Transparent surfaces for repelling oil underwater made from silica

Researchers in China have taken inspiration from fish scales and skeleton flowers to make a transparent underwater surface that stays clean by repelling oil.

In air (a) the surface is misty but underwater (b) it has high transparency and repels oil

Light scattering means that many synthetic oil-repellent surfaces are opaque, limiting their use. A transparent, oil-repellent surface would have applications in biology and underwater optics, including in diving goggles and cameras. Now, Feng Chen’s research group at Xi’an Jiaotong University has developed such a material.

Fish repel oil by trapping water within their scales to create a self-cleaning, oil-repellent coat and prompted part of the idea behind the work. Chen’s other brainwave was triggered by Diphylleia grayi – also known as the skeleton flower.

The skeleton flower on a sunny day (a) and in the rain (b)

In air Diphylleia grayi’s petals appear white, but on contact with water they become transparent. This change is not due to a pigment but loose cell structure in the plant petals. On sunny days the air–liquid interface of the petals causes diffuse reflectance, endowing the petals with a white colour, whilst on rainy days water enters the petals, yielding a water–water interface, increasing light transmission so they turn transparent.

By using femtosecond laser ablation to create rough nanostructures on a silica glass surface, Chen and his team have made a surface that combines both of these systems – it turns transparent and repels oil when in water. Chen’s student, Jiale Yong, highlights the value of ‘realising the importance of having a trapped water layer’ as they hope to ‘apply the transparent oil-repellent material in underwater oil-repellent optical devices’.

Laser-induced microstructures in the glass mimic fish scales and plant cells

Nanophotonics researcher, Hewei Liu from the University of Wisconsin-Madison, US, is impressed by the bio-inspired method referring to it as a smart idea. ‘The transparent oil-repellent surface provides researchers more choice to design and fabricate novel underwater optical devices,’ he adds. And Xu Deng, who specialises in innovative bio-inspired materials at the Lawrence Berkeley National Lab, US, says the work is an ‘important step towards finding an easy and reliable way to make oil-repellent materials’.