Unraveling the interplay of leaf structure and wettability: A comparative study on superhydrophobic leaves of Cassia tora, Adiantum capillus-veneris, and Bauhinia variegata

In this article, superhydrophobic leaves of Cassia tora, Adiantum capillus-veneris (ACV), and Bauhinia variegata are reported for the first time, and the wettability of these leaf's surfaces was correlated with their surface morphology at micro- and nanoscale. Field Emission Scanning Electron Microscopy (FESEM) images of the surfaces were used to get surface morphological information at the micro-nanoscale structures. A special drying method was implemented to ensure the minimal structural collapse of these surfaces under the high vacuum of FESEM. FESEM images of Cassia tora leaves showed widely spaced, low aspect ratio nanopetals distributed on bumpy blunt microfeatures, responsible for high contact angle hysteresis, and high roll angle measured on the Cassia tora leaves. ACV leaves showed the presence of micrometer-scale spherical morphology made of nanoscale hair-like features. These hierarchical re-entrant surface features generated a very high contact angle and low roll-off angle. Leaves of Bauhinia variegata showed similar superhydrophobic and self-cleaning properties. However, surface features were different, which consisted of a higher aspect ratio and closely spaced nanopetals uniformly distributed over flat surfaces consisting of micro-scale ridges. Our comprehensive investigation covers a detailed analysis of droplet impact studies, shedding light on the intricate dynamics governing droplet behavior on these superhydrophobic surfaces. Furthermore, we extended our analysis to encompass droplet impact on macrostructures to assess their influence on droplet receding and rebound phases. Notably, it was observed that only the microstructure of Cassia tora had a discernible impact on the receding and rebound phases of droplets. Additionally, our experiments examining maximum spreading diameter demonstrated good agreement with established models, further strengthening the scientific basis of our findings. These findings not only contribute to the advancement of our understanding of surface wetting phenomena but also bear practical implications for the development of water-repellent and self-cleaning materials.