8. Botanical Leaves: Groovy Terrain

Rice leaves repel water so efficiently that they are considered to be one of the few superhydrophobic materials (Feng et al., Adv Mater 14(24):1857–1860, 2002). These properties are very useful in a variety of applications, which is why scientists have been trying to mimic the surface of rice leaves to create water repellent materials for a wide range of uses. In this chapter, the various properties of superhydrophobic surfaces are explored and a comparison of surface structure is drawn between rice leaves and lotus leaves, which have anisotropic and isotropic properties, respectively. When we examine rice leaves, we can see how the roll-off angle—the critical angle at which the droplet begins to roll—is significantly lower in the longitudinal direction of the leaf than in the perpendicular direction of the leaf. This anisotropy originates from the multi-scale roughness of the surface (Lee et al., Adv Funct Mater 23(5):547–553, 2012). Though both surfaces are covered in a series of micropapillae that secrete epicuticular wax crystals that cause effective water repellency, the arrangement of the papillae on the lotus and rice leaves differ (Barthlott and Neinhuis, Planta 202(1):1–8, 1997). However, the hierarchical structures of papillae on both have been found to be almost identical (Woodward et al., Langmuir 16(6):2957–2961, 2000). The engineering applications of the superhydrophobic and anisotropic properties exhibited by rice leaves includes coatings on microwave and radio antennas (Antonini et al., Cold Reg Sci Technol 67(1–2):58–67, 2011), as well as self-cleaning glass developed using physical techniques such as ion etching and chemical techniques such as plasma-chemical roughening (Park et al., ACS Nano 6(5):3789–3799, 2012), which can be used in buildings as well as in the encapsulation of solar cells. Future applications are also being explored, such as in the way drugs are delivered inside the human body and in the field of micro-machines.