HOW do you clean up nanoparticles? Obviously, you sweep them up with a nanobrush.
Fabricated from millions of carbon nanotubes and resembling a tiny toothbrush, a nanobrush has been made that can clean very small surfaces and paint the inside of capillary tubes that are thinner than a human hair.
Their small size is not the only advantage carbon nanotubes have over traditional bristles, says brush designer Pulickel Ajayan at Rensselaer Polytechnic Institute in Troy, New York. Bristles are often made from animal hair, synthetic fibre and metal wire. But each has its limitations. Metals corrode and weaken, hair is not very strong and synthetic fibres melt.
Advertisement
Carbon nanotubes, on the other hand, are 30 times stronger than steel, yet five times less dense. They are highly elastic, resilient to heat, have large surface areas and, to top it off, conduct electricity. This ability makes them highly suitable as the contact brushes used to drive electric motors, says Ajayan.
Ajayan and Anyuan Cao, in collaboration with colleagues at the University of Hawaii in Honolulu, have designed brushes that have a silicon carbide fibre as the stem on which carbon nanotubes grow in a single row of bristles, like a toothbrush, or in groups of bristles, more like a toilet brush. The base of the stem is coated in gold, which acts like a 鈥渉andle鈥� and also inhibits the growth of nanotubes. Typically the smallest the bundle of bristles can measure is a few micrometres in diameter (Nature Materials DOI: 10.1038/nmat1415).
鈥淭he brushes have been used to sweep up 50-nanometre particles鈥�
The brushes have been used to sweep particles as small as 50 nanometres in diameter off a plain surface, and clean another surface pitted with microscopic grooves. The researchers also attached the gold handle of a three-pronged brush to the shaft of a small electric motor and cleaned the inside of a 300-micrometre-wide capillary by rotating the bristles. Then by coating the spinning brush with a red dye they painted the inside of the cavity.
It seems to be very good at cleaning up small particles, says Ajayan, although he is still not sure why. The aim now is to try and apply the brushes to more specific microelectronic and biomedical applications.
Nanofriction
Nanotechnologists may need to revise their understanding of friction if they want their tiny devices to work properly. At the molecular scale, friction forces can rise to 10 times normal.
Our understanding of what happens when very small objects come in contact is mainly based on theory. So Binquan Luan and Mark Robbins at Johns Hopkins University in Baltimore, Maryland, built a computer simulation of the mechanics of how molecules interact. Because a smooth surface is actually quite rough at the atomic scale, the contact areas and stresses between two nanoscale objects can increase or decrease by a factor of 2, the researchers found, while friction can increase by an order of magnitude (Nature, DOI: 10.1038/nature03700).
