Scientists at the TIFR Centre for Interdisciplinary Sciences (TCIS), Hyderabad, and Central Electrochemical Research Institute, Karaikudi, have developed a material which, when coated on other substrates, can render them both water repellent (hydrophobic) and oil repellent (oleophobic). This is extremely useful in improving the applicability of a range of products, from car wind-shields to medical aids such as surgical implants and other tools. In fact, the material is superhydrophobic, as its water-surface contact angle is above 170 degrees. Materials which repelled water and oil have been engineered earlier, but these required treatment with very high temperatures, and even then, formed sheets which were many layers thick. The ultrathin, superhydrophobic sheets mentioned above are formed by just one layer of atoms, and so, are transparent as well.
The analogy of a water-repellent lotus leaf goes only so far, “When you put low surface tension organic liquids or oils on a lotus leaf, it will completely wet the leaf. But these nanolayers can also bring roughness in to the surface along with low surface energy, because of which low surface tension liquids will also roll off from the surface, just like water does,” says T.N. Narayanan, now at TCIS, Hyderabad, in whose lab these materials have been developed.
Dr T.N. Narayanan and collaborators made fluorinated graphene oxide from flurographite by oxidising and exfoliating it. Exfoliating the graphite, which is a stacked array of carbon atoms, causes it to expand and form individual sheets of graphene – a state of carbon in the form of thin sheets comprising single layer of carbon atoms. This looks like a powder to the naked eye, but when viewed through a transmission electron microscope, shows up as transparent thin sheets. Realising that in order to be useful to industry, this material needed improved binding capacity, or adhesion to substrate, the group developed it further. They mixed it by hand with polydimethyl siloxane to make an ink which could be sprayed on to the surface which was needed to be coated.
Going further, the researchers improvised on a chemical vapour deposition technique to directly deposit the fluorographene on to the substrate thereby removing the need for a binder. Describing this as an “accidental discovery,” Dr Narayanan adds, “Recently, we found that ultrathin films of fluorographene can be made on any substrate using a facile method at relatively higher temperature of 500 degrees Celsius. Ultrathin superhydrophobic films were made on various substrates from glass to metals in large area using this vapour deposition. This will be beneficial in many fields in coating technology…”
Oil drilling machines use these sort of superhydrophobic films to prevent corrosion. The research group has been approached by one such company for use in its oil drilling machines where reducing the thickness of the coating is economically enormously beneficial. The film may be used in surgical implants and surgical tools, too, as due to its extreme hydrophobicity, biofilms do not form on the surface of these tools, acting as an anti-bacterial agent.
“We are on the way to ensuring its protection via proper IP. A few companies and persons already have shown interest in different aspects of this material… We are collaborating with leading institutes such as University of California, USA and Bar-Ilan University, Israel, etc. for developing different applications of this material,” adds Dr Narayanan.
