Nanotechnology Under Your Fingertips - Touching Nanoscience Everywhere
Our interactions with everyday materials are surface interactions â€” when we reach out and touch a table, or a glass, or clothing, we are experiencing the surface of these materials.Our interactions with everyday materials are surface interactions. When we reach out and touch a table, or a glass, or clothing, we are experiencing the surface of these materials; it is surfaces that we must take care of â€“ we must protect them from damage and keep them clean. In some cases, the properties of surfaces can be undesirable. For example, the surface of an automobile window or eyeglasses that become foggy in humid weather. Nanotechnology can provide methods for changing these surface properties without altering the bulk properties of the material.
Striking examples of the latest research findings were reported at the recent meeting of the American Chemical Society in Washington DC. At this meeting, Michael Rubner, Director of the Center for Materials Science and Engineering at MIT, has been working to understand and mimic nanostructured surfaces found in nature to create practical and durable surface coatings with novel properties. The lotus leaf has a surface topology that includes micron-scale hills and valleys decorated with nanometer sized oily particles; in essence, the lotus leaf is self-cleaning. The hills and valleys limit the surface contact area available to water condensing on the surface. The nanometer size particles prevent absorption of water into the valleys, resulting in a super-hydrophobic material. Several groups have been working to reproduce this surface structure, but as Rubner points out, â€śMost of the methods to date are expensive, or substrate limited, or require the use of harsh chemicals.â€ť The approach he advocates is to create alternating layers of anionic and cationic polyelectrolytes by alternately applying solutions of poly(acrylic acid), PAA and poly(allylamine hydrochloride), PAH. By dropping the pH of the solution after multiple layers have been applied the researchers change the charge of the polymers and initiate phase separation. The material self-assembles into a microporous structure which templates the deposition of 50nm SiO2 nanoparticles, which are reacted with semifluorinated silane to create a durable super-hydrophobic surface. The result: water droplets roll off immediately. Companies have already been working on perfecting this type of surface structure: in 2002, Pilkington of Australia developed Activ, the worldâ€™s first self-cleaning, laminated glass using DuPont Butacite PVB.
Another nature-inspired example for Professor Rubner is the Namibian desert beetle, the Stenocara. This beetle has an even more sophisticated nanostructured surface than the lotus leaf, including super-hydrophilic areas (to encourage absorption of water from the desert air) and super-hydrophobic areas (so that water droplets are formed and flow efficiently to the beetles mouth). Rubner found that if several layers of SiO2 nanoparticles are deposited, he could also reproduce a super-hydrophilic surface. By using smaller particles (7nm), the surface treatment becomes transparent, and after sintering can create a durable coating for glass. So the water does not form droplets (fog) on a super-hydrophilic surface, it instantly spreads. â€śOur coatings have the potential to provide the first permanent solution to the fogging problem,â€ť claims Rubner.
Another presenter at the meeting, Roger Wang, a researcher at the Textile and Clothing Institute at the Hong Kong Polytechnic Institute, claims inspiration from a 1951 film staring Sir Alec Guinness. In â€śThe Man with a White Suit,â€ť a scientist invents a fabric that won't get dirty or ever wear out. But when he tries to commercialize his new material, he faces opposition from clothing manufacturers who fear that everlasting clothing will destroy their business overnight. More than half a century later, the team at the Textile and Clothing Institute, lead by professors John H. Xin and Xiao Ming Tao, has developed a nanostructured surface treatment for clothing fibers based on a combination of self-assembled fluorinated surfactants and zinc oxide nanorods that partially fulfills the dream of the â€śWhite Suit.â€ť The fabric is not everlasting, but it is self-cleaning. "The treated cotton fibers remain water repellent even after 20 or 30 wash cycles," notes Wang. "They are also oil repellent, retaining an American Association of Textile Chemists and Colorists (AATCC) oil repellency number of more than 5.5 even after 20 wash cycles," he explained. This means that a wide range of "dirt" will be repelled from the cloth. NanoTex and U-Right are examples of companies already commercializing such fabric technologies for use by textile manufacturers. For textile companies, nanotech products that are compatible with its current production technologies are highly desirable, since most of these companies do not have large R&D budgets for nanotechnology.
The utilization of self-assembly processes to create micro and nanostructured surfaces and to template the deposition of nanoparticles will be discussed at one of the upcoming courses to be offered at the NSTI Nanotechnology Course Series in Washington, D.C. Six two-day nanotechnology courses will be taught by leading instructors from around the world on October 17 and 18, 2005. To learn more about the variety of courses being offered and to register, please visit http://www.nsti.org/courses/.