IIT Guwahati researchers have developed a superhydrophobic coating that can make the surface behave like a lotus leaf where water droplets roll off easily or a rose petal where the droplets stick till the angle of tilt goes beyond 20 degrees. The coating, which can be spray-painted on diverse surfaces of any geometry and size, has several potential applications.

A polymeric coating that is extremely water-repelling (superhydrophobic) and will allow water to roll off from the surface like in the case of a lotus leaf or stick to the surface as in the case of rose petals has been synthesised by a two-member team from the Indian Institute of Technology (IIT) Guwahati.

It can be spray-coated on various surfaces (glass, plastic, metal, wood and concrete) of diverse chemical composition, texture (smooth or rough surface), geometry (plain sheet or complex shape such as shoes), and size. The team led by Dr. Uttam Manna from the institute’s Department of Chemistry found the coating’s water-repelling property remained intact even when subjected to severe physical and chemical abuse.

By modulating the functionality of the coating with small amine molecules, the coated surface was made to behave either as non-adhesive superhydrophobic coating (where water rolls off as in lotus leaves) or adhesive superhydrophobic coating where the droplets stick to the surface like in the case of rose petals. The results were published in the Journal of Materials Chemistry A.

“There is a fundamental difference in the way the trapped air is present at the interface between the surface and water droplets and this makes the coated surface either adhesive or non-adhesive superhydrophobic,” says Dr. Manna. In the case of the non-adhesive coating, the trapped air is continuous and uniformly distributed. This leads to the trapped air minimising the contact area between the water droplet and the substrate. With adhesive coating, the trapped air is not continuous and the contact area between the water droplet and the substrate is more thereby causing the droplets to stick to the surface up to 20 degrees tilt angle.

Importance of pentanol

The superhydrophobic coating was prepared by mixing a polymer (branched poly(ethyleneimine)) and a reactive small molecule (dipentaerythritol penta-acrylate) in different alcoholic solvents — ethanol to pentanol.

“We were able to achieve a stable coating only when pentanol solvent was used,” says Dr. Manna. The polymer and the small molecule react rapidly in the presence of pentanol. Also, the volatility (rate of evaporation) of pentanol is way less compared with other alcoholic solvents.

“As pentanol evaporates slowly, it allows the reaction between the polymer and small molecule to be completed thus making the coating to be stable and uniform,” says Koushik Maji from the Department of Chemistry at IIT Guwahati and first author of the paper. “With other solvents, the reaction is slower and the solvents evaporate quickly leading to less stable coating (cracking and peeling).”

The coating is highly chemically reactive and this makes it possible to post modify the coating with amines containing small molecules to make it either adhesive or non-adhesive superhydrophobic. Unlike in other cases, the reactants themselves get covalently cross-linked thus making the use of external binder or cross-linker for stability redundant.

Potential applications

The superhydrophobic coating has diverse applications depending on whether it is made adhesive or non-adhesive. The non-adhesive superhydrophobic coating can be used for oil-water separation and making the surface self-cleaning.

Since the coating remains adhesive up to 20 degrees of tilt, the adhesive superhydrophobic coating can be used in open microfluidic devices for diagnostic purposes where controlled transfer of aqueous droplets without any loss in volume is needed.

Physically durable, chemically stable

The physical durability was tested by rubbing the surface ten times with sandpaper, dropping 200 grams of sand on the coated surface from 20 cm height, and manually scratching the coated surface with a sharp knife. Small portion of the coating was lost only when the researchers used an adhesive tape to peel off the coating, but superhydrophobicity remained intact. The chemical stability was tested by exposing the coated surface to artificial seawater, river water, highly acidic (pH 1) and alkaline (pH 13) for 100 hours. The water repelling nature of the coated surface was unaffected even when exposed to UV irradiation for 30 continuous days.

Published in The Hindu on March 17, 2018