Introduction
The lotus plant has a number of uses. One of the commonly applied uses is for religious purposes. For instance, the seeds of the lotus plant normally are used to create strings of prayer beads. Each bead has its own spiritual meaning. This is very widespread in the Asian countries such as India, China and Japan. The plant offers extraordinary purity and significant beauty [2]. Other applications of the Lotus plant include aesthetics and temperature regulation where the leaves are used as hats.
Restaurants are common places where waste generation is frequent. The type of surface present forms an important part in the establishment of restaurants. Customers can evaluate the quality of restraint based on the level of cleanliness and service. For cleanliness, surfaces need to be easily cleaned. The lotus effect seeks to explain why having artificially developed surfaces that mimic the lotus effect can be very beneficial in various human applications.
It is not common to find a plant in nature that offers significant benefits in terms of repelling water. The Lotus plant is one such plant that may offer such a use. The water repelling characteristics of the Lotus plant allow it to grow in muddy areas and remain spotless. Scientists believe the microscopic structure and surface chemistry of the leaves of the lotus plant prevent it from getting wet. Such characteristics can be applied in the development of fabrics that have the ability to repel water and stains. For instance, common coffee spills or rain on clothes can be avoided. Consequently, this can have a tremendous effect in terms of the efficiency of people in situations where stains and spills are common. Frequent changing because of the common tea or coffee spills can be avoided. It is not ordinary to find such a sanitation system in nature.
Figure 1: Lotus Leaf Repels Water
The Lotus Effect
The ability of the Lotus plant to repel water has received a lot of interest from many scientists especially Wilhelm Barthlott. At the University of Bonn in Germany, Barthlott led the way in the development of the lotus effect [2]. In 1970, Barthlott used the electron microscope to study the self-cleaning properties of the lotus leaves.
According to observations made by Barthlott, specific characteristics such as the waxy nature of the leaf surface and the tiny bumps it has, are the main causes of the repellant nature of the lotus leaves. Wax has the property to prevent water percolation on any surface, and this situation is similar to what happens on the surface of the lotus leaves. The water droplets landing on the leaf surface form spherical droplets, which reduce the area of contact with the leaf surface [3]. On the other hand, on a normal non-repellent leaf surface, the area of contact of the water droplets with the leaf surface is big, which allows more contact with the leaf surface. The bumps referred to as papillae prevent dirt particles from sticking to the leaf and thus the particles have a higher chance of sticking to the rolling water droplets [3]. The bumps reduce the contact area for the water droplets. The presence of the trapped air between the leaf bumps and the water increases the contact angle of the lotus leaf. Water attractive surfaces have a contact angle of less than 30 degrees while, for water repellant surfaces such as the lotus plant leaves, the contact angle is bigger than 90 degrees [6].
Figure 2: Micro-View of the Lotus Leaf
During the initial observation of the lotus leaf, Barthlott had not foreseen what a promising future his research would bring, despite having established that the properties of the lotus leaf would be applied artificially. Application artificially would imply the use of microscopic bumps, which tend to reduce the contact angle. This application encouraged Barthlott to patent the idea of developing surfaces with microscopic bumps that enhanced self-cleaning ability of the material. This was later established as Lotus Effect.
The development of a surface to have a lotus effect is not easily accomplished. The hydrophobic material repels everything, which then brings about the issue of making it stick or not to repel the object itself. To address this problem, Barthlott developed a ‘honey spoon’, which consisted of minute rough silicone particle surface that enabled honey to roll off, leaving the surface clear. Based on this new development, big chemical industries realized that the application of the method was possible. The major application of the Lotus effect is in the paint industry. In 1999, Sto AG, a German Multinational Company, developed StoLotusan. StoLotusan paint and is credited to be among best German inventions in currently. StoLotusan paint, which is used in on the front walls for buildings. Additionally, it can also be applied on other external walls such as the sides and back provided it is exposed to a constant flow of water. Ideally, the StoLotusan paint is not recommended for interior walls as the lack of free flowing water causes dust particles to accumulate on the wall. Furthermore, indoor cleaning methods for walls surfaces, which entails the use of cleaning and scrubbing agents, damage the Lotus effect. Application of the StoLotusan paint on walls ensures that the wall remains dry. Because of the lotus effect, water and dirt cannot remain on the surface of the wall. Subsequently, water just simply rolls off carrying any loose dust particles. Therefore, any habitat for undesired microorganisms such as fungi and algae is avoided. The normal masonry paint allows dust accumulation, which when combined with water creates a damp environment for the survival of algae and fungi.
Inspirations from Biology for technology
The Lotus effect has been designed to imitate the behavior of the lotus leaves. The lotus effect has influenced biological engineering known as biomimicry, which involves the study and replication of nature’s significant behaviors to address human challenges. Its application has been used in the development of self-cleaning materials, paint and windows. In its initial research stages of the application of the lotus effect, the studies were confined to only waxy surfaces. However, with time, the area has been expanded to include things such disinfection possibilities, wettability and self-cleaning abilities. The development of superhydrophobicity surfaces can be done in several ways, which also has led to possibilities in developing superhydrophilicity surfaces. The common material used in superhydrophilicity is titanium oxide, which has self-cleaning abilities. Because of the superhydrophilicity, water on the surface of titanium oxide does not form spherical balls; rather it spreads on the whole surface and in the process collects and carries away all the dirt particles. Additionally, titanium oxide surfaces are known to be resistant to fogging. Furthermore, disinfection and freshening ability of the titanium oxides adds to its self-cleaning ability. Disinfection occurs through the photocatalytic action of titanium oxide. The photoactivated titanium oxide involves the degradation of the cell wall and cytoplasm membrane of the bacteria or microorganism [7]. This occurs because of the manufacture of reactive oxygen species, which include hydrogen peroxide and hydroxyl radicals [7]. The consequence is leakage of cellular contents, which is followed by a complete mineralization of the organism [7]. The freshening ability because of the photocatalysis due to using titanium oxide in nanoscales, removes any odor that may develop from the breaking down of organic material in the reactions.
The applicability of the lotus effect and the superhydrophilicity of titanium oxide are not common situations. Researchers have experimented on the idea of developing surface materials that can incorporate both superhydrophilicity and superhydrophobic effect. This can then be modified in such a way that it is possible to move from one effect to another depending on the particular situation that seems more efficient for self-cleaning. Companies such as Fujishima and Watanabe have envisioned the combinations of the two effects. The titanium oxide is used on surface material to increase the lotus effect of the surface material. However, the idea seems non-applicable because of the photocatalytic activity of the titanium, which would act on the waxy layer of the lotus surfaces and in the process clean the surface of any wax thus destroying the lotus effect. The adhesive forces between wax and water are normally weaker compared to cohesive forces between water droplets. As such, the result is to increase the contact angle of the surface. Thus when high concentrations of titanium oxide is used, there photocatalytic mechanism causes the degradation of the waxy surface, which reduces the contact angle. The companies found out that only small amounts of titanium oxide is required to ensure that such attacks are avoided and to enhance the lotus effect without compromising the high contact angle and repellant ability of the surface.
Other compounds have also been used in the development of hydrophilic materials. Research at the M.I.T. by Rubner and Cohen indicated that small changes have the ability to determine whether the final result of material development is superhydrophilic or superhydrophobic. Rubner and Cohen used layers of positively charged polyelectrolytes and silica particles (negatively charged), by stacking them in alternating layers. Consequently, they added a coating of silicone to provide a hydrophobic effect. However, prior to the addition of the silicone, the layered material had already developed hydrophilic characteristics. This was attributed to the silica layers that created a surface that soaked up any water present on the surface. This observable fact was referred to as nanowicking where the nanopores in the silica layer formed a form of a sponge. In situations where the layers were completely filled with water, the water would start to flow from the edges.
This application is common in glass, which consists of layers of silica. The superhydrophilic layers of silica ensure that fogging does not develop and maintain the transparent nature of the glass. This discovery can be applied in the development of bathroom mirrors and windshield mirrors for vehicles during the cold mornings. The advantage of application of the silica layers as a hydrophilic material over the titanium oxide is that the silica layers can function well in light and darkness.
The Future
Increase in technology has allowed researchers to continue experimenting on new applications of the lotus effect further than the normal self-cleaning abilities. Restaurants have adopted the use of hydrophobic and hydrophilic surfaces in kitchen areas where a lot of water is used, and waste generated. Furthermore, the development of easy cleanable clothes is quite common. People prefer to purchase such products as it saves them time to change or clean up the stained cloth. In cabin fittings, the lotus effect is significant in ensuring the coatings shed any water droplets. Further, this helps in the removal of any contaminants present. Biomimicry is being applied in other fields such as development of swimsuits that resemble the ability of sharks to reduce friction. Therefore, the lotus effect will continue to be influential in the development of biomimicry in other aspects of plants and animals if researchers continue to conduct additional experiments. This sentiment is greatly acknowledged by Barthlott, who notes that other plants and animals may also have some useful characteristics that can be applied in the day-to-day human challenges. Based on his current research of superhydrophobicity, Barthlott has established that certain plants such as the Pistia and the Salvinia can lock in air on the leaf surfaces. Such a property is applied in the development of clothes that have the ability to remain dry when underwater for four days.
Works Cited
[1] Bill Sayoran “The Lotus Effect: A Manifestation of Divine Purity.” The Fountain Web.<http://www.fountainmagazine.com/Issue/detail/the-lotus-effect-a-manifestation-of-divine-purity>
[2] Peter Forbes “lf-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology” Scientific American Web <http://www.scientificamerican.com/article/self-cleaning-materials/>
[3] “The Magic Lotus Leaf - Nature's Nanotechnology” Psi Web. <http://www.p2i.com/blogs/articles/magic_lotus_leaf_natures_nanotechnology >
[4] “HYDROPHOBIC SURFACE ALLOWS SELF-CLEANING: SACRED LOTUS”AskNature Web.<http://www.asknature.org/strategy/714e970954253ace485abf1cee376ad8#.UwsC9fldWOM>
[5] ArunnNarasimhan “Lotus Leaves, Hydrophobic and OmniphobicSurfacer” Web. < https://home.iitm.ac.in/arunn/lotus-leaves-hydrophobic-and-omniphobic-surfaces.html >
[6] Beilstein J. Nanotechnology “Superhydrophobicity in perfection: the outstanding properties of the lotus leaf” Beilstein Journal, Web http://www.beilsteinjournals.org/bjnano/single/articleFullText.htm?publicId=2190-4286-2-19
[7] Foster, Howard A., et al. "Photocatalytic Disinfection Using Titanium Dioxide: Spectrum And Mechanism Of Antimicrobial Activity.” Applied Microbiology & Biotechnology 90.6 (2011): 1847-1868.
