Difference in wettability of lotus leaves in typical states and its mechanism analysis

Abstract: Recent studies have made great progress on the superhydrophobicity and self-cleaning functions of lotus (nelumbo nucifera) leaves. However, studies on the self-cleaning function of lotus leaves at different states are still lacking. It is still unclear about whether the self-cleaning function of the lotus leaves remains at different states. In this study, the wettability of lotus leaves under four typical states, including fresh leaves, old leaves, sick leaves and dried-up leaves, was compared to explore the hydrophobic mechanism of lotus leaves. Wettability of different lotus leaves was determined. Another measurements included contact angle tests, scanning electron microscope (SEM) measurement for the microcosmic patterns of the lotus leaves, and the chemical analysis of the wax extracted from the leaf surface were done. From the contact angle results and the distribution of water drops on the lotus leaves under different states, it was found that the water droplets were of different wetting degree at different lotus leaves. On the fresh lotus leaves, water could roll together into an almost perfect sphere, and roll freely on the leaves. On the old lotus leaves, some of water droplets could form approximate spheres although the edges of the leaves were curled. The hydrophobicity of dried-up leaves remained well on the surface without peeling. However, the water droplets spread very fast on the sick leaves, displaying the hydrophilicity. The contact angle values of the fresh leaves, old leaves, and dried-up leaves were 153°±3°, 141°±3°, 150°±3°, respectively, showing hydrophobicity or superhydrophobicity. The contact angle value of the sick leaves was 58°±2°, displaying hydrophilicity. The results above were consistent with the observations. According to SEM measurements to visualize the surface microstructure of lotus leaves, the fresh lotus leaves had micro- and nanoscale hierarchical structures, for example the papillae microstructures were covered by wax clusters. The microcosmic pattern of dried-up lotus leaves was mostly similar except that papilla and wax clusters were slightly converged. The surface of the old lotus leaves was much rougher, which was composed of batten-like convexes, papillae and wax. Different from fresh and old lotus leaves, the sick lotus leaves had the surface with irregular linear fiber network structure rather than the papilla + wax hierarchical structure. In addition, obvious difference of the surface wax content determined by comparing the wax extraction results was also observed for these four different kinds of lotus leaves. The wax content was only 1.03% for the sick leaves, much lower than that for the fresh (1.98%), old (1.94%), and dried-up (2.09%). In summary, the results here showed that the difference in the wettability for these four different lotus leaves could be attributed to the different microstructures of the leave surfaces as well as the difference in the surface wax contents. Among them, the microcosmic pattern was the major factor affecting the hydrophobicity of the lotus leaves.