A leaf from nature

High-voltage power lines often risk short-circuiting and damage because of dirt and dust build-up on their ceramic or silicone insulators.

But that problem could soon be a thing of the past thanks to researchers at the Georgia Institute of Technology, who have developed a self-cleaning insulator coating that mimics the natural, non-stick surface of the lotus leaf.

The water and dirt-repellent exterior contains a combination of silicone, fluorocarbons and inorganics such as titanium dioxide and silicon dioxide that is able to withstand ultraviolet radiation.

The self-cleaning power lines are one of several lotus surface applications being studied by the team. The plant’s unique properties have intrigued researchers for decades.

Although it prefers to grow in muddy water, its leaves remain clean because of an unusual combination of a water-repelling surface, with micron-scale hills and valleys and nanometer-scale waxy bumps that create rough surfaces. Water droplets roll off the leaf’s surface like mercury, taking mud, tiny insects and contaminants with them.

The team first attempted to duplicate the two-tier surface three years ago using organic materials like polybutadiene. ‘Organics are very good, but their carbon bonds tend to break down under high intensive UV,’ said project leader CP Wong, a professor at Georgia’s School of Materials and Engineering.

That led the team to try a combination of inorganics, such as titanium dioxide and silicon dioxide, along with silicone and fluorocarbon to mimic the lotus surface. ‘Titanium dioxide and silicon dioxide tend to be very robust in terms of UV degradation,’ explained Wong. The researchers’ other application studies for lotus-like surfaces are being supported by, among others, the National Science Foundation and NASA.

The prototype has shown excellent durability in more than 1,500 hours of testing, said Wong. ‘We’ve simulated the coating in high voltage and condensed salt fog tests, which mimics power lines near the coast.’

The team is being funded by the National Electric Energy Testing Research and Applications Centre (NEETRAC), a consortium of over a dozen US energy companies. Wong said they are all keen to implement the surface as soon as possible.

The researchers also hope a lotus coatings application will prevent ‘sticking’ of micro-electromechanical systems (MEMS). Wong said that when moisture is trapped in between the structures of MEMS and substrates they tend to stick. This in turn deforms the structures and results in device failure. ‘All we do is put a thin layer of coating on a nanoscale and it solves the problem,’ he said.

Wong said the researchers are also looking into lotus surfaces for biological applications. One study is investigating the use of surfaces on implantable medical devices to prevent cells from attaching and forming blood clots. If successful, this application could replace anti-clotting materials that are coated on to implantable devices like stents.

Another study is attempting to use carbon nanotube bundles to create the surface bumps needed to prevent dust from accumulating on the solar panels of spacecraft on the moon or Mars — where there is no rain.

Researchers reckon that arranging patterns of nanotube bundles a few microns apart and applying a weak electrical charge will keep dust away and maximise efficiency.

‘We are also looking into applying it to astronaut’s space suits,’ said Wong, adding that space particles attaching to astronauts’ suits has been a problem since Man’s first moon walk in 1969.

‘One of the problems when they get back are the particles collected on their suits start diffusing around the whole lunar modules,’ he said. ‘By applying the surface to the suit, we will hopefully mitigate this problem.’