Beyond Self-Cleaning: Switchable Surfaces

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This story is a supplement to the feature "Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology" which was printed in the August 2008 issue of Scientific American.

By switching the hydrophobicity of precise locations on a surface, scientists hope to control fluids moving through networks of microscopic channels on so-called microfluidic chips.

Researchers at Pohang University of Science and Technology in South Korea attached a molecule based on azobenzene on top of polymer-silica multilayers [see box on preceding page]. A hydrophobic group on the end of the molecule, along with the roughness of the layers, makes the surface superhydrophobic (left). In ultraviolet light, however, the molecule bends, burying the hydrophobic group and making the surface superhydrophilic (right). Visible light promptly restores the original condition.

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On a treated surface, water clings to the areas made superhydrophilic by square spots of ultraviolet light. Water placed on other regions forms the characteristic nearly spherical drops of the lotus effect.

Credits: Kilwon Cho Pohang University of Science and Technology (Switchable Surface); Ann Sanderson (Illustrations)

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