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From the Labs: Nanotechnology

New publications, experiments and breakthroughs in nanotechnology–and what they mean.
February 19, 2008

Coating That Repels Oil
New materials clean themselves, elimi­nating the need for soap and water.

A new coating made of microscopic threads can repel a variety of liquids, including water (dyed blue), methanol (green), octane (red), and methylene iodine (clear).

Source: “Designing superoleophobic surfaces”
Gareth H. McKinley, Robert E. Cohen, et al.
Science 318: 1618-1622

Results: Researchers at MIT and the Air Force Research Laboratory at Edwards Air Force Base in California have made novel materials that cause oil to bead up and form near-spherical droplets that easily roll or even bounce off surfaces. The researchers also analyzed the mechanisms behind the materials’ oil-repellent properties and developed design rules that could be useful for making similar materials in the future.

Why it matters: The researchers’ oil-repellent surfaces could make rubber hoses and engine seals more durable by preventing them from absorbing oil and swelling. Eventually, the detailed design rules could help scientists develop materials for other applications–such as transparent, self-cleaning displays, something cell-phone companies have been working on for years.

Methods: The air force researchers first developed new molecules containing high concentrations of fluorine atoms. When applied to a surface in a thin film, the molecules cause oil to bead up. The MIT researchers found a way to blend these molecules with commercial polymers and enhanced the liquid-­repelling properties of the blended material by spinning it into microscopic threads. These threads accumulate on a surface, creating a rough, air-trapping network that alters the contact angle between the material and oil, causing the oil to bead up even more than it would on a flat film.

Next Steps: The polymeric surfaces aren’t ideal: for one thing, they’re opaque. The researchers hope that the design rules they developed will allow other researchers to create super-oil-repellent materials that overcome current limitations.

Better Lithium-Ion Electrodes
Silicon nanowires could increase the storage capacity of batteries.

Source: “High-performance lithium battery anodes using silicon nanowires”
Yi Cui et al.
Nature Nanotechnology 3: 31-35

Results: Researchers at Stanford University demonstrated that silicon nanowires used as anodes in lithium-ion batteries have five to eight times the energy-storage capacity of the graphite anodes normally used in the batteries. The researchers also showed that the nanowires can absorb and release lithium ions quickly over many cycles without breaking apart.

Why it matters: The advance could lead to greater storage capacity in lithium-ion batteries. Such batteries work by shuttling lithium ions between the cathode and the anode (the positive and negative electrodes) as the batteries are charged and discharged. Silicon has long been considered a promising electrode material because it can, in theory, hold 10 times as many lithium ions as graphite. But as silicon absorbs lithium ions, it swells to many times its original volume. Over several cycles, this normally causes silicon electrodes to break apart and stop functioning properly. The silicon nanowires, however, were able to swell to four times their original size and remain intact, demonstrating that silicon could be a practical material for battery electrodes.

Methods: The researchers distributed gold nanoparticles on a stainless-steel substrate. When they exposed the nanoparticles to silane, a gas containing silicon, the gold catalyzed the growth of silicon nanowires. The researchers then tested the nanowire electrodes. They also studied the composition and structure of the nanowires.

Next Steps: The researchers are developing other ways to make the silicon nanowires, with the goal of finding techniques that are less expensive and thus potentially more practical for commercial manufacturing. Better cathodes also need to be developed before the full benefits of the new anode materials can be realized.

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