US researchers have designed an ultra-thin coating that is hydrophobic and self-heals when scratched or dented. The nano-scale thick coating was developed with steam power plants in mind, where durable yet thin coatings would deliver energy gains. Such hydrophobic materials have potential applications in self-cleaning, anti-fogging, anti-fouling and anti-icing surfaces, they report.
Most durable hydrophobic coatings comprise perfluorocompounds (PFCs) on a mechanically robust matrix, forming a composite coat often more than 10µm thick. Moreover, dents or pinpricks can allow water to seep in and degrade the matrix. Also, there are increasing concerns over the persistence of PFCs as environmental pollutants.
Self-healing coatings do not need to be as tough as fluorinated compounds, as they repair themselves if damaged. ‘You get a material that can reconfigure itself and can flow and respond to damage by self-healing,’ says research leader Chris Evans at the University of Illinois, US. The new material would not hold a macroscopic shape, but it is ideal for thin coatings. Even for coatings with a nanoscale thickness of less than 10nm, they were shown to retain their hydrophobicity after scratching, cutting, indenting and steam condensation, the researchers report.
The new coating is based on a commonly used silicone polymer – polydimethylsiloxane (PDMS), altered by adding a network of dynamic covalent bonds to its backbone (Nature Comm., doi: 10.1038/s41467-021-25508-4). These special covalent bonds can exchange and allow strands of the network to switch places.
The researchers’ focus is on boosting the efficiency of steam power plants, which are the largest producers of electricity globally. They envisage using their ultra-thin, hydrophobic coating in the condensers of such plants. Placed on the surface of the condensers, this would make them more water-resistant and better at forming droplets, and therefore boost heat transfer.
‘Cool air will condense on the surface and if it does this in a way that forms a film, that’s bad, because it will be typically many microns thick, and now your heat transfer is not very efficient,’ explains Evans. ‘You would like the water to form droplets and roll off, and that is exactly what this coating does.’
The thin film was deposited using dip-coating on a variety of surfaces, but the researchers are now investigating different ways to deposit it on substrates. The team is hopeful that they will license their coating technology.
‘The chemical system used here is pretty much known and quite old – essentially the same system is used in “silly putty”, or “bouncing putty” – although it has not been used in this particular application,’ notes Johan Winne, an organic chemist at Ghent University in Belgium. ‘As a chemist, it is nice to see a new application of chemistry that was long known to be special, but so far only useful for toys, and some niche applications. The paper could help to further revive interest in this chemistry.’
‘The coating is impressive because durability is one of the biggest challenges to practically use hydrophobic coatings in many applications,’ comments Evelyn Wang, a mechanical engineer at Massachusetts Institute of Technology, US. ‘The work has demonstrated proof of concept of durable hydrophobicity on test substrates under a variety of conditions,’ she adds. ‘While the approach seems promising, demonstrating the ability to deposit these coatings at scale and achieving performance enhancements for a specific application will be important.’
Image credit: Abobe Stock