The team, led by Hui Wu at Tsinghua University, Beijing, China, developed a new electrospinning technique that aligns hydrophobic polymer nanofibres as they form from solution by collecting them on a thin silver wire. When they collected the fibres on to a flat surface instead of the wire, the fibres formed in a random arrangement.
The artificial water strider can float on the water surface while carrying a weight 100 times heavier than the real insect
Wu showed that the random arrangement of fibres resulted in a hydrophobic surface like that of the lotus leaf, on which water droplets can roll in all directions with equal ease, known as isotropic wetting. But he found that the aligned fibres formed a surface on which water droplets roll off preferentially in one direction. This behaviour, known as anisotropic wetting, is displayed by bamboo leaves, goose feathers and the legs of water strider insects, but it has never before been successfully mimicked by man.
'We believe we are the first team to realise surfaces with anisotropic wetting in two or three directions in large scale by nanofibres. And we also successfully made artificial water strider's legs with fibrils for the first time,' says Wu.
Wu's artificial water strider can float on the surface of a water bath while carrying a weight of one gram, 100 times heavier than the real insect. The supporting force achieved could be a new record, Wu says: 'It was more than 200 dynes per centimetre, which is higher than that of any previously reported man-made water strider.'
Wu says he believes the technology could have some exciting uses. 'It may find its application in microfluid control, non-wetting and self-cleaning surfaces, and also biologically inspired water strider robots, which could serve as a new type of aquatic vehicles,' he says.
Polymer expert Vladimir Tsukruk, from the Georgia Institute of Technology, Atlanta, US, is enthusiastic about the work. 'This is very intriguing research. This technology-friendly approach of creating a three dimensional surface with distributed anisotropic wettability looks appealing for applications such as sophisticated tissue engineering. Also, it could be useful for fluid separation processes and microfluidic arrays with distributed droplet motion control,' he comments.
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