Abstract
Hierarchically structured flower leaves (petals) of many plants are superhydrophobic, but water droplets do not roll-off when the surfaces are tilted. On such surfaces water droplets are in the “Cassie impregnating wetting state”, which is also known as the “petal effect”. By analyzing the petal surfaces of different species, we discovered interesting new wetting characteristics of the surface of the flower of the wild pansy (Viola tricolor). This surface is superhydrophobic with a static contact angle of 169° and very low hysteresis, i.e., the petal effect does not exist and water droplets roll-off as from a lotus (Nelumbo nucifera) leaf. However, the surface of the wild pansy petal does not possess the wax crystals of the lotus leaf. Its petals exhibit high cone-shaped cells (average size 40 µm) with a high aspect ratio (2.1) and a very fine cuticular folding (width 260 nm) on top. The applied water droplets are in the Cassie–Baxter wetting state and roll-off at inclination angles below 5°. Fabricated hydrophobic polymer replicas of the wild pansy were prepared in an easy two-step moulding process and possess the same wetting characteristics as the original flowers. In this work we present a technical surface with a new superhydrophobic, low adhesive surface design, which combines the hierarchical structuring of petals with a wetting behavior similar to that of the lotus leaf.
Highlights
Plant surfaces provide a large diversity of hierarchically designed structures with various functions [1,2]
We present the superhydrophobic surface of the wild pansy Viola tricolor (Figure 1), with a low tilt angle (TA) and discuss the influence of papillae morphology and the dimensions of cuticular folding on the petal wetting state
One may assume that the replicas display the real shape of the fresh petal surface structures
Summary
Plant surfaces provide a large diversity of hierarchically designed structures with various functions [1,2]. Both describe superhydrophobic surfaces with high adhesive forces to water, and this means that the wetted surface area is smaller than in the Wenzel model but larger than in the Cassie–Baxter model. In contrast to the lotus surface with air pocket formation between cell papilla, wax crystals and salient water droplets [18], the petal surface seems to prevent air pocket formation and droplets penetrate into the cuticular folds by capillary forces.
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