​What is The Lotus Effect in Detail? An Example of SiO2 and Other Hydrophobic Materials

The Lotus effect means the self-cleaning properties of the lotus flower that come out as a result of ultra-hydrophobicity. In this method, all the dirt particles are extracted out by water droplets because of the miniature work done on the surface as a result of which the adhesion of droplets is minimized.

However, these properties are not only found in this flower but a few other flowers as well. They have a huge impact on the factors in which they play a role despite being at a certain level. The importance and applications of the lotus effect in the field of nanotechnology are quite huge and are drastically increasing over time. Their basic function is to cleanse the surface and make it water free so that it can be prevented from any wetness in the future too. The entire lotus effect is created by the layer of wax that is present on the lotus leaf as that layer of the wax exhibits the water-repelling properties and further activates the whole process of continuing it to a larger scale. This is the origin of the lotus effect. When researchers saw the adaptability of this effect, they further classified it so that it can be utilized in a better way and on a larger scale.

Introduction

The lotus flower or leaves of Nelumbo gives an effect known as the lotus effect, which has self-cleaning characteristics due to the ultra-hydrophobicity, which the leaves display. Water droplets pick up the particles of dirt because of the surface’s nano and microscopic architecture. That architecture helps in reducing the adhesion of the droplet to that surface. In certain insect’s wings and some plants like cane, Alchemilla, Opuntia (prickly pear), and Tropaeolum (nasturtium) also have characteristics of self-cleaning and ultra-hydrophobicity. Wilhelm Barthlott and Ehler studied ultra-hydrophobic micro-nano structured surface's self-cleaning characteristics in 1977 and named those characteristics as the 'lotus effect'. In 1986, Brown developed perfluoropolyether and perfluoroalkyl ultra hydrophobic materials to handle biological and chemical fluids.

In 1964, Dettre and Johnson used rough hydrophobic surfaces to study the ultra hydrophobicity phenomenon. A theoretical model was developed because of their work in which PTFE telomere or paraffin coated the glass beads, which were used in different procedures. Since 1990, many different biotechnical applications have risen.

Functional Principle

Water’s high surface tension results in droplets turn into an almost spherical shape. As minimal surface area is contained by the sphere, this shape reduces the solid-liquid surface energy. When liquid contacts with the surface, the surface gets wet due to the adhesion forces. The droplet's fluid tension and surface's structure determines whether the wetting is incomplete or complete. The surface's hydrophobic water-repellent double structure is the reason for self-cleaning characteristics. The process of self-cleaning results when the adhesion force and contact area between the droplet and surface are majorly lessened. The covering waxes and characteristic epidermis (cuticle as the outermost layer) forms the hierarchical double structure. Papillar of 10 to 15 μm width and 10-20 μm height is possessed by the lotus plant's epidermis. Epicuticular waxes are imposed on the width of papillae. Being hydrophobic, these superimposed waxes created the double structure's second layer. Regeneration of this system is common. For the surface’s water repellency functioning, the responsible party is this biochemical characteristic.

Measurement of Hydrophobicity

The contact angle of the surface can measure the surface’s hydrophobicity. If the contact angle is high, the hydrophobicity will be high too, and vice versa. If the surface has a contact angle greater than 90°, it is hydrophobic and if the contact angle of the surface is less than 90°, it is hydrophilic. The surface of the plants, having a contact angle of 160°, is known as ultra hydrophobic. Ultra hydrophobic means that only 2-3 percent of the droplet's surface is in contact, which is the typical size. If the plant has a double structure surface, for instance, the lotus, then a 170° contact angle can be reached, however then in that case, 0.6 percent is the contact area of the droplet. All of this results in a self-cleaning effect.

Picking up of Dirt Particles

Water droplets pick up the particles of dirt, which have very less contact area. The particles are therefore removed from the surface easily. If the surface is contaminated, then the adhesion between the rolling water droplet and the dirt particle (doesn’t matter what chemistry) is much high as compared to the adhesion between the surface and the particle. On general materials like stainless steel, this cleaning effect has been displayed when a superhydrophobic surface is formed. The self-cleaning effect is not for the organic solvents as it depends on the water’s high surface tension. Thus, against graffiti, the surface’s hydrophobicity provides no protection. Acting as a defense against pathogens like the growth of algae or fungi, this effect holds major importance for animals, insects, and plants which can't clean themselves properly. Self-cleaning has other positive effects too, like preventing the plant surface's area from contamination which is exposed to light and leads to lessened photosynthesis.

Technical Application

The ultra hydrophobic surfaces having a property of self-cleaning is due to the chemical-physical characteristics at the nanoscopic to microscopic scale instead of just the leaf surface’s particular chemical characteristics. The usage of this effect on the surfaces that are made by men arose this possibility. Usage was supposed to be done by mimicking nature in a way that would be general instead of specific.

Further Developments

Fabrics, roof tiles, paints, coatings, treatments, and other surfaces are developed by some nanotechnologists which can stay clean and dry themselves by replicating like the lotus plant does as it has the self-cleaning characteristics. If the composition contains micro-scale particulates, then by using them with special silicone or fluorochemical treatments on the structured surfaces, this can be attained. For producing the lotus effect, other than chemical surface treatments, femtosecond pulse lasers sculpt the metals as the other treatments remove as time passes. At any angle, the black color of the material remains, so combining this with the self-cleaning characteristics will result in a low maintenance solar thermal energy collector. Metal’s high durability is capable of being utilized in the self-cleaning latrines for lessening the transmission of disease.

Microwave Antennas

Ice and snow buildup and rain fade can be majorly reduced by the super-hydrophobic coatings when they are implemented to microwave antennas. Patterned ultra hydrophobic surfaces can significantly enhance bioanalysis that’s based on the surface. For water’s funneling to a basin to be used in irrigation and in drew harvesting, the hydrophobic and superhydrophobic characteristics have been utilized.

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Lotus Effect in Nanotechnology

In graphic mode, the lotus effect is seen as a drop that instead of being absorbed, stays on the surface. The Lotus effect refers to self-cleaning and water repellency. Its origin is from a ‘lotus flower’. It is a mystery for scientists as no one has explained why that flower doesn’t get wet. The lotus flower’s hydrophobic property has been copied for many beneficial effects on human life. It is nanotechnology research's main task.

Reproduction of Lotus Effect

On different surfaces like metal, glass, plastic, wood, and stones, the lotus effect has been reproduced through technological innovation’s chemical products. In personal, domestic, and business sectors, these achievements have had a very good effect as they contributed to enhancing the appearance of various materials, extending their lifetime, and helping in avoiding the continuous investment in cleaning and maintenance.

For instance, in windshield and body, a car can be very well protected, durability can be extended, and on rainy days, fog can be avoided. In textiles, liquid repellency is very beneficial as it lightens incidents like wine spillage and prevents stains. Because of its various advantages, a great impact is made by Lotus effects. Now the product’s commercialization has been globally spread, leading to nanotechnology becoming one of the most powerful markets. A broad quantity of products is offered by Nanografi that are related to nanotechnology in this nanotechnology’s preponderant rise.

Importance of the Lotus Effect

Lotus Effect has a lot of importance when it comes to creating self-cleaning materials. So much importance that the biologists, Barthlott and Neinhuis, gave the idea under the name ‘Lotus-Effect’. From that time, many physicists and botanists studied Lotus Effect to understand it better and to find technological applications that are possible. Superhydrophobicity is a physical characteristic whereas hydrophobicity (water repulsion) is a chemical characteristic. Both characteristics can be differentiated by the contact angle between the water and the surface. Hydrophobic characteristics are obtained if the contact angle is between 150 and 90 degrees whereas superhydrophobic characteristics are obtained when the angle of contact is more than 150 degrees, as it causes the effect to amplify and results in being impossible to wet surface.

Nanotechnology and Lotus Effect

Nanotechnology is the theory that hypothesizes the option of controlling the matter’s structure by working on the types of connections that is between the atoms. Therefore, when looking for solutions by studying the structure of matter, nanotechnology can be used.For measuring atoms and molecules, the nanometer scale is used. -1 nanometer is related to a meter’s billionth.

Application in Nanotechnology

Nanotechnology is used in different fields, ranging from the field of textiles to optics, from the field of electronics to construction, and from robotics to medicine. By observing nature and its remarkable microscopic inventions, the idea came of making mirrors based on the refractive index’s periodic modulation for optical telecommunications applications through the study of all pigments is born, or the idea of replicating Gecko leg tissue's morphology to create materials which permit perfect adherence to the surfaces, or the idea to create materials that self-clean and copy the Lotus plant surface's microscopic morphology.

Nanotechnology can be brought into our homes through the products of Nanotechnology. For instance, nanotechnology products, that are based on silicon, utilizes nanoparticles of silicon that forms bonds of molecular-level in the silicon-based substrates like stone, ceramics, and glass. This modifies the substrate’s surface structure for forming a breathable and uniform active barrier, therefore making it Hydro-Oil Repellent. As a specific structure of the molecule is possessed by each product, there are also specific nanotechnological products for each surface which binds to the support's structure at the level of the molecule, forming an effect that preserves it from the ease and wear of cleaning and protects it against the action of atmospheric agents, oil, and water. That effect is an invisible barrier effect.

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Products with Molecular Activity

With solutions with high technological value and molecular activity, nanotechnology products are proposed by Lotus Effect, as lubricants and degreasers for cars, motorcycles, bicycles, and professional technical cloths, and in the sector of the maintenance, cleaning, and protection of external and internal supports like cement, stone, tiles, bricks, fabric, wood, ceramic, stainless steel, and glass. Therefore, because of the material's high repellency for water, like the flower plant's leaves, the lotus effect is a precise capacity for self-cleaning.

Researchers and scientists were inspired by the research on Lotus leaf, it resulted in the production and design of some very useful and beneficial substance in various technological applications, ranging from the field of transport to the field of biomedicine. Reproducing the lotus effect in floors, fabrics, paints, tiles, and other surfaces have been naturally tried. In conclusion, like Albert Einstein, we have to be more observant about our surroundings and environment and allow ourselves to be amazed and inspired to understand it.

Lotus effect via an example of SiO2 is explained further and a few other hydrophobic materials as well. These examples explain the lotus effect in-depth and give us an overview of the importance of the lotus effect.

Example of SiO2

In academia and industry, an attractive interest is created by hydrophobic coatings because of their remarkable self-cleaning characteristics due to their property of repelling water. By roughening the low surface energy materials, the highly hydrophobic surfaces have been made, based on the lotus leaf effect and other natural phenomena. Its surface free energy is determined by the coating’s chemical composition. One of the strong organic bonds is the C-F bond. One of the effective ways of lessening surface free energy on the molecular structure's architecture and enhancing water impermeability is introducing the fluorinated component in the coating. This addition will improve the coating's hydrophobic characteristics due to the C-F bond’s low polarizability and small dipole, together with the large free volume. For improving the roughness of the surface, including nanoparticles deposited, chemical vapor deposition, electrochemical deposition, plasma etching, etc., numerous methods have been implemented. Also, corrosion resistance is increased when the nanoparticles are added, for instance, nanoparticles of ZnO, nanoparticles of SiO2, and nanoparticles of Ti.

Effect of the SiO2 particle sizes on the water contact angles of coatings

The SiO2 particle sizes influence on the coating’s water contact angles are explained, with a constant of 1 wt % SiO2 nanoparticles. The coating’s water contact angles are affected when SiO2 is added with various sizes of particles. 62.8 is the minimum coating angle whereas 85.6 is the largest contact angle. Mainly, two factors determine the paint film’s water resistance: the paint film’s solid surface energy and structure. Different structural scales are made by SiO2 of different sizes.

Effect of the contents of SiO2 nanoparticles on the water contact angles of coatings

Scientists investigated the effect of 18 nm SiO2 (S103583) contents on different coatings' water contact angles. When SiO2 content increases from 0.5 to 2.o wt%, the coating’s water contact angles also increases gradually. Particularly, when SiO2 nanoparticles of 1,5 wt% and 2.0 wt% are introduced, they advance the composite coating’s hydrophobicity significantly, therefore, inducing the coating’s water contact angles higher than 90°.

Although, a lot of addition of SiO2 results in the decrease of the water contact angles as some of the paint film’s outer edge was exposed to some of SiO2. The paint film’s hydrophobicity decreases because of the exposed hydrophilic SiO2. When 1.5 wt% is the amount of SiO2 content, the emulsion's viscosity increases, and a taste is presented. When SiO2 weight content is 2.0 wt%, the hydrophobic effect’s benefits are not that obvious in comparison to when the weight content is 1.5 wt%. 1.5 wt% is used as appropriate content for taking the hydrophobic effect and coating process into consideration.

Waterborne fluorine-containing epoxy coatings

SiO2 nanoparticles-modified waterborne fluorine-containing epoxy coatings exhibited a higher contact angle. Coating’s corrosion resistance was enhanced on SiO2 addition and it also made coating display great thermodynamic stability.

Super-hydrophobic diatomaceous earth

Diatom’s microscopic skeletal remains are contained by the Diatomaceous earth. The skeleton of diatoms is made up of hydrated silicon dioxide. It has both micro and nano-porosity and a nano-roughness that greatly increase either its water absorption or its water repellency, based on its surface chemistry. Naturally, it is super hydrophilic because the chemistry of its surface is naturally hydrophilic. But it can become superhydrophobic if the DE is treated with a hydrophobic silane due to its texture’s amplification effect and nano-porosity and its hydrophobic surface chemistry. This superhydrophobic nano-porous powder is called superhydrophobic diatomaceous earth, or simply SHDE. A 160 to 170-degree contact angle can be possessed by the surface which consists of SHDE powder. The angle can be as high as 175° too.

Water marbles

When hydrophobic silane completely functionalizes the particles, they become superhydrophobic water doesn’t wet them. But if only partially functionalization occurs, then each of the particles can attain both of the behaviors, super hydrophilic and superhydrophobic. Water marbles are made when the drops of water interact with these particles.

Resin marbles

At Oak Ridge National Laboratory, the researchers said that resin marbles can be formed when the superhydrophobic nano-textured silica (for instance, SHDE) interacts with the molten powder-coat resins. The electrostatic process is used for coating surfaces with the dry polymer resins in the powder-coat process. Once the charged resin particles sufficiently cover an electrically grounded surface, the coated surface is placed in an oven where the particles of resin become molten and make a cured and uniformly coated powder resin surface.

The process of the electrostatic application won't be adversely affected by a blend of powder-coat dry resins and SHDE, since the charging of SHDE happens the same as the charging of dry resin particles does. The SHDE interaction with molten resin functions like the water marble forming process during the curing process. Resin is wet partially by SHDE while the molten resin is repelled by most of the SHDE. Resin marbles are completely formed if the SHDE proportion to resin is huge enough (for instance, 25% SHDE to resin), then without joining other molten resins, the molten resin cures.

Result of the resin marbles

As a result, a surface is formed which contains such cured resin marbles that are completely unbound to the substrate or the other resin marbles. If SHDE proportion to resin decreases to 5% then most of the molten resins will flow but a uniform coating won’t be formed because of the SHDE’s resin-phobic/philic nature.

An extremely porous interconnected surface with micro-porosity can be formed by the blended powder-coat resins with SHDE because of the resin marble’s restricted interaction. The coating’s entire volume got micro-porosity extended all over it. Moreover, the outer surfaces of the micro-pore are coated with SHDE, forming a surface nano-porosity and super hydrophobic effect all over the coating’s entire volume.

Conclusion

Thus, it is evident that the literal meaning of something can be more clearly defined via certain examples. So is the case in this article, the lotus effect has been further well explained by the example of SiO2 and some other hydrophobic materials to put a little more emphasis on the importance of the lotus effect and how it is certainly helping build the industry. As evident through certain examples and experiments, the results and outcomes of the lotus effect are quite striking. That is why researchers took this opportunity and formulated advancements so that the industries can take benefit from this whole phenomenon.

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References

https://www.matec-conferences.org/articles/matecconf/pdf/2017/44/matecconf_cmpse2017_08005.pdf

https://aip.scitation.org/doi/pdf/10.1063/1.5141617

https://www.researchgate.net/publication/280115341_Superhydrophobic_materials_and_coatings_A_review

https://www.researchgate.net/publication/261516233_Superhydrophobic_Surfaces_Developed_by_Mimicking_Hierarchical_Surface_Morphology_of_Lotus_Leaf

https://cutt.ly/JJGXwB7