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

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/Fw water 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 a 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.

On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics

“…The water contact angle over Wall05 exhibits a marked system-size dependence even up to 256,000 water molecules (128,000 beads) Larger drops exhibit a less pronounced but nevertheless noticeable size dependence. It is interesting to note that the effect of this scale up is an increase in the hydrophobicity of this surface in correspondence with previous simulations [32] and experimental studies at the macroscale. For this substrate, a system with 1,024,000 water molecules shows an apparent limiting contact angle of 74.30° [8].…”

Surface tension of ab initio liquid water at the water-air interface

“…73 , the different surface tensions between the current study and Ref. 73 can be attributed to the artificial truncation of the LJ and electrostatic forces. First, we compared the surface tensions by changing the Gaussian basis set (DZVP, TZV2P, and QZV3P) using the BLYP XC functional and Grimme's D3 van der Waals correction.…”

“…73 However, since the LJ and electrostatic interactions were both truncated beyond the cutoff length of 10 Å in Ref. 73 , the different surface tensions between the current study and Ref. 73 can be attributed to the artificial truncation of the LJ and electrostatic forces.…”

“…Note that this value is also smaller than the 70.8 ± 1.9 mN/m reported previously. 73 However, since the LJ and electrostatic interactions were both truncated beyond the cutoff length of 10 Å in Ref. 73 , the different surface tensions between the current study and Ref.…”

Wettability of water droplet on misoriented graphene bilayer sructure: A molecular dynamics study

“…at equilibrium. The extraction of contact angle, θ C , is based on the most popular circular-fitting method, 21,[25][26][27] and is illustrated in Figure 1(c). At equilibrium, the contact angle θ C is calculated through the best circular fit to the positions of the outer layer water molecules with an extrapolation to the surface of top graphene layer, and water molecules within 0.8nm to the graphene surface are not included in the circular fit to avoid the influence from density fluctuations near the interface.…”