Creation of "Rose Petal" and "Lotus Leaf" Effects on Alumina by Surface Functionalization and Metal-Ion Coordination.

Functional differences between superhydrophobic surfaces, such as lotus leaf and rose petals, are due to the subtle architectural features created by nature. Mimicry of these surfaces with synthetic molecules continues to be fascinating as well as challenging. Herein, we demonstrate how inherently hydrophilic alumina surface can be modified to give two distinct superhydrophobic behaviors. Functionalization of alumina with an organic ligand resulted in a rose-petal-like surface (water pinning) with a contact angle of 145° and a high contact angle hysteresis (±69°). Subsequent interaction of the ligand with Zn2+ resulted in a lotus-leaf-like surface with water rolling behavior owing to high contact angle (165°) and low-contact-angle-hysteresis (±2°). In both cases, coating of an aromatic bis-aldehyde with alkoxy chain substituents was necessary to emulate the nanowaxy cuticular feature of natural superhydrophobic materials.