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'Lotus leaf' membrane for oil spill
- Lo Tien Yin

LOTUS EFFECT: The membrane created by Dr Liu Xiaogang (inset) and the team which could be further developed to perhaps clean up oil spills in future.
Asians are probably familiar with the lotus leaf - the fresh ones lend flavour to food, while the dried ones can be used to wrap food. But scientists have gone beyond food and taken lessons from the lotus leaf. Although lotuses prefer muddy rivers and lakes, the leaves and flowers remain clean. Scientists studied them and found that they have a natural cleaning mechanism. Discovered in 1982 by Wilhelm Barthlott, the mechanism involves microscopic structure and surface chemistry of the leaves which enable water droplets to roll off their surface like mercury, taking with them dirt such as mud and tiny insects. This property has led to discovery of coating treatments which allow surfaces to stay dry and clean.

But recently, Dr Liu Xiaogang, Department of Chemistry has taken the "lotus effect" one step further. He stumbled upon a novel membrane which can both repel water and absorb oil - much like lotus leaves. Recalled Dr Liu: "I was doing my postdoctoral studies at the Massachusetts Institute of Technology, working with my colleague, Dr Jikang Yuan on a membrane coated with organic silane (a form of hydrocarbon with silicon content) molecules when we accidentally dropped water onto the membrane. We found that the membrane repels water droplets like lotus leaves but absorbs oils in large capacity. "

"It struck us that these materials could be used for oil and water separation. I continued to work on these membranes after I moved to NUS. After two years of hard work, we finally understood why the membrane selectively absorbs oils," he added.

The NUS team in collaboration with MIT, scientists from the National Institute for Materials Science, Japan as well as Institute of Materials Science, University of Connecticut, USA have published their findings in Nature Nanotechnology online (June 2008).

In their paper, they presented a self-assembly method for constructing thermally stable, free standing nanowire membranes. These membranes can selectively absorb oils up to 20 times the material's weight in preference to water, through a combination of superhydrophobicity and capillary action. What's more, the authors wrote, the nanowires that form the membrane structure can be re-suspended in solutions and subsequently re-form the original paper-like morphology over many cycles.

"Our results suggest an innovative material that should find practical applications in the removal of organics, particularly in the field of oil spill cleanup. The membranes when placed into a water and oil solution, will only absorb oil. Its high uptake capacity of up to 20 times is due to the porous internal structure of the membrane. Imagine a sponge that only soak up oil," he added.

There are limitations to their current method however - the cost for synthesis. "It may cost a little bit higher than traditional polymer materials. But this membrane can be recycled, thus lowering its cost in the long run," said Dr Liu.

The team plans to optimise the synthetic conditions for a broad range of materials other than manganese oxides that can be used for oil separations. They will also be developing a membrane system that will act as gas filters for the removal of hydrocarbon vapour.