Hot-wire probes having dimensions of the order of nanometers have been proposed for measuring temperatures (and possibly velocities) in boundary-layer flows at spatial resolutions much finer and distances from walls much smaller than have been possible heretofore. The achievable resolutions and minimum distances are expected to be of the order of tens of nanometers — much less than a typical mean free path of a molecule and much less than the thickness of a typical flow boundary layer in air at standard temperature and pressure. An additional benefit of the small scale of these probes is that they would perturb the measured flows less than do larger probes.

Nanoscale Hot-Wire Probes would be used to characterize a flow at several distances from a surface, typically ranging from tens of nanometers to a few micrometers. The lateral dimension, W, of the array of probes would be made much less than the expected characteristic dimension of lateral nonuniformity of the flow. The distance from the surface (L) of the most distant probe would be made much less than the expected thickness of the boundary layer of the flow.
The hot-wire components of the probes would likely be made from semiconducting carbon nanotubes or ropes of such nanotubes. According to one design concept, a probe would comprise a single nanotube or rope of nanotubes laid out on the surface of an insulating substrate between two metallic wires. According to another design concept, a nanotube or rope of nanotubes would be electrically connected and held a short distance away from the substrate surface by stringing it between two metal electrodes. According to a third concept, a semiconducting nanotube or rope of nanotubes would be strung between the tips of two protruding electrodes made of fully conducting nanotubes or ropes of nanotubes. The figure depicts an array of such probes that could be used to gather data at several distances from a wall.

It will be necessary to develop techniques for fabricating the probes. It will also be necessary to determine whether the probes will be strong enough to withstand the aerodynamic forces and impacts of micron-sized particles entrained in typical flows of interest.

The proposed probes may or may not be based on the same principles as those of larger hot-wire anemometer and thermometer probes. The potential for unexpected effects (e.g., quantized current or heat flow) in the nanometer size range makes it necessary to perform research on the basic physics of these probes in order to be able to calibrate them, operate them under conditions (e.g., constant current, or constant temperature) that will yield useful data, and interpret their thermal and electrical responses in terms of flow parameters.

This work was done by Ken T. Tedjojuwono and Gregory C. Herring of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category. LAR-16223.

NASA Tech Briefs Magazine

This article first appeared in the January, 2003 issue of NASA Tech Briefs Magazine.

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