Distinct partial and complete droplet bouncing states on (super) hydrophobic leaf surfaces with ultra-low- to high-adhesion textural features

Abstract

We report on surface texture and tilting-based microdroplet impact dynamics on four distinct plant species, namely, Kalanchoe, Ziziphus, Mesua, and Litchi. Herein, conditions responsible for partial and complete rebound states are predicted after having analyzed droplet-shape evolution for distinct cases of (super)hydrophobicity and surface adhesion. The impacting water droplet with low Weber numbers (We ∼ 2.8) displays complete rebound characteristics for all the leaf-types under study. With an augment in We (∼14.4 and 25.8), low adhesion, superhydrophobic Kalanchoe leaf reveals a complete rebound feature, whereas, high-adhesion, superhydrophobic Ziziphus leaf exhibited an unusual partial rebound state, giving off “parent” and sticky “residual” parts. Interestingly, the dynamic partial wetting transition from the Cassie-to- Wenzel state is noted in this species upon droplet impact following the Weber number criterion, 5.7<Wecri<8.6. Furthermore, with an increase in surface inclination from ∼0° to 30°, the partial- to complete droplet rebound feature was seen on the Ziziphus leaf. In contrast, near superhydrophobic and high-adhesion Mesua and Litchi leaf surfaces showed complete rebounding features and oscillation prior to attaining static equilibrium. The maximum spreading parameter (βmax) ranges from ∼1.2 to 1.9 for all the leaf-types and with varied We (∼2.8, 14.4, and 25.8) and that it falls only marginally with an increase in tilting, witnessed for the highest We. Consequently, the contact time, τc alters from ∼12.2 to ∼10.2 ms within mentioned ranges of surface inclination, and We. Insights on droplet dynamics have potential in microfluidics, anti-icing, and greener pesticide spray management.