Rippled coating could improve fuel efficiency on aircraft

Researchers at Surrey University have used near-infrared radiation to selectively treat a colloidal dispersion of polymers in water to create various solid riblets.

Military aircraft have long used ‘efficient’ coatings on the fuselage — while, more recently, some commercial airliners belonging to carrier EasyJet were treated with a similar coating. This method essentially smooths out microscopic hills and valleys that naturally occur in paint coating.

The primary effect of this is to prevent the build-up of debris, including ice, dirt and seawater, but there is also a modest drag-reduction effect. However, the process is uniform and completely unselective, whereby anionic nanoparticles are pulled electrostatically into the pores.

By contrast, the Surrey researchers aim to introduce arrangements of so-called ’riblets’ in sizes ranging from less than a millimetre to a couple of centimetres.

They use a process called infrared radiation-assisted evaporative lithography to treat a colloidal dispersion of polymers. Essentially, the hotter spots evaporate more quickly and the plastic particles are then guided there as the evaporating water is replaced.

‘The infrared does two things, it drives the lateral flow by modulating evaporation rate but there’s also enough heating from that to sinter the particles, so that’s how we’re able to make a hard coating,’ said project lead Prof Joe Keddie of Surrey.

‘What we have done is a lot of work on controlling the pattern, we’re able to control the height — so peak to valley — or the distance between the riblets, which you would call the pits.’

The researchers have demonstrated the technique using a simple near-infrared lamp and a physical mask, but they have now started to experiment with the use of lasers.

Ultimately it could lead to a more automated throughput, whereby CAD designs inferred by computer modelling are uploaded to a laser array that then lays down the correct texture.

‘You have to work with someone modelling fluid dynamics to get the right arrangement, so in different positions in a ship you will probably need different types of riblets and different orientations of the riblets,’ Keddie said.

The actual production method is also reasonably energy efficient when compared with convection ovens, and could also find a use in the manufacture of microlenses or by artists and designers, he added.