Experiments were performed in which liquid droplets were videographically recorded impacting horizontal superhydrophobic surfaces. The superhydrophobic surfaces were micropatterned with alternating ribs and cavities and coated with a hydrophobic coating. The following surface types were also tested for comparison: smooth uncoated, micropatterned uncoated, and smooth coated surfaces. Three liquid types were used: pure water, ethanol, and a 50/50 water/glycerine mixture. Acquired data demonstrated that the maximum droplet spread diameter exhibited a greater Weber number dependence than that previously reported in the literature. The time delays between impact and maximum spread and between impact and ejection of a vertical jet were characterized, and it was found that experiments with hydrophilic surface behavior follow somewhat different trends than those with hydrophobic behavior, and that there are modest differences between superhydrophobic and hydrophobic surfaces. When analyzing the velocity of the issuing vertical jet, a region of micro-jets was observed with velocities that, under certain conditions, can exceed 15 times the impact velocity. The experimental data acquired were also compared to two recent models from the literature and it was determined that the models do not adequately account for surface anisotropy or apparent slip at the solid-liquid interface. The experiments also showed that instabilities resulting in fingering are dependent upon surface and fluid type, but not contact angle. The onset of peripheral splashing was observed, in general, to occur at a lower Weber number as contact angle increased for the differing surfaces. For surfaces with rib and cavity features, the droplet spread and retraction were generally observed to be asymmetric with spread and retraction faster along the length of the ribs. The occurrence of two-pronged and oscillating jets for water/glycerine tests was also observed for all patterned surfaces. Lastly, an interesting spread pattern with four liquid droplets clustered at about 30° from the perpendicular direction was observed for all fluid types on patterned surfaces for high Weber numbers.



College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering



Date Submitted


Document Type





John Pearson, superhydrophobic, droplet, impingement