The Lotus Effect

Water is vital to every living being, yet on occasions in which it is abundant, it could also disturb the balances that an organism operates on. Throughout the course of evolution, a variety of organisms have developed physical traits that protect them from this excess water. Using inspiration from these organisms, scientists in the modern era have been able to study the interactions between water and different surfaces, and develop their own waterproofing technologies. 

Usually, waterproofing is achieved through a coating that acts as a barrier, such as protective materials on rain coats, or even the fat that is found in the outermost layers of our skin cells; however, there are other ways of repelling water, such as using the concept of hydrophobicity, which can be found in the interaction between water drops and lotus leaves. In this article, we will be analyzing the engineering feats of mother nature, and figuring out how researchers are able to mimic the nanostructure of lotus leaves in order to reduce drag on aircraft, provide antibacterial surfaces and protect metals from rust.

The Genius of Mother Nature

Lotus plants live in water and sprout from mud. This is a problem, as if mud sticks to the lotus leaves and blocks sunlight, the plant is unable to photosynthesize. In order for the lotus leaves to stay mud-free in such an environment, lotus leaves have a special structure that prevents water and mud from sticking. This makes it so that when water comes in contact with a lotus pad, the water droplets stay as spherical as possible. The water droplet also slides off easily when the leaf is tilted at a slight angle, leaving no residue behind, ensuring that the leaf remains dry. This interaction between the surface of a lotus pad and a water droplet is called a hydrophobic interaction. 

 The reason lotus pads are hydrophobic is due to its nanostructure, and how it interacts with the molecular structure of water. The surface of a lotus leaf appears to be smooth at first glance, and will continue to appear smooth until it is looked at through a microscope. Then, it becomes apparent that the surface of the leaf is actually bumpy, made up of small pillars, whose surface is also made of smaller pillars around 200 nm-1 in size. These pillars trap a thin layer of air trapped under the water and reduces the contact between the leaf and water, giving the water less area to stick to.

 However, this structure would be useless if not for the forces in play between the water molecules themselves. The reason water stays in the form of a droplet instead of sinking through the nanostructured pillars is due to the polarity of a water molecule, or the attractive force of it. Because water molecules constantly attract each other, it forms a sticky film around the water droplet that allows it to stay intact. This is what allows the water droplet to remain spherical instead of spreading out. The combination of a water droplet’s ability to retain its spherical shape, and the nanostructures on the lotus leaf ensures the least possible contact with the water droplet.

Copying Nature

There are several ways how this phenomenon could be mimicked by humans. One of them is the soft lithographic imprinting technique, in which liquid polymer is cast on the Lotus leaf to create a usable mold. Another technique is the the Sol-gel process, in which a solution is turned into a moldable gel. The third method: Layer by layer assembly is done by depositing layers of material on top of each other in a controlled manner. Finally, the etching method is when a metal is left in acid with a patterned protective shield that allows the acid to eat away a specific spots in the metal creating the desired pattern.

Applications

There are many usages of hydrophobic materials such as the creation of self cleaning surfaces, materials that reduce drag, and materials that resist corrosion. Self-cleaning materials are primarily needed in hospitals as the water repelling properties make it easier for the removal of pathogens that reside in water.

 Hydrophobic materials could also be used to reduce drag for aircraft. Drag can be caused by water droplets and fluid from insects, which then solidifies on the aircraft. Hydrophobic materials decrease the contact between the aircraft and water/dead insect fluid it encounters, allowing these fluids to roll off instead of sticking. 

Finally, hydrophobic properties can reduce corrosion caused by water. Corrosion happens when materials (typically metals) chemically react with the environment around them and start decaying (rusting is an example of this). Since hydrophobicity reduces contact with water and creates a layer of air that protects the metal, it significantly slows down this process. 

Conclusion

The potential of a mimicked lotus leaf is endless, as its properties have numerous applications. However, lotus leaves are not the limits of the capability of nature. Some other desirable properties engineered by nature include the adhesive properties of gecko feet, the optical/mechanical properties of butterfly wings, and even the ability of self healing. As we progressively create more complicated materials and technology, we should not hesitate to look to nature for inspiration, as nature is the best engineer with four billion years of experience. 

Works Cited:

Bokov, D., Turki Jalil, A., Chupradit, S., Suksatan, W., Javed Ansari, M., Shewael, I. H., Valiev, G. H., & Kianfar, E. (2021, December 24). Nanomaterial by sol-gel method: Synthesis and application. Advances in Materials Science and Engineering. Retrieved January 15, 2023, from https://www.hindawi.com/journals/amse/2021/5102014/

Definitions for hydrophilicity, hydrophobicity, and superhydrophobicity ... ACS Publications . (2014, February 20). Retrieved January 15, 2023, from https://pubs.acs.org/doi/10.1021/jz402762h

Garg, P., Ghatmale, P., Tarwadi, K., & Chavan, S. (2017, May 26). Influence of nanotechnology and the role of nanostructures in in Biomimetic Studies and Their Potential Applications. MDPI. Retrieved January 15, 2023, from https://res.mdpi.com/biomimetics/biomimetics-02-00007/article_deploy/biomimetics-02-00007-v3.pdf

Jeyan, J. V. M. L., & Mohan, A. P. (2016, January). A study of superhydrophobic nature of Lotus leaves and its applications ... Research Gate . Retrieved January 15, 2023, from https://www.researchgate.net/publication/328118496_A_Study_of_Superhydrophobic_Nature_of_Lotus_Leaves_and_its_Applications_on_Aircraft