“Lotus effect” is a coined term for self-cleaning properties. The excellent water repellency and self-cleaning ability of lotus leaves have inspired the construction of manmade self-cleaning surfaces. The lotus effect is based on the micro/nano-structures creating roughness on the surface and the hydrophobic wax coating on the lotus leave. These properties are mimicked to obtain similar self-cleaning properties through various different methods. The lotus effect inspired surfaces are utilized in numerous applications ranging from self-cleaning windows and high performance solar cells.
Natural systems have been a source of inspiration to the human race for a very long time. Not only they have affected the arts and day-to-day living, but also they have had a significant impact on engineering applications. The sophisticate properties and structures of organisms have been shaped by evolutionary pressure over a long period of time. So, it does make sense to benefit from this evolutionary wisdom while developing novel products and coming up with solutions to complicated problems of engineering. Sometimes, we might even find solutions to the problems we didn’t even know about. Self-cleaning surfaces are an example of this situation. For so long, it was accepted that cleaning required active effort to remove dirt and that surfaces could not clean themselves. However, when it was discovered that plants have “self-cleaning” properties, this conception had to be revised. The leaves of the Lotus plant (Nelumbo Nucifera) have become the face of these self-cleaning properties, so much so that the self-cleaning effect is often claimed as the “Lotus effect”. Lotus plant is considered as a symbol of purity in China for more than 2,000 years since it emerges from swamps and exists in filthy environments but still manages to keep its leaves free of all the dirt around it. Even though the lotus plant has been a center of attraction for a very long time, the mechanism of the lotus effect has remained a mystery until the invention of scanning electron microscopy. After that, various studies have investigated the lotus effect with the purpose of incorporating this self-cleaning property to man-made products. The self-cleaning effect of lotus leaves is believed to be mainly based on the superhydrophobicity of Lotus leaves. Superhydrophobic surfaces inspired by the lotus effect shows a high static contact angle above 150°, a contact angle hysteresis of less than 10°, low roll-off angles, and low-water holding capacity exhibit extreme water repellence and self-cleaning properties. When water drops fall onto the surface of lotus leaves, they form spherical drops with high contact angles and roll-off from the surface collecting dirt and other particles. Studies on the lotus effect have attributed this hydrophobic behavior to the nanostructure of the lotus leave and epicuticular waxes covered across its surface.
Effect of Nanostructure of Self-Cleaning Properties
On smooth surfaces, it is not possible to exceed 120° contact angles by means of chemical hydrophobicity. Hence, superhydrophobicity requires surface roughness on nano and micro scales. This surface roughness is perfectly observed on the Lotus leaves with scanning electron microscopy. Lotus leaf surfaces possess randomly distributed micro-papillae with diameters ranging from 5 μm to 9 μm. Additionally, each papilla and the joining surfaces between them contain fine branch-like nanostructures with a diameter of approximately 120 nm. These surfaces are called “low energy surfaces”. The extended nanostructures prevent the underside of the leaf from getting wet while the combination of micro-papillae and Nanosized hair like structures creates pockets of air between the papillae thus, water can interact with only the peaks of the rough surface instead of wetting the entire surface. Air pockets significantly decrease the capillary forces between the droplet and surface. As a consequence, the droplet takes an almost spherical shape and usually rolls off easily. Spherical droplets on rough surfaces hold onto the dirt particles and drag them away instead of rolling over them. This is the result of very low adhesion forces between the dirt particles and the surface of Lotus leaves. When a droplet falls onto the surface, the particle yields to the prevalent capillary forces and adheres to the droplet instead of the surface. Consequently, it is carried away with the rolled off water droplet. Even strongly hydrophobic substances like soot are easily washed off with water only due to the Lotus effect.
Effect of Epicuticular Waxes on Self-Cleaning Properties
All terrestrial plant species are covered with a waxy layer called cuticle which is crucial to prevent uncontrolled water losses from the leaves. Self-cleaning plants have an additional epicuticular wax layer. Epicuticular waxes are self-assembled crystals consist mainly of hydrocarbons, primary and secondary alcohols, and ketones. This additional layer provides hydrophobic properties to the surfaces due to the low surface energy hydrocarbons in their structure. The low energy surface lowers the force that keeps the water in contact with the solid. Only the liquid which has a surface tension equal to or lower than the critical surface tension of a solid can spread on this solid surface. As a result, water droplets form a spherical shape which leads to a lower wet area and lower roll-off angle. The size of wax crystals ranges from 200 nm to 5 μm. On the other hand, the shape of wax crystals has very little importance on the hydrophobic properties.
The Lotus effect relies on both of these parameters, however, the studies have also shown that the desired hydrophobic effect can be achieved through just the surface roughness. It was also demonstrated that it is possible to obtain the superhydrophobic Lotus effect on hydrophilic surfaces by tuning the roughness and morphology of the surface.
What Are The Methods For Imitating The Lotus Leaf Surface?
Self-cleaning technology has been inspired by Lotus leaves to develop similar surface structures and self-cleaning properties. Both of the components of the Lotus effect, surface roughness, and low energy surface, hold an important place in developing self-cleaning surfaces. Hence, there are two different approaches to the matter at hand. The self-cleaning surface inspired by the Lotus effect can be obtained by either constructing a micro-/nanocomposite hierarchical structure on an initial hydrophobic surface or modifying the low surface energy materials on the micro-/nanocomposite hierarchical structure surface.
Techniques to fabricate micro/nano-structures on a hydrophobic surface include lithography, etching, anodization, and template replica. Lithography methods which include X-ray, photon, E-beam, and soft lithography have been widely employed for the synthesis of self-cleaning surfaces. Lithography offers precise control of the surface structure allowing the production of surfaces patterned with different shapes and different sizes. Furthermore, the obtained template is easy to be fabricated and can be used many times. Anodization is a metal treatment method in which the surface of a metal and its alloy form an oxide film through the impressed anodic current in the electrolyte solution. The treatment of surface results in random three dimensional shapes creating superhydrophobicity. Another effective method for obtaining micro/nanostructures on the surface is etching. Etching methods include plasma etching, laser etching, and chemical etching. It is a straightforward and effective way of fabricating rough structure surface by etching surface with corrosive gas plasma or laser. By the etching methods, it is possible to obtain a superhydrophobic metallic surface with a content angle of 150°. The etching is a highly selective and rapid method for creating surface roughness however, it has a high cost and not appropriate for large scale applications. The template replica method involves creating a negative template directly from the lotus leave and then creating a copy of the lotus leaf surface by using this negative template. Scientists have created a PVC surface with a 157° contact angle and a 3° roll-off angle showing superhydrophobicity and self-cleaning properties. The template replica method is cost effective, fast, simple, and accurate method of obtaining desired surface roughness for self-cleaning surfaces. This method is also suitable for large scale applications.
Techniques to fabricate superhydrophobic coatings include sol-gel, dip coating, self-assembly, electrochemical, and chemical/ physical vapor deposition. In the sol-gel method, precursors of chemically active compounds form sols in appropriate solvents and become dry gel after concentrating, aging, and drying. For some materials, nanostructures form after the removal of solvent and result in superhydrophobic and self cleaning properties. The most frequently used materials in the sol-gel method are SiO2, TiO2, and Al2O3. It is possible to obtain alumina gel films with a contact angle of 165° by applying the sol-gel method. This method is simple, low-cost, and applicable on a large scale however, the process takes a long amount of time, result in solvent contamination, and the surface structure is not controllable. Dip coating is a simple method applied on various different surfaces such as glass, polycarbonate, and poly(methyl methacrylate) (PMMA). The coating materials can be silica, zinc oxide, indium tin oxide, NaCl, and many other materials. Through this simple method, self-cleaning properties are attained by various different materials. The electrochemical deposition has also attracted attention for the production of self-cleaning surfaces. The advantage of electrochemical deposition is its versatility to prepare microscale and nanoscale structures. It is possible to obtain superhydrophobic surfaces with a 173° contact angle through this method. Another widely used method for the preparation of superhydrophobic surfaces is electrostatic spinning. This method offers the preparation of micro-/nanofiber and great control over the morphology through manipulating the ratio and concentration of the spinning solution. Furthermore, electrostatic spinning is a low-cost method that is suitable for large scale applications. Self assembly is a simple and inexpensive method to prepare micro- and nano- dual-scale superhydrophobic surface structures with the self-cleaning property. The self assembly method also provides control over the morphology structures and contact angles over 150°. Chemical/physical vapor (CVD) deposition is a popular method in various different application fields due to its easy process and parameter control. Similarly, it has also attracted attention for the preparation of superhydrophobic self-cleaning surfaces. CVD method includes the deposition of hydrophobic materials such as ZnO and poly(tetrafluoroethylene) (PTFE) onto inherently rough surfaces such as carbon nanotube (CNT) to create superhydrophobic surfaces. The bonding between the substrate and deposited film is significantly strong showing good stability and durability. High contact angles up to 164° are obtained through this method. Even though this method is simple and offers strong substrate-film interactions, the high cost limit it only suitable for the fabrication of some special materials.
Each of these methods is different approaches to the preparation of superhydrophobic self-cleaning surfaces inspired by the Lotus effect. They can be utilized individually or in combination with each other. Hybrid production methods offer great control over the desired parameters and broaden the range of materials used in the production.
Application Areas of Lotus Leaf Inspired Self-Cleaning Surfaces
The understanding of the Lotus effect and development of production methods for self-cleaning surfaces have created a great number of opportunities in various fields. The most significant application areas of the Lotus effect are exterior construction materials, corrosion resistance coatings, textiles, solar cells, sensors, drag reduction, anti-icing coatings, anti-fogging coatings, and anti-reflection coatings.
Exterior Construction Materials
The self-cleaning and water-repellency properties of Lotus effect inspired surfaces have attracted great attention in exterior construction materials such as paints, window glasses, coatings. The self-cleaning coatings and paints are utilized on the exterior walls of buildings to minimize the damping due to water sorption, growth of mold, mildew, and algae on the building surface. Furthermore, due to the high water repellence of self-cleaning coatings, the water droplets roll-off from the surface collecting dirt and dust creating a self-cleaning surface. The self-cleaning water-repellent coatings are also utilized on the roof of buildings to rapid drainage of ice-melt waters and rain waters; minimize the formation of algae and bacteria improving the life of the material. Another application of lotus effect inspired coatings on building materials is self-cleaning window coatings. Not only self-cleaning coatings offer ease of use but also they provide optical clearance since water droplets do not accumulate on the surface of the windows. In addition to exterior construction materials, self-cleaning surfaces are also utilized for interior materials such as flooring materials. Lotus effect inspired coatings on flooring materials endows water-repellent properties improving the durability and lifetime of the material. The use of self-cleaning surfaces allows buildings to maintain their aesthetic appearance unaltered over time, which is very important in urban environments. Furthermore, self-cleaning technology results in large decreases in maintenance costs. The lotus effect self-cleaning coatings are also utilized on pavements and sidewalks for fast drainage providing safe driving conditions and cleaner sidewalks. The superhydrophobic self-cleaning surfaces are also utilized on the bridge structures such as piers, docks, railings, dividers, and abutment to enhance their service life with greater water resistance.
Corrosion Resistant Coatings
Superhydrophobic coatings have also attracted attention in industrial applications due to their anti-corrosive properties. Metallic surfaces coated with superhydrophobic materials show excellent corrosion resistance hence, improved durability and lifetime. These metallic materials are utilized in the food industry and self-cleaning machine parts.
The utilization of self-cleaning properties in the textile industry is a no-brainer. Self-cleaning textiles provide convenience, ease of use, and reduction of the use of harsh chemicals such as detergents. Coating fibers with reactive SiO2 nanoparticle monolayers and nonfluorinated polymers creates a self-cleaning surface on the textile which allows the removal of dirt with water and oil-based liquids. This superhydrophobic coating can be permanently anchored to the fiber boundary due to the chemical attachment of the nanoparticles and polymers to the surface. The application of the Lotus effect on textiles provides a range of superhydrophobic self-cleaning products to resist spills, repel and release stains, and resist static
The superhydrophobic self-cleaning surfaces are greatly utilized in solar cell applications. The water-repellency and dirt removing properties of self-cleaning properties provide transparency and antireflective properties to the organic solar cells. The multifunctional surface results in an enhancement of photovoltaic power conversion efficiency due to the reflection suppression and transmittance enhancement.
Sensors are prone to the accumulation of pollutants and clogging. Especially membranes suffer from clogging and resulted in a decrease in performance. The self-cleaning surfaces inspired by the lotus effect are utilized to eliminate the clogging of membranes by passively and constantly removing dirt and pollutant from the membrane surface. The self-cleaning coatings are also utilized on sensitive infrared nanosensors in bioimaging and optoelectronic devices.
Dragging poses a critical problem for the movement of materials. The Lotus leaf inspired materials are found to be drag reducing as well as self-cleaning. The fluidic drag reduction of the superhydrophobic surface is based on the superior water repellency of the surface, which dramatically reduces the interaction on the solid/water interface and generates a thin layer of air to establish a shear-free air/water interface. Based on these observations, the lotus effect can be utilized for the coatings of the underside of ships and other underwater structures.
Anti-icing and Anti-fogging Coatings
Fogging frequently occurs on the surface of cold windshields, eyeglasses, safety glasses, and ski goggles due to the accumulation of water molecules on the surface of materials. The incorporation of superhydrophobic coating onto these surfaces inhibits the accumulation, provides anti-fogging and self-cleaning properties. Similarly, icing causes problems such as mechanical failures in cold environments and high altitudes. Even though it is a complex problem, it is possible to aid icing on surfaces by carefully adjusting the effect of wettability, roughness, structure, and morphology of the self-cleaning surfaces.
The Lotus effect self-cleaning is a fascinating phenomenon in nature. The superhydrophobic self-cleaning properties of lotus leaves have astonished scientists and inspired various studies. The lotus effect is based on two different surface properties of the lotus leaves; the extended micro/nano-structures on the surface of the leaves and the epicuticle wax coating throughout the surface. The branch like nanostructures creates air pockets between the surface of leaf and water droplets decreasing the contact area between the leaf and the water molecules drastically. The low surface energy of hydrocarbons in the epicuticle waxes decreases the forces that keep the water in contact with the solid. The combination of these two surface properties causes water droplets to form a sphere like structure and easily roll-off from the surface. The roughness and hydrophobicity of the surface allow dirt and dust particles to adhere to water droplets rather than the solid surface of the lotus leaf providing an effective self-cleaning property to the lotus leaves. This superhydrophobic behavior requires a water contact angle above 150°, contact angle hysteresis of less than 10°, low roll-off angles, and low-water holding capacity. Scientists have come up with various different methods of imitating these surface properties and obtaining self-cleaning surfaces. These methods are either focused on obtaining micro/nano-structures on a readily hydrophobic surface or coating a rough surface with a hydrophobic material. The surface roughness is created by lithography, etching, anodization, and template replica. On the other hand, the hydrophobic coatings are obtained by techniques such as sol-gel, dip coating, self-assembly, electrochemical, and chemical/ physical vapor deposition. These methods can be applied individually or in combination with each other. The resulting Lotus effect inspired superhydrophobic self-cleaning surfaces have been utilized in various different application areas such as exterior construction materials, window glasses, corrosion resistance coatings, textiles, solar cells, sensors, drag reduction, anti-icing coatings, anti-fogging coatings, and anti-reflection coatings. Even though the studies are increasing rapidly, the lotus effect self-cleaning surfaces are relatively novel applications holding many other undiscovered opportunities.