Since the introduction of the ‘Lotus concept’ in 1992 [1
], the lotus leaf became the archetype for superhydrophobicity and self-cleaning properties of plant surfaces and a model for technical analogues [3
] . Lotus (Nelumbo nucifera
) is a semi-aquatic plant and develops peltate leaves up to 30 cm in diameter with remarkable water repellency. As an adaptation to the aquatic environment – some of the leaves float occasionally on the water surface – the stomata are located in the upper epidermis. The lower epidermis consists of convex cells covered with wax tubules and contains only few stomata. The upper epidermis features the distinctive hierarchical structure consisting of papillae with a dense coating of agglomerated wax tubules, which is the basis for the famous superhydrophobicity ().
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 (more ...)
However, a hierarchical surface structure which induces strong water repellency and contact angles above 150° is not a special feature of lotus leaves. It has been known for a long time that plant surfaces covered with epicuticular wax crystals are water repellent, and that this feature is enhanced when the epidermis has additional structures such as papillae or hairs [5
]. Neinhuis and Barthlott (1997) [7
] presented an overview of more than 200 species with contact angles >150° and their surface morphologies. Many studies, in which the properties of lotus leaves were compared with those of other superhydrophobic plants, have shown the superiority of the upper side of the lotus leaf. A standard tool for the determination of wettability or water repellency is the measurement of the static contact angle by the ‘sessile drop’ method. Neinhuis and Barthlott (1997) [7
] for example, measured contact angles on the lotus leaf of 162°, which are among the highest of the compared species, but many other (43%) of the tested superhydrophobic plants also showed contact angles between 160 and 163°. Even some species with flat epidermis cells but with a dense layer of epicuticular wax crystals, such as Brassica oleracea
or some Eucalyptus
species, can exhibit contact angles >160°. Thus, the contact angle alone is not suitable for a differentiated comparison of superhydrophobic samples. Other values such as contact angle hysteresis or roll-off (tilting) angle show more clearly the differences between the species. Mockenhaupt et al. (2008) [8
] compared the tilting angles and the stability of the superhydrophobicity of various plants under moisture condensation conditions. Only the lotus leaves showed no significant loss of water repellency when water vapour condensed on the surface of the cooled samples at 5 °C. Wagner et al. (2003) [9
] examined the morphology of the epidermal structures and the wettability with liquids of varying surface tension such as methanol–water mixtures. They reported the lowest wettability by these liquids for the lotus leaves in comparison to other species. They also described the unique shape of the papillae and a very high papillae density (number per area). Chemical analyses [10
] and crystal structure analysis by X-ray diffraction [11
] showed unique properties of the epicuticular wax of the lotus. The high content of nonacosanediols leads to a high melting point as well as a strongly disturbed crystal structure which is the basis for the formation of tubules. The visualization of the contact zone between leaves and droplets with cryo-scanning electron microscopy demonstrated the extremely reduced contact area for lotus [12
]. Zhang et al. (2008) [13
] made detailed measurements of the water repellency of the papillose lotus leaf surface in comparison with the non-papillose leaf margin. The importance of the nanoscopic wax crystals for the water repellency was demonstrated by Cheng et al. (2006) [14
]. They reported a strong decrease of the contact angle after melting of the waxes. A limited air retaining capability of submersed lotus leaves was reported by Zhang et al. (2009) [15
] after the leaves were held at a depth of 50 cm for 2 h. Bhushan et al. (2010) [4
] used the surface structures of the lotus leaf as model for the development of artificial biomimetic superhydrophobic structures.
It became obvious that the outstanding and stable superhydrophobicity of the lotus leaf relies on the combination of optimized features such as the surface topography, robustness and the unique properties of the epicuticular wax. The aim of this article is to integrate the relevant features of the lotus leaf, and to compare them with superhydrophobic leaves of other plant species in order to illustrate their significance.