Unravelling the anisotropic wetting properties of banana leaves with water and human urine

Abstract

We report the heterogeneous and anisotropic wetting behaviors of Banana (Musa) leaves on adaxial (top) and abaxial (bottom) surfaces with water and human urine. Both surfaces comprise of varying number densities of micro ridges, micro stomata, micro flakes and nano hairs while the adaxial surface also constitutes discrete visible crest lines. The Wenzel roughness on abaxial and non-crest adaxial are 1.5 ± 0.05 and 1.18 ± 0.008, respectively as measured by combining optical profilometry and atomic force microscopy. A key scientific development in this work is to incorporate the measured Wenzel roughness in the surface energy estimations using Neumann's, Owens-Wendt-Rabel-Kaelble (OWRK) and Wu's models. The surface energy is 29 ± 2 mJ/m2 (OWRK) and 26 ± 1 mJ/m2 (Wu) on the adaxial surface. Nevertheless, the abaxial surfaces are characterized by smaller surface energy values of 20 ± 4 mJ/m2 (OWRK) and 24 ± 3 mJ/m2 (Wu). The measured static contact angles with water and urine are 110 ± 4° and 88 ± 4° (crest line), 130 ± 4° and 107 ± 5° (non-crest adaxial), 121 ± 4° and 120 ± 2° (abaxial) indicating hydrophobicity and urine-phobicity. The water droplets (2.5 to 10 µl) exhibited anisotropic dynamic wettability as they only rolled off parallel to the micro-ridges but stuck when slid perpendicular to the ridges even for specimen tilts up to 90°. Thus, this study unravels the anisotropic wettability of banana leaves due to the presence of micro-ridges which can be biomimicked for providing anisotropic liquid gliding. Also, this can aid in designing the spraying conditions of urine as a natural pesticide/fertilizer in the banana orchards.