Stearic acid-coated fly ash surface can be made to behave like rose petals or lotus leaves.
Researchers at the Indian Institute of Technology (IIT) Hyderabad have developed a cheap waterproofing material by coating fly ash, a waste by-product from coal-based thermal power plants, with stearic acid, which is a surfactant. While fly ash is extremely water-loving (hydrophilic), it turns into a highly water-repelling surface once coated with stearic acid.
The stearic acid has a hydrophilic part called the head and hydrophobic portion called the tail. While the head of stearic acid which is hydrophilic binds to fly ash particles, the water-repelling tail remains free. Numerous free hydrophobic tails of stearic acid makes the fly ash surface water repellent.
Fly ash varies in size from 100 nanometres to a few microns and can thus provide surfaces with different roughness based on the size of the particles chosen. The stearic acid-coated fly ash surface can be made to behave like one of the two naturally occurring water-repelling materials — rose petals or lotus leaves — by the varying the surface roughness.
In the case of a rose petal the surface repels the water but at the same time the structure of the petal ensures that the water droplets adhere to it. Thus, even when inverted, water droplets do not fall down. On the other hand, in the case of the lotus leaf, water droplet rolls off easily.
“When we used particles of more or less uniform size, the inter-particle space is more and water penetrates and sticks to the surface like in the case of a rose petal and remains adhered to the surface even when we tilted it to 90 and 180 degrees,” says Dr. Atul Suresh Deshpande from the institute’s Department of Material Science and Metallurgical Engineering and corresponding author of a paper published in
Chemistry Select.
The topography changes completely when fly ash particles of different sizes are used. The smaller particles tend to fill the gap between the larges ones thus resulting in a tighter packing with very little gap between particles. “Air pockets tend to form when the gap between particles reduces making it difficult for water to get into the gaps. Water droplets are suspended on top of the air pockets and so has less adhesion to the surface. So when the surface is tilted slightly (5 degree) the droplets tend to roll off easily,” says Dr. Mudrika Khandelwal from IIT Hyderabad and a co-author of the paper.
“By using a combination of stearic acid and surface roughness we were able to achieve superhydrophobicity (where the water contact angle is more than 150 degree),” says Urbashi Mahanta, a PhD student from IIT Hyderabad and first author of the paper. “We used a simple process of particle distribution to achieve the two types of hydrophobicity.”
The extent of hydrophobicity increased in both surfaces when the drying temperature was increased from 60 degree C to 80 degree C. The melting point of stearic acid is over 69 degree so when heated to 80 degree C the surfactant melts leading to better coverage and therefore smoother surface compared with samples treated to 60 degree C. Samples with tight and loose packing when treated to 80 degree C showed more hydrophobicity. The stearic acid-coated fly ash can be used as a waterproofing material. Owing to its hydrophobic nature, the surface can be easily cleaned. One can use an adhesive to apply the stearic acid coated fly ash on a surface,” says Dr. Deshpande.
More work is needed to test the mechanical and chemical durability of the fly ash waterproofing material.
