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Switchable lotus effect creates self-cleaning surfaces

07 Sep 2006

A material that becomes super-water-repellent when illuminated with ultraviolet light could act as a self-cleaning surface in countless applications.

Scientists in Japan have developed a material that becomes water repellent when it is illuminated with ultraviolet light. The properties of the material mimic those of the lotus blossom - a flower which is renowned for being immaculately clean. What's more, the effect is reversible as team can switch the material back to its original state using visible light. (Angewandte Chemie International Edition 10.1002/anie.200602126)

The secret behind the lotus effect is a special microstructure on the plant's leaves. This structure forces water droplets to form beads which roll off of the surface picking up any dirt particles they encounter.

To date, scientists have not been able to duplicate this natural phenomenon but now this synthesized material could find uses wherever a self-cleaning surface is required. Windows and car bodywork are just two examples that spring to mind although the team believes there are many more.

Kingo Uchida from Ryukoku University and Shinichiro Nakamura from Mitsubishi Chemical Group Science and Technology Research Center synthesized their material from a family of compounds known as diarylethenes. "The thickness of the film is between 20 and 40 micrometers," Uchida told optics.org. "When we illuminate the surface with UV light at 254 nm from a mercury lamp, fibrils start to grow within five minutes but one day later is enough to observe superhydrophoboicity."

Uchida and colleagues switch the material back to its original form by illuminating it with light in the range 450 to 600 nm. "It takes only 10 minutes for the material to return," commented Uchida.

The effect originates from changes in the molecular structure. The diarylethene molecule is made of three rings that are hooked together. Ultraviolet light initializes isomerization which creates a fourth ring.

Uchida explains that the isomer with the closed fourth ring crystallizes in the form of needles. Visible light sets off the reverse reaction: the ring re-opens, and the needles disappear.

Uchida adds that he is only studying this phenomenon for scientific interest and does not have any immediate plans to commercialize the research.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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