5. Lotus Leaves: Humble Beauties

The lotus flower is famous around the world across many cultures and religions. For example, it is considered to be a sacred flower by Buddhists, symbolizing cosmic harmony and spiritual illumination. It is even believed to have birthed the sun, an Egyptian myth inspired by the fact that the flower opens in the morning and closes by evening (The Flower Expert, Lotus flowers, http://​www.​theflowerexpert.​com/​content/​aboutflowers/​exoticflowers/​lotus, 2005). What many people do not know, however, is that its counterpart—the wide, flat leaf of the lotus plant—albeit not as bright and delicate as the flower, is a hidden beauty all its own. The lotus leaf has been the subject of great interest in recent scientific research because of its superhydrophobicity, which is otherwise known as the Lotus Effect (Forbes, Self-cleaning materials: lotus leaf-inspired nanotechnology, http://​insurftech.​com/​docs/​links/​Related-Papers/​Article-1-Scientific-American-Self-Cleaning-Materals-Lotus-Effect.​pdf, 2008). The lotus leaf’s superhydrophobic, self-cleaning surface is seen as an evolutionary advantage that benefits the life and longevity of the plant in a variety of ways, such as decreasing the leaching of nutrients and reducing dust and contaminating particles that can inhibit photosynthesis (Solga et al., Bioinsp Biomim 2:S126–S134, 2007). The wettability of the lotus leaf surface is one mechanism that contributes to this intriguing characteristic. Wettability is determined by measuring the contact angle between the liquid and the surface, and contact angle is governed by Young’s law (McHale et al., Beilstein J Nanotechnol 2:145–151, 2011). The surface roughness of lotus leaves also plays an important role in the leaves’ superhydrophobicity and is characterized by a hierarchical structure consisting of papillae and nano-scale, tubule-like asperities that minimize contact area (Bhushan and Jung, Prog Mater Sci 56(1):1–108, 2011). The wonders of the Lotus Effect have been taken advantage of in a wide variety of markets, ranging from self-cleaning windows to stain-resistant clothing. Self-cleaning paint has also been developed based on the Lotus Effect, which helps prevent the growth of numerous fungi, algae, and bacteria that attach to and colonize building structures, diminishing the need for harmful biocides (Solga et al., Bioinsp Biomim 2:S126–S134, 2007). As is evident in this chapter, despite being the seemingly humble counterpart to the elegant lotus flower, the lotus leaf boasts many remarkable characteristics that engineers have been inspired by and have tried to mimic.