Lesson from Lotus Leaf:Photoreversibly Switchable Smart Surface
Professor Kilwon Cho and Doctor Ho Sun Lim of the Department of Chemical Engineering discovered the smart surface that can switch reversibly from extreme water-hating (superhydrophobic) to dramatic water-loving (superhydrophilic) via exposure to ultraviolet or visible light (J. Am. Chem. Soc., 2006, 128, 14458). Recently, this smart surface was introduced with the subject, “Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology” in a hot issue of materials science field in Scientific American.
The lotus grows in muddy water, but its leaves, when they emerge, are seemingly never dirty. The effect is caused by the combination of two features of the leaf surface: its waxiness and the microscopic bumps (a few microns in size) that cover it. The innumerable bumps on a lotus leaf transform its waxy surface into an extremely water repellent or superhydrophobic one – the contact angle exceeds 150 degrees, material and water on it forms nearly spherical droplets with very little surface contact. Raindrops roll easily across such a surface, removing all dirt. Since the lotus has this exceptional purity, many researchers have been interested in synthetic self-cleaning materials, some of which are based on this ‘lotus effect’. And this research has also broadened into an entirely new science of wettability, self-cleaning and disinfection.
Especially, smart surfaces that respond to external-stimuli are one of the newly attractive research fields due to their great advantages in wide applications. Their tunable wettability might be achieved by many means: ultraviolet light, electricity, temperature, solvent and acidity.
In 2006 Professor Cho’s research group achieved complete switchability by adding a compound based on the azobenzene molecule to the top of a silicapolyelectrolyte multilayer film. A hydrophobic group on the end of the azobenzene molecules, along with the roughness of the layers, makes the surface superhydrophobic, but under ultraviolet light the azobenzene compound changes chain configuration from straight-chain (trans) form to bent (cis) one and converts it to superhydrophilic. Visible light promptly restores the original condition.
Selective exposure to light carves the pattern on the surface, and the surface has been designed to attract or push away water in waffle-shaped pattern. Light irradiation made the exposed surface hydrophilic. In these regions, water spreads out over as much surface as possible. In the remaining hydrophobic parts, water gathers itself into a sphere to avoid contacting the surface. Scientific American has suggested that this kind of control could have major applications in the field of microfluidics, such as the microarrays now used for drug screening and other biochemical tests. For instance, hydrophilic pathways could be closed or opened by switching parts of them to be hydrophobic of hydrophilic.
Professor Kilwon Cho (Department of Chemical Engineering) received his Ph.D in polymer science from the University of Akron in 1986, and worked as a researcher at IBM Research Center. He has been with POSTECH since 1988.
Department of Chemical Engineering