Non-Aerodynamic Applications of Weakly-ionized Air Plasmas  
Weakly-ionized plasmas are not only the subject of increased interest as a means to improve the performance of aircraft through airflow control [l], but they are an integral part of various other devices and manufacturing processes. A brief outline is here given of the most common uses of weakly-ionized plasmas in manufacturing (surface treatment, surface cleaning, arc welding, etc) as well as of promising new applications in medicine (wound treatment).
Plasma Surface Treatment
Atmospheric weakly-ionized air plasmas are commonly used to obtain good adhesion on materials that have a low surface energy such as plastics and rubber by “energizing” the surface (i.e., increase its polarity).

Example of surface modification by an atmospheric plasma. Dyne Technology, June 2012.

Once the surface polarity is increased by the plasma, it then becomes possible to apply paint, bond, varnish to or to adhere some coating to the plastic or the rubber.
Plasma Surface Cleaning
Weakly-ionized air plasmas are also commonly used to clean surfaces especially in the semiconductor industry. For instance, in the youtube video shown below, an air plasma is used to clean a glass surface from grease and dirt while not damaging the glass itself.

An inductively-coupled weakly-ionized air plasma is used to clean a glass surface from dirt and grease.

How can a plasma clean a surface without damaging it? When the air is subject to a relatively strong radio-frequency wave, some free electrons are accelerated and impact neutrals liberating other electrons which are themselves accelerated and impact other neutrals hence creating more electrons and so on. Such an electron avalanche not only results in the air being ionized (and hence becoming a plasma), but also liberates in the process some excited species. These excited species and the highly energetic electrons then interact with the glass surface and vaporize the grease and dirt. The surface itself is not damaged because the plasma is “cold” and in non-equilibrium. That is, although the electrons are highly energized and have a temperature typically exceeding 8000 K, the neutrals have a temperature of 300-350 K. Because the neutrals compose more than 99.99% of the plasma matter, the plasma feels cold to the surface and does not damage it. Weakly-ionized non-equilibrium plasmas hence provide a unique way to clean hard to access surfaces or surfaces that are microscopic in size (such as in the manufacturing of semiconductors for instance) without damaging the surface itself.
Arc Welding
An important fabrication process of cars, ships, metal-framed buildings and other metal devices, arc welding consists of bonding two pieces of metal together through heat produced by an electric arc. The quality of the weld depends on several key parameters such as the diameter of the electrode, the power and voltage supplies, the distance between the electrode and the weld, etc. The heat generated by arc welding is created through a current flowing between two electrodes through a gas. Because one electrode is forcibly a cathode, and because the region near the cathode (i.e. the cathode sheath) is characterized by a low conductivity, a large amount of Joule heating is deposited in the gas in the vicinity of the cathode, hence resulting in melting of the cathode, the metal parts to be joined, or both. The simulation of the arc welding process can be done using the same physical model as used in plasma aerodynamics [l] and using the same numerical method accounting for the motion of the neutrals as well as the motion of the charged species. Thus, the computationally-efficient plasma model developed by our group [l] could also be used to simulate more efficiently arc welding.
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