Ion wind

Ion wind, ionic wind, coronal wind or electric wind is the airflow induced by electrostatic forces linked to corona discharge arising at the tips of some sharp conductors (such as points or blades) subjected to high voltage relative to ground. Ion wind is an electrohydrodynamic phenomenon. Ion wind generators can also be considered electrohydrodynamic thrusters.

An electrostatic pinwheel, or corona spinner, attached to a small Wimshurst machine (electrostatic generator) is shown stationary and rotating

The term "ionic wind" is considered a misnomer due to misconceptions that only positive and negative ions were primarily involved in the phenomenon. A 2018 study found that electrons play a larger role than the negative ions during the negative voltage period. As a result, the term "electric wind" has been suggested as a more accurate terminology.[1]

This phenomenon is now used in an MIT ionic wind plane, the first solid state plane, developed in 2018.

History

B. Wilson in 1750[2] demonstrated the recoil force associated to the same corona discharge and precursor to the ion thruster was the corona discharge pinwheel.[3] The corona discharge from the freely rotating pinwheel arm with ends bent to sharp points[4][5] gives the air a space charge which repels the point because the polarity is the same for the point and the air.[6][7]

Francis Hauksbee, curator of instruments for the Royal Society of London, made the earliest report of electric wind in 1709.[8] Myron Robinson completed an extensive bibliography and literature review during the 1950s resurgence of interest in the phenomena.[9]

In 2018, researchers from South Korea and Slovenia used Schlieren photography to experimentally determine that electrons, in addition to ions, play an important role in generating ionic wind. The study was the first to provide direct evidence that the electrohydrodynamic force responsible for the ionic wind is caused by a charged particle drag that occur as the electrons and ions push the neutral particles away.

In 2018, a team of MIT researchers built and successfully flew the first-ever prototype plane propelled by ionic wind.[10]

Mechanism

Net electric charges on conductors, including local charge distributions associated with dipoles, reside entirely on their external surface (see Faraday cage), and tend to concentrate more around sharp points and edges than on flat surfaces. This means that the electric field generated by charges on a sharp conductive point is much stronger than the field generated by the same charge residing on a large smooth spherical conductive shell. When this electric field strength exceeds what is known as the corona discharge inception voltage (CIV) gradient, it ionizes the air about the tip, and a small faint purple jet of plasma can be seen in the dark on the conductive tip. Ionization of the nearby air molecules result in generation of ionized air molecules having the same polarity as that of the charged tip. Subsequently, the tip repels the like-charged ion cloud, and the ion cloud immediately expands due to the repulsion between the ions themselves. This repulsion of ions creates an electric "wind" that emanates from the tip, which is usually accompanied by a hissing noise due to the change in air pressure at the tip. An opposite force acts on the tip that may recoil if not tight to ground.

A vaneless ion wind generator performs the inverse function, using ambient wind to move ions, which are collected yielding electrical energy.

See also

References
  1. Zyga, Lisa (February 7, 2018). "What causes ionic wind?". Phys.org. Retrieved August 27, 2018.
  2. DIVISION OF ENGINEERING: ENGINEERING ASPECTS OF ELECTROGASDYNAMICS*- by MC Gourdine - 1968 - Transactions of the New York Academy of Sciences - Wiley Online Library
  3. Demo 41-06 Pinwheel - Video Encyclopedia of Physics Demonstrations Archived 2014-04-16 at the Wayback Machine
  4. Electrostatic pinwheel:Van de Graaff makes pinwheel spin - Lecture Demonstrations - UC Berkeley Physics
  5. Fun with Static Electricity, Elliott County 4H - Science Fun, Inc. - Alan Kuehner (ret.) Brookhaven National Laboratory
  6. Electostatic pinwheel - Indiana University Demo Reservation
  7. Electrostatic Pinwheel, European, later 19th century - The Bakken - A Library and Museum of Electricity in Life Archived 2014-04-16 at the Wayback Machine
  8. Robinson, M. (1962, May). History of the electric wind. American Journal of Physics, 30(5), 366-372.
  9. Robinson, M. (1960, June 8). Movement of Air in the Electric Wind of the Corona Discharge. (AD0262830)
  10. Chu, Jennifer (November 21, 2018). "MIT engineers fly first-ever plane with no moving parts". MIT News. Retrieved November 21, 2018.
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