Direct generation of measurable voltages and currents is possible when a fluids flows over a variety of solids even at the modest speed of a few meters per second. In case of gases underlying mechanism is an interesting interplay of Bernoulli's principle and the See beck effect: Pressure differences along streamlines give rise to temperature differences across the sample; these in turn produce the measured voltage.
Description of Carbon Nanotube Flow Sensors
The electrical signal is quadratically dependent on the Mach number M and proportional to the Seebeck coefficient of the solids. This discovery was made by professor Ajay sood and his student Shankar Gosh of IISC Bangalore, they had previously discovered that the flow of liquids, even at low speeds ranging from 10 -1 meter/second to 10 -7 m/s (that is, over six orders of magnitude), through bundles of atomic-scale straw-like tubes of carbon known as nanotubes, generated tens of micro volts across the tubes in the direction of the flow of the liquid.
Results of experiment done by Professor Sood and Ghosh show that gas flaw sensors and energy conversion devices can be constructed based on direct generation of electrical signals. The experiment was done on single wall carbon nanotubes (SWNT).
These effect is not confined to nanotubes alone these are also observed in doped semiconductors and metals.The observed effect immediately suggests the following technology application, namely gas flow sensors to measure gas velocities from the electrical signal generated.
Unlike the existing gas flow sensors, which are based on heat transfer mechanisms from an electrically heated sensor to the fluid, a device based on this newly discovered effect would be an active gas flow sensor that gives a direct electrical response to the gas flow.
One of the possible applications can be in the field of aerodynamics; several local sensors could be mounted on the aircraft body or aerofoil to measure streamline velocities and the effect of drag forces.
Energy conversion devices can be constructed based on direct generation of electrical signals i.e. if one is able to cascade millions these tubes electric energy can be produced.
As the state of art moves towards the atomic scales, sensing presents a major hurdle. The discovery of carbon nanotubes by Sujio Iijima at NEC, Japan in 1991 has provided new channels towards this end. A carbon nanotube (CNT) is a sheet of graphene which has been rolled up and capped with fullerenes at the end.