Flexible Cold Atmospheric Plasma Jet Sources

From the source

1. What an atmospheric-pressure plasma jet is, stated plainly. It is the afterglow from a dielectric barrier discharge projected into ambient air, carried by a noble gas or an admixture with reactive gases. Its elementary parts are streamers — dipolar charged structures propagating between anode and cathode — and the guided streamers flowing out of the discharge region form the plasma column, which transits from a filamentary to a diffuse homogeneous mode, typically when a set of adjacent streamers overlap.Section 1, Introduction, opening paragraph

2. Two forces decide where a bundle of jets goes. The four key aspects of technological relevance are the discharge voltage waveform, pulsed against sinusoidal alternating current; the timing of each jet; the gas flow rate, one to ten litres per minute; and the inter-nozzle distance, a few millimetres. Two main interactions then govern the trajectories: hydrodynamic, which is attractive, and electromagnetic, which is repulsive. Divergence grows at lower flow rates and larger voltages; heavier gases such as argon rather than helium reduce it, because heavy plasma atoms carry more inertia; and in the limit of close-packed nozzles the air boundary layer between neighbours vanishes and the jets merge into a single brighter column.Section 2.1, Multi-jet interactions

3. The deflection is repulsion between plasma bullets, and it can be tuned. The jets push each other apart because the guided ionisation waves — the elementary constituents of the jet, detectable by fast intensified-camera imaging — repel electrostatically. Synchronising the voltage signal supplied to each jet changes the time of flight of those bullets and therefore the repulsion; in a crossed-flow two-beam experiment at ninety degrees the interaction potential could be tuned simply by choosing the same or opposed polarities for the two jets.Section 2.1, paragraphs on voltage waveform and bullet repulsion

4. Plasma sources that bend, with their numbers. A close-packed micro-jet source moulded in silicone with inserted rod electrodes, driven at 20 kilohertz, produced collimated micro-jets in square matrices from 2 by 2 up to 8 by 8 at sub-millimetre nozzle spacing with no observable jet-to-jet interaction. A silica-aerogel foil carrying one to five millimetre-sized nozzles, driven by 15 kilohertz alternating current at around 10 kilovolts peak to peak through copper tape electrodes, gave voltage, current and emission spectra qualitatively unchanged when bent concave or convex to a curvature radius of about 5 millimetres, at powers under 5 watts — low enough for temperature-sensitive samples such as biopolymers and organic tissue. A polyethylene cylinder 5.4 centimetres across, fed axially with helium through millimetre radial channels, fires jets outward in all directions.Sections 3.1 and 3.3

5. How a jet is carried down a metre of tube, and how far it can go. Threading a thin floating copper wire down a plastic tube, ending a few millimetres before the exit, propagates the electrical power to the tip in a transmission-line fashion with negligible losses, so there is no plasma along the tube and the jet at the exit stays near room temperature at the usual settings, about 10 kilovolts at about 10 kilohertz. Without the wire the discharge occupies the entire tube. The review gives an order-of-magnitude ceiling on length: taking the longest plasma-chemistry timescale as a tenth of a second to a second, a flow of one litre per minute and a bore of one millimetre — a linear velocity of about 20 metres per second — the maximal length is velocity times time, about 10 metres, which is consistent with the tube devices reported.Sections 4.1 and 4.2

6. What the flexible sources have already done, and what the authors want built next. A close-packed micro-jet array treating burn wounds in rat models reduced burn size to 20 percent of the original in two weeks against 80 percent for the untreated control, with tissue histology pointing to accelerated healing through regulation of anti-inflammatory processing; a flexible plasma gun through a narrow capillary produced a significant in-vivo reduction of pancreatic tumour in mice; and a gold-coated capillary kept working after being bent two hundred times through plus and minus 180 degrees. The milestone the authors set is a thin foil enclosing the circuit elements and gas microchannels — a multi-jet plasma shower that adapts its shape to any sample, dosed against the thicknesses of epidermis, dermis and hypodermis. The named next steps are miniaturised piezo-transformer drivers, flexible multi-jet headers for lithography on non-flat surfaces, and diffuse jets through flexible porous structures.Sections 3.1, 4.2, 5.1 and 5.2

The way in

How to cite it

Carles Corbella, Sabine Portal, Michael Keidar (2023) Flexible Cold Atmospheric Plasma Jet Sources doi:10.3390/plasma6010007

Where it sits in the curriculum

• Plasmoids, charge clusters and the orbs