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SCIENTISTS in the US have created the ultimate non-stick surface, created by a thin layer of nanobubbles.

The bubbles are formed in trillions of nano-sized cavities which cover the material surface. "Our results explain how these nanocavities trap tiny bubbles which render the surface extremely water repellent," says study leader Antonio Checco, physicist at the US Department of Energy’s Brookhaven National Laboratory. The research could lead to a new class of non-stick materials for a range of applications, including improved-efficiency power plants, speedier boats, and surfaces that are resistant to contamination by germs.

Brookhaven physicist Benjamin Ocko, says the 'superhydrophobicity' effect occurs when air bubbles remain trapped in the textured surfaces, thereby drastically reducing the area of liquid in contact with the solid. This forces the water to ball up into pearl-shaped drops, which are weakly connected to the surface and can readily roll off, even with the slightest incline.

The non-stick surface was created by treating sheets of silicon with a thin layer of wax which renders them hydrophobic, which is then ‘pockmarked’ via diblock copolymer-assisted lithography.

The process works as follows: A thin (~40 nm thick) film of diblock copolymers – long molecules made of two distinct segments that repel each other – is deposited on the silicon surface which is then baked at around 150ºC to let the polymer molecules move around and find their equilibrium configuration. The repulsion between the two blocks of the polymer causes phase separation inside the film, leading similar blocks of different molecules to self-assemble, forming nanoscopic domains.

“In our case one of the blocks forms cylinders that are 20 nm across, 40 nm apart, hexagonally ordered and oriented perpendicularly to the silicon substrate,” Checco tells tce. “The cylinders were chemically degraded and removed to produce a polymer mask containing many cylindrical voids. By exposing this mask to an etching gas, we were able to etch nanocavities in the silicon surface. The cavities have the same size and periodicity as the polymer mask.”

The pockmarking drastically increases the surface area of the material which makes it considerably more hydrophobic than it already is. “It is energetically very costly for the water to fill the cavities because the surface of contact between water and solid would be very large,” Checco says. “Hence the shape of the surface and its hydrophobic nature ‘encourage’ the nanobubbles to form.”

Water barely penetrated the cavities, no matter how shallow or deep they were. Direct observation of the bubbles showed that they are only around 10 nm across and, unlike larger micrometre-sized bubbles, have nearly flat tops. "This flattened configuration is appealing for a range of applications because it is expected to increase hydrodynamic slippage past the nanotextured surface," Checco says. "Moreover, the fact that water hardly penetrates into the nano-textures, even if an external pressure is applied to the liquid, implies that these nanobubbles are very stable."

The team hopes that others will build on their insights into the nanoscale aspects of superhydropobicity and apply the knowledge to the design of future superhydrophobic non-stick surfaces.