Beyond primitive bionic surfaces: Bionic structures coupled surface with superhydrophobicity and programmable directional adhesion

The Cassie state surface, exemplified by lotus and rice leaves, and the Wenzel state surface, exemplified by rose petals, are both superhydrophobic due to their unique structures. The objective is to expand the primitive wettability function of a single bionic structure by coupling various bionic structures in the corresponding wetting state region on an entire surface. The rice leaf–rose petal structures coupled (RRSC) aluminum surfaces were fabricated by micro-milling. The rice leaf wall substrate forms an air layer in the Cassie state. The Wenzel roughness is controlled by the rose petal crown. The RRSC surface is more hydrophobic than a single bionic surface, and it can also be programmed for adhesion and directional diffusion. An augmentation of up to 0.57° in contact angle and 3.34° in run-off angle per micrometer of rose petal crown width is exhibited. A model for identifying the wetting states on rough surfaces is developed, and the effect of two bionic structures on wetting states is analyzed quantitatively. The rose petal crown is observed to retain the air layer by impeding droplet penetration. By determining the free energy and adhesion work, the superhydrophobicity and programmable adhesion originate from the cooperative interaction and internal competition between two wetting states, respectively. This article presents a new theoretical thermodynamic model for analyzing wetting states, wettability, and adhesion on rough surfaces. The bionic structures coupled strategy is proposed to exceed the primitive wettability of bionic surfaces by coordinating natural surface characteristics in different wetting states.