Indian Institute of Technology Hyderabad have used an industrial waste product – fly ash – to develop water repellent structures that mimic the characteristics of lotus leaves and rose petals. Such ‘superhydrophobic’ (super water repellent) structures have significant industrial applications such as anti-fouling paints, anti-sticking surfaces for antennae, self-cleaning coatings for automobiles, stain-resistant textiles and anti-soiling architectural coatings, among others.
The practical implications of the work by the IIT Hyderabad team are compelling. Not only is the water repellent product much cheaper than other superhydrophobic coatings developed so far but also serves towards up-cycling an industrial waste material – fly ash – into an industrially useful product, which serves the double purpose of side-revenue and waste management.
This research was jointly undertaken by Dr Mudrika Khandelwal and Dr Atul Suresh Deshpande, Department of Materials Science and Metallurgical Engineering, IIT Hyderabad. Their research paper, co-authored with Urbashi Mahanta, was recently published in the reputed peer-reviewed journal Chemistry Select.
Speaking about their research, Dr Mudrika Khandelwal said, “The water repellence properties of the lotus leaf and rose petal have been known for a long time, and scientists have explored ways in which these structures can be made artificially for various applications. Mimicking natural structures for engineering applications was not easy. In order to artificially replicate a natural phenomenon, scientists must understand the relationship between the natural structure and the purpose it serves.”
Explaining further, Dr Mudrika Khandelwal highlighted the difference in the water repellent natures of lotus leaf and rose petal. While both lotus leaf and rose petal are water repellent, drops slide off the lotus leaf, carrying with them any dirt and debris on the leaf, but adhere to the rose petal and do not fall off even when the petal is inverted. This difference arises from the differences in the structures. the surface of the rose petal consists of microstructures that have larger spacing, and a smaller density of nanostructures than the surface of a lotus leaf.
Attempts to replicate the superhydrophobicity of nature have involved altering the water adhesion behaviour of surfaces. Existing methods use costly materials or sophisticated instruments or suffer from scalability issues. This prompted the IIT Hyderabad researchers to look at cheap materials that can be modified to obtain different water adhesion properties.
Highlighting the economic aspect of their research, Dr Atul Suresh Deshpande said, “We have used an inexpensive material, fly ash, a waste product from the coal industry, to obtain hydrophobic surfaces with tailorable water adhesion behaviour. The particle size of fly ash is between 100 nanometres to a few micrometres, a size range that is suitable for generating rough surfaces that are hydrophobic. However fly ash itself is not water repellent, and therefore, we (researchers) coated stearic acid on the fly ash particles.”
“Using these surface-modified fly ash particles, the researchers were able to obtain surfaces with different adhesion properties. Whether the water drops roll-off (‘lotus leaf effect’) surfaces coated with the fly ash or stuck to the surface (‘petal effect’) depended upon when the stearic acid was coated on the fly ash particles. “Coating the fly ash particles with stearic acid before depositing the fly ash on the surface leads to the lotus leaf effect, whereas depositing fly ash particles and then coating them with stearic acid resulted in the rose petal effect”, explained Dr Deshpande.
Dr Mudrika Khandelwal said, “Our method for producing water repellent surfaces is quick and highly scalable as it does not require any sophisticated instrument.” The synthesis process is so simple that even an untrained individual can apply it to obtain superhydrophobic surfaces at a low cost.
Dr Deshpande added that the superhydrophobic (super-water-repellent) coatings produced using fly ash can be used to protect concrete structures from rain and moisture-induced damage. In addition, these materials can be used for water harvesting applications.
