The effect of pillar surface fraction and pillar height on contact angles using molecular dynamics

Predicting hydrophobicity is very important in manufacturing products with self-cleaning properties. This study focuses on the effect of graphite surfaces with pillars of different surface fractions and heights on the contact angles of a nano-sized water droplet. This study used molecular dynamics simulations to investigate the hydrophobic properties of water droplets on the nano-scale. The contact angles were calculated and averaged over time for each case. Results showed the droplets in either the Wenzel state or the Cassie state. In general, as the pillar height increases the static contact angle increases to a certain point when the pillar height no longer has a significant effect on the contact angle. Over all, the smaller the pillar surface fraction the larger the change in the contact angle as the pillar height increased. As pillar surface fractions decreased from 36%, simulated contact angles undershot Cassie predictions at increasing amounts due to part of the water droplet sagging below the pillar tops. Graphite displayed anisotropic characteristics due to its layered structure which caused the contact angles to increase as surface roughness increased even though its Young contact angle was less than 90 degrees.