Superhydrophobic coating could protect your sofas

Materials World magazine
,
1 Apr 2015

An unusually robust superhydrophobic paint developed by UCL researchers could have applications from car windscreens to hospital floors. Simon Frost reports.

Slow-motion videos of red wine miraculously bouncing off superhydrophobic white shirts and cream carpets are all the rage online, but what they don’t show is that those coatings are notoriously weak. Now, researchers at UCL have developed a robust paint that maintains its self-cleaning properties when subjected to abrasion and even contact with oil – normally the ‘kryptonite’ for hydrophobic materials. 

The research team, led by Professor Ivan Parkin, Head of Chemistry at UCL, combined a scaffold formed of two different sizes of titanium dioxide nanoparticles with a fluorosilane coating, which gives the paint the low surface energy required to repel water. 

The lotus effect 

‘In nature, these superhydrophobic surfaces are already out there,’ says Professor Claire Carmalt,
co-author of the paper, which was published in Science in March 2015, with Parkin, Yao Lu and others. ‘The lotus leaf appears to have a waxy, smooth surface to the naked eye, but the microstructure is formed of a series of tiny protrusions that, combined with a waxy coating, create superhydrophobic properties. The low surface energy means that when water drops onto the leaf, it forms very spherical shapes like marbles that are supported by these protrusions. When they roll across the surface, they act like miniature vacuum cleaners, picking up dirt and bacteria. So the leaves always look very clean, even when they’re located in dirty water,’ she explains. 

Man-made superhydrophobic materials work on the same principle, but typically suffer from mechanical weakness. ‘Whether you apply them to a hard or soft surface, they can normally be rubbed off quite easily,’ Carmalt adds. ‘We tried adhering our coatings using double-sided sticky tape, which worked surprisingly well – we carried out a series of abrasive tests, from finger wipes to sandpaper and were impressed with how well the tape adhered the paint to the surface.’ The problem with what she describes as the Blue Peter approach is that it limits the applications, so the team moved to spray adhesives. ‘You can easily apply spray adhesives to much larger surface areas and to both hard surfaces, such as metals and glass, and soft surfaces, such as cotton and paper.’ 

An artist’s spray gun was used to apply the coating onto hard surfaces, a syringe to small surface areas, and the researchers even dip-coated balls of cotton wool in the paint. ‘One of my favourite experiments was dipping the coated cotton wool into blue-dyed water. It was absolutely pristine white when we pulled it out – it hadn’t wetted at all.’ 

A slippery state

What about the vulnerability of superhydrophobic coatings in the presence of oil? Carmalt explains, ‘The surfaces normally lose their water repellency even when partially contaminated with oil, and this is largely because the surface tension of oil is lower than that of water, so it penetrates the surface. We found that even after our coated surfaces were contaminated with oil, water still formed droplets. I wouldn’t say they’re still superhydrophobic, but a slippery state is achieved – the oil gradually penetrates the surface when immersed, but it remains rough enough to retain its self-cleaning properties.’

Potential applications of the coating include car paintwork, home furnishings, mould-resistant paint and easy-clean surfaces that could reduce the risk of hospital-acquired infections. Another application is self-cleaning glass, but UCL won’t be putting window cleaners out of business any time soon – the thickness of the paint means that it has low transparency. ‘We’re going to do further studies to see whether we can create more transparent coatings that could be used for windows as well as anti-fogging car windscreens,’ Carmalt says. The next step is to undertake longer-duration durability tests as well as experimenting with alterations in the titanium dioxide scaffold.