Droplet Impact on the Superhydrophobic Surface with Singularities

Reducing the contact time of an impacting droplet is highly desirable in various industrial fields including anti-icing. With the straightforward upscaling advantage, singularities on superhydrophobic surfaces can induce an annular rebound with a limited reduction in contact time. To break this limitation and further reduce contact time, this study focuses on optimizing the singularity number and arrangement. The effects of the singularity number and dimensionless spacing (l* scaled by the droplet diameter) on the dynamic and contact time characteristics of a droplet impacting the superhydrophobic surface are experimentally studied under varying Weber numbers (We). The experimental results indicate that in comparison to the single singularity, two singularities with l* < 1.0 can generate two liquid rings with four lateral liquid subunits due to the impalement at the high We region. Owing to the reduced equivalent diameter of the subunit, increasing We results in a gradually decreased contact time and accordingly breaks the limitation. However, the liquid film cannot be pierced at l* > 1.0 with a limited reduction. Considering the further reducing potential at l* < 1.0, four singularities are explored without a further reduced contact time due to the formed central liquid film. Using an additional central singularity, the central liquid film is pierced promoting its annular rebound. In consequence, five singularities significantly break the limitation in contact time, particularly a 61.7% reduction to the superhydrophobic flat surface at l* < 1.0.