Such devices differ from a conventional plasma apparatus, where the plasma is typically confined by electrodes or generated inside the chamber at a reduced pressure. The electrodes are used to ionize a flowing noble gas, which is then released into the ambient air or toward a target at atmospheric pressure conditions. Plasma jet devices are typically designed with dielectric tubing and a discharge electrode, either in the form of a single electrode inside the tubing or two ring electrodes outside the tubing. For example, the flow rate in a helium plasma jet at 0.2 standard liters per minute (slpm) corresponds to a neutral helium gas flow rate of approximately 0.47 slpm without discharge 10. ![]() Therefore, this phenomenon resembles the increase in the flow rate or gas speed. This is observed from the flow trajectories, which is additionally further lowered toward the horizontal plane of the flow. Nevertheless, when the electrical discharge, or so-called plasma, is generated inside the same flow, the gas speed of the flow increases. The flow trajectories are lowered toward the horizontal plane when the neutral gas flow is faster. In the jets, where a horizontally ejected neutral helium gas flow is used, the gas flow trajectories, or so-called free jet boundary, are typically bent upward due to the buoyant force. In addition to the aforementioned electric wind actuators, the extraordinary gas flow is manifested also in plasma jets operating at atmospheric pressure 9, 10, 11. Because of the possibility of producing a gas flow using only electrical energy without mechanical generation, these types of discharges have been investigated and applied in aerodynamic applications as alternative flow controllers. The electric wind generally arises in non-thermal air discharges, such as DC corona discharge and surface dielectric barrier discharge 6, 7, 8 in principle, the charged species in weakly ionized air are accelerated by the electric field and transfer their momentum to neutrals via high-frequency collisions. One of the well-known cases of this effect is the electric wind (also called ionic wind), which is created by an electrohydrodynamic (EHD) force in electrically charged fluids such as weakly ionized plasma and ionic solutions. This charged particle–neutral coupling is also applied in practical engineering. The neutral drag induces an electric field through the charge separation between ions and electrons under the geomagnetic field, while the ion drag resulting from the electric and geomagnetic fields exerts a force on neutrals 2, 3, 4. In planetary or stellar atmospheres (in which electrons, ions, and neutrals coexist in sufficient densities), including those of the Earth, the ion–neutral coupling profoundly affects ion and neutral particle kinetics and their properties 2, 3, 4, 5. In recent decades, state-of-the-art experimental approaches with knowledge-intensive instruments have enabled the discovery of numerous unexpected pieces and evidence that relate this natural phenomenon and its relevant mechanisms to c–n coupling. ![]() Hauksbee in the 1700s, this phenomenon rapidly became a popular scientific subject and attracted the attention of notable scientists, including of M. After the first observation of c–n coupling by F. Solving the hydrodynamic problem of such phenomena has been of paramount importance for centuries and requires expertise in a wide range of disciplines. Collisional coupling (that is momentum and energy exchange) between charged particles and neutral particles (c–n) can significantly impact any natural phenomena involving weakly ionized gases.
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