Animals and humans alike flee to shelter as the first peal of thunder cracks across a cloud-ridden sky. After the storm takes its toll, light breaking out from behind the gray reveals tiny droplets clinging to exposed surfaces; trees, roof trimming, and umbrellas all drip with the remnants of a heavy rain. However, water hitting a lotus leaf simply rolls off, leaving the leaf as pristine and dry as it was before.
The water also carries with it most contaminants, as these pollutants are more attracted to the water than to the leaf’s rough surface . The plant’s inherent “cleanliness” and untouchability inspired the ancient Chinese to associate it with purity and promise, coining a phrase that roughly translates to “rises out of mud but is unpolluted” . Today, the lotus leaf’s unique properties inspire polymer and material scientists and promise many environmental and medical advancements.
Scientists researching the lotus leaf attribute its untouchability to a property called superhydrophobicity. A surface is superhydrophobic when water on the plane forms liquid droplets with contact angles of over 150 degrees . Hierarchical structures of micro-scale papillae and wax-like nano-scale branched structures give the leaf’s surface a characteristically high degree of roughness. These bumps minimize the contact area between the lotus leaf and the water, and air in between the bumps further increases the hydrophobicity. As a result, water coming into contact with the superhydrophobic leaf rolls off easily, taking dirt and other small particles with it .
This self-cleaning ability of superhydrophobic materials has inspired the construction of spoons, paints, and fabrics; a superhydrphobic t-shirt can be doused in ketchup and mustard and still retain its spotless white color . Recently, study of such surfaces has expanded to biofouling to create surfaces that can resist the aggregation of biological organisms.
The Dangers of Biofouling
Fouling due to proteins, bacteria, and marine organisms poses global economic and environmental concerns . The binding of microorganisms to submerged surfaces can form conditioning layers, providing platforms for larger and more complex aquatic species to accumulate. Biofouling on marine vessels increases surface roughness and hydrodynamic drag, causing decreased speed and maneuverability, increased fuel consumption, and increased greenhouse gas emission . As a result, these vessels consume up to 86% more power, and fouling alone costs the US Navy an estimated $1 billion per year . The operational and maintenance costs, as well as the danger of significant hull damage, highlight the need for an effective antifouling coating.
Antifouling surfaces have medical, technological, and industrial applications as well. Medical equipment such as catheters, prosthetic devices, and contact lenses risk infection from biofilm formation . Affected patients may be required to undergo antibiotic therapy and surgical replacement of contaminated devices . Antifouling surfaces help ensure the efficacy of biological implants and the accuracy of biosensors and in-vitro diagnostics . In the food packaging and storage industry, the accumulation of microbes and bacteria constitute great health and financial concerns. Surfaces that resist biofouling would also be highly beneficial in hospital apparel, water purification systems, and materials for patterned cell culture .
Although biocidal coatings involving antibiotics, quarternary ammonium salts, and silver are traditionally used to combat biofouling, their practicality is diminished due to resistant pathogenic strains and environmental regulations . Toxic antifouling strategies like biocidal paints release significant amounts of contaminants worldwide, so an environmentally friendly method of preventing biofouling must be developed.
Researchers and engineers turn to superhydrophobic surfaces for their self-cleaning quality to solve these environmental, medical, and economic costs of biofouling.
Besides anti-fouling surfaces, nanotechnology companies incorporating the lotus leaf’s design have succeeded in engineering various products that repel water and other contaminants. Nano-Care, for example, is a finish applied to fabrics that threatens to make washing machines obsolete. The nano-size particles on the clothing fibers mimic the physics of the lotus leaf’s papillae, making such materials self-cleaning. This technology places superhydrophobic outdoor structures such as awnings and sails in high demand as well. Similarly, a microscopic silicone surface gives the “honey spoon” a roughness that allows sticky substances to roll off the surface without leaving anything behind .
Additionally, self-cleaning paint for buildings poses new possibilities for combatting pollution. Such superhydrophobic surfaces can repel damaging particles that are normally deposited on public structures through air or rain. For example, StoLotusan façade paint for buildings, developed in Germany, introduced the technique to great success. Other ideas for the future include unfoggable mirrors and microfluidic chips that can be controlled by manipulating the surface’s attraction or repulsion to water .
Superhydrophobicity could change the way we live, the clothes we wear, and the Earth’s resistance to pollution. Further advancements lie in harnessing the innate characteristics of nature, like those of the lotus leaf, to understand these surfaces.
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Image Credit: Retrieved September 12, 2014 from: G. J. Bulte. “Lotus leaf with waterdrops.” Wikimedia Commons. December 4, 2010.  Retrieved September 12, 2014 from: William Thielicke. “Lotus effect.” Wikimedia Commons. June 12, 2007.