Wetting of Water and Water/Ethanol Droplets on a Non-Polar Surface:  A Molecular Dynamics Study

Spreading of sessile water droplets on a solid graphite surface has been studied using atomistic molecular dynamics techniques. The system is in the partial wetting regime and a straightforward scheme is applied to extract the contact angle from the atomistic simulations. Even for droplets containing a thousand molecules, the calculated contact angle for water is 83°, which is close to the experimental macroscopic value. On addition of ethanol, the contact angle decreases to 30° and we approach complete wetting. Time-averaged structural analyses show that the ethanol molecules form a monolayer on top of the solid surface. A few Å above the solid surface, the ethanol molecules are preferentially located at the liquid/vapor interface rather than in the bulk of the droplet.