A Molecular Dynamics Simulation of Water Droplet in Contact with a Platinum Surface

A water droplet in contact with a platinum surface was simulated by the molecular dynamics method. Water molecules were modeled with the well-known SPC/E model and the platinum surface was represented by three layers of harmonic molecules. The phantom molecules were used beneath the layers of platinum surface in order to mimic the constant temperature heat bath. As the potential function between water and platinum surface, two different models developed by Spohr & Heinzinger (1988) and Zhu & Philpott (1994) were employed. These potential models were derived from the extended Hükel calculations between a water molecule and a platinum cluster. As the initial condition, a water droplet with the density of liquid water at 350 K was placed on the center of the platinum surface. For the initial 100 ps, velocity scaling was used for the temperature control. After this initial rough control, only the phantom heat bath was used as the temperature control at 350K. During the velocity-scaling temperature control in 100 ps, the liquid vapor interface was already in semi-spherical shape. Then, the gradual spreading of the water droplet was observed. In the spreading process of a liquid droplet on a platinum surface, the area of contact region between water and platinum expanded just in proportion to the one-third power of time at early stage and later to the one-fifth power of time. On the other hand, for Lennard-Jones fluid, it was reported that the spread of interfacial area was proportional to the logarithmic or square of time. Even though the water droplet finally spread to a monolayer film on a fcc (111) surface with the S-H potential, a stable droplet structure on a monolayer film was realized with the Z-P potential as shown in Fig. A-1 and Fig. A-2. This is the first realization of stable droplet structure on the monolayer film with molecular dynamics level calculation. The mechanism of the ‘drop on film’ structure was explained by the very concentrated monolayer film of water. The dense monolayer repulses the other molecules, so that the effective potential of the surface is weakened. Hence, the layer concentration of the monolayer leads to the larger contact angle (less wettable). Furthermore, comparing three different platinum surface structures, (111), (100) and (110), the contact angle varied drastically depending on the density of the monolayer and it was largest on the fcc (100) surface. Figure A-1 Snapshot of water droplet on fcc(111) platinum surface (Z-P potential). 0 10 20 30 40 50 60 0 10 20 30 40 50 Radius [Å] H ei gh t [ Å]