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05 February 2003

Carbon nanotubes as flow sensors?

Carbon nanotubes may soon see use as a flow sensor as researchers draw on electrokinetic phenomena and slip boundary conditions that offer an in-depth understanding of microfluid flow in restricted microchannels.

Complex experiments have now demonstrated that the Coulombic effect, involving direct scattering of free charge carriers from fluctuating Coulombic fields of ions or polar molecules in the flowing liquid, is stronger than the phonon drag effect in generating electric current/voltage.

The outcome: A model has emerged for a practical flow sensor, capable of downsizing to small dimensions as short as a nanotube.

A new avenue has come about that can gauge flow in tiny liquid volumes, with high sensitivity at low velocities and exceptionally rapid response times.

"Many proposed applications of carbon nanotubes depend on simple mechanical or electrical properties, but nanotubes also have the potential to be devices in their own right-namely sensors," said Shirley Savage, a contributor to industry research firm Frost and Sullivan's Technical Insights' "Nanotech Alert" newsletter.

Another prospective development might entail building a voltage/current source in a flowing liquid environment. The resultant miniature energy conversion device is likely to have biomedical applications.

Latest analysis seeking to understand fluid flow in microchannels uniquely combines an equation for electrokinetic effects with the familiar Navier slip condition. The ensuing equation predicts fluctuating flow in a circular microchannel. This helps resolve general time-dependent problems and offers better understanding of fluid flow in microchannels with hydrophobic walls.

The concept permits the insertion of other models of slip besides those predicted by Navier's formulation. It is also of immense use because at present, almost all potential microchannels are likely to have circular cross-section and hydrophobic walls.