension should be taken into account.