Superhydrophobicity in perfection: the outstanding properties of the lotus leaf

Abstract

Lotus leaves have become an icon for superhydrophobicity and self-cleaning surfaces, and have led to the concept of the 'Lotus effect'. Although many other plants have superhydrophobic surfaces with almost similar contact angles, the lotus shows better stability and perfection of its water repellency. Here, we compare the relevant properties such as the micro- and nano-structure, the chemical composition of the waxes and the mechanical properties of lotus with its competitors. It soon becomes obvious that the upper epidermis of the lotus leaf has developed some unrivaled optimizations. The extraordinary shape and the density of the papillae are the basis for the extremely reduced contact area between surface and water drops. The exceptional dense layer of very small epicuticular wax tubules is a result of their unique chemical composition. The mechanical robustness of the papillae and the wax tubules reduce damage and are the basis for the perfection and durability of the water repellency. A reason for the optimization, particularly of the upper side of the lotus leaf, can be deduced from the fact that the stomata are located in the upper epidermis. Here, the impact of rain and contamination is higher than on the lower epidermis. The lotus plant has successfully developed an excellent protection for this delicate epistomatic surface of its leaves.

Keywords: Lotus effect; epicuticular wax; leaf surface; papillae; water repellency.

Figure 1

(a) Lotus leaves, which exhibit extraordinary water repellency on their upper side. (b) Scanning electron microscopy (SEM) image of the upper leaf side prepared by ‘glycerol substitution’ shows the hierarchical surface structure consisting of papillae, wax clusters and wax tubules. (c) Wax tubules on the upper leaf side. (d) Upper leaf side after critical-point (CP) drying. The wax tubules are dissolved, thus the stomata are more visible. Tilt angle 15°. (e) Leaf underside (CP dried) shows convex cells without stomata.

Figure 2

Epidermis cells of the leaf upper side with papillae. The surface is densely covered with wax tubules. (a) SEM image after freeze drying. (b) Light microscopy (LM) image of thin section of an embedded sample. Assuming a contact angle of >140°, for example, the area of heterogeneous contact between single papillae and water (marks) is small in comparison to the epidermis cell area.

Figure 3

SEM images of the papillose leaf surfaces of Nelumbo nucifera (Lotus) (a), Euphorbia myrsinites (b), Colocasia esculenta (c), and Alocasia macrorrhiza (d). Lotus has the highest density of papillae with varying heights and the smallest diameter of the papillae. The papillae of the other species have larger diameters and are covered with different wax types: wax platelets (E. myrsinites and C. esculenta) and a wax film (A. macrorrhiza) which covers cuticular foldings.

Figure 4

The contact between water and superhydrophobic papillae at different pressures. At moderate pressures the water intrudes into the space between the papillae, but an air layer remains between water and epidermis cells (a). The superhydrophobic surface of the papillae causes a repellent force (‘re’). When the water recedes, then the papillae lose contact one after the other (b, c). At a certain water level, the meniscus is flat and the force is neutral (‘n’). Just prior to the separation an adhesive force (‘ad’) arises at the almost horizontal area of the papilla tip, which is small on tips with intact wax crystals and larger when the wax is damaged or eroded. On artificial superhydrophobic structures with equal height (d) the adhesive forces during water receding occur simultaneously at all contacts.

Figure 5

Measured forces between a superhydrophobic papilla-model and a water drop during advancing and receding. The images corresponding to the marks (arrows) in the diagram show the repellent (a) and adhesive (b) meniscus. (c) Papilla-model tip shown with SEM.

Figure 6

Papillose and non-papillose leaf surfaces with an intact coating of wax crystals: (a) Nelumbo nucifera (Lotus); (b) Euphorbia myrsinites; (c) Brassica oleracea; (d) Yucca filamentosa. Even the non-papillose leaves are superhydrophobic. The contact angle of B. oleracea can exceed 160°.

Figure 7

Traces of natural erosion of the waxes on the same leaves as in Figure 6: (a) Nelumbo nucifera (Lotus); (b) Euphorbia myrsinites; (c) Brassica oleracea; (d) Yucca filamentosa. On the papillose leaves (a,b) the eroded areas are limited to the tips of the papillae. On non-papillose cells, the damaged areas can be much larger (c,d), causing stronger pinning of water droplets.

Figure 8

Test for the stability of the waxes against damaging by wiping on the same leaves: (a) Nelumbo nucifera (Lotus); (b) Euphorbia myrsinites; (c) Brassica oleracea; (d) Yucca filamentosa. On the papillose surfaces only the waxes on the tips of the papillae are destroyed. The waxes between the papillae are protected and remain intact. On the non-papillose surfaces, most of the waxes are destroyed, adhesion of water drops (pinning) is strongly increased, and the superhydrophobicity is lost.

Figure 9

SEM and LM images of cross sections through the papillae. Lotus (a,b) and Euphorbia myrsinites (c,d) have almost massive papillae, those of Alocasia macrorrhiza (e,f) have a relatively thick outer wall; the epidermal cells of Colocasia esculenta have thin walls (g,h). The arrow in (b) marks a stoma.

Figure 10

Epicuticular wax crystals in an area of 4 × 3 µm². The upper side of the lotus leaf (a) has the highest crystal density (number per area) of wax crystals and the smallest spacings between them. Lotus upper side (a) ca. 200 tubules per 10 µm²; (b) Lotus underside ca. 63 tubules per 10 µm²; (c) Euphorbia myrsinites ca. 50 platelets per 10 µm²; (d) Yucca filamentosa ca. 17 platelets with over 80 jags per 10 µm²; (e) Brassica oleracea ca. 22 rodlets and tubules, and (f) Eucalyptus macrocarpa ca. 50 tubules per 10 µm². The larger spacing between the wax crystals of the other surfaces compared to the lotus upper side is obvious.

Figure 11

Chemical composition of the separated waxes of the upper and lower side of the lotus leaf. The upper side wax contains 65% of various diols and only 22% of nonacosan-10-ol (C₂₉-10-ol), 13% was unidentified; the underside wax contains 53% nonacosan-10-ol and only 15% of various diols. Alkanes (18%) were only found in the underside wax and may be an essential part of the underlying wax film.

Figure 12

X-ray diffraction diagram of upperside lotus wax. The ‘long spacing’ peaks indicate a layer structure which is common in aliphatic waxes. The broad ‘short spacing’ peak at 2θ = 27° indicates a strong disorder in the lateral package of the molecules.

Figure 13

Model of a wax tubule composed of layers of nonacosan-10-ol and nonacosanediol molecules. The OH-groups (red) occupy additional space so that the dense package is disturbed and the layer is forced into a curvature which leads to the formation of a tubule. The polar OH-groups are hidden in the layer, only the CH₃-groups appear at the surface of the layers and tubules.