Molecular dynamics simulations of the contact angle between water droplets and graphite surfaces
Fluid Phase Equilibria 332 (2012) 173? 177
Danilo Sergi*, Giulio Scocchi, Alberto Ortona
University of Applied Sciences (SUPSI), The iCIMSI Research Institute, Galleria 2, CH-6928 Manno, Switzerland
The authors of the paper used molecular dynamics (MD) simulations to investigate the wettability of graphite. This was done by determining the equilibrium contact angle and relating this to the modified Young equation.
The study used LAMMPS for their simulations. At first, a cubic box with length 30 ? contained 512 molecules of water, which was modeled using SPC-Fw. Equilibration time under NPT ensemble lasted for 200 ps, with 1 fs time step. Temperature and pressure were kept constant at 298.16 K and 1 atm respectively. But for the actual determination of wetting characteristics, NVE conditions were used for 0.5 ns.
Upon equilibrating water, the next simulation done was the wetting experiment. The oxygen atoms of water and carbon atoms of graphene were made to interact under Van der Waal’s forces. Equilibration of this setup lasted for 0.5 ns under NVT. Production simulations ran for 1 ns, dumping frames at 0.5 ps intervals.
Properties calculated from the simulations include water density, oxygen-oxygen radial distribution function (RDF), and contact angle. The size of droplets range from 30 to 80 ?. The contact angle measurements of these droplets were compared to the ones predicted by the Young equation. Moreover, the authors were able to obtain macroscopic contact angle from the average of the largest droplets. It was also observed that the transition for reaching the macroscopic limit starts with the 60-? radius.
Contribution and application:
The topic of this paper, which is wettability of graphite, doesn’t really have practical importance to what our lab is currently researching. However, this paper shows detailed procedures for calculating density of water as well as RDF and contact angle. In addition, the authors also provided details about the simulations which are very useful for learning how to do MD simulations.
By: Hannah Ebro