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Abstract PubMed PMID
Abstract
Saint Elmo's fire and lightning are two known forms of naturally occurring atmospheric pressure plasmas. As a technology, nonthermal plasmas are induced from artificially created electromagnetic or electrostatic fields. Here we report the observation of arguably a unique case of a naturally formed such plasma, created in air at room temperature without external electromagnetic action, by impinging a high-speed microjet of deionized water on a dielectric solid surface. We demonstrate that tribo-electrification from extreme and focused hydrodynamic shear is the driving mechanism for the generation of energetic free electrons. Air ionization results in a plasma that, unlike the general family, is topologically well defined in the form of a coherent toroidal structure. Possibly confined through its self-induced electromagnetic field, this plasmoid is shown to emit strong luminescence and discrete-frequency radio waves. Our experimental study suggests the discovery of a unique platform to support experimentation in low-temperature plasma science.
Keywords:
hydrodynamic shear; luminescence; toroidal plasmoid; tribo-electricity; water jet.
Copyright © 2017 the Author(s). Published by PNAS.
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Conflict of interest statement
The authors declare no conflict of interest.
Figures
Fig. 1.
Close-up views in the flow…
Fig. 1.
Close-up views in the flow field of the impinging jet in air. (…
Fig. 1.
Close-up views in the flow field of the impinging jet in air. (
A) Photograph of the laminar jet in open air impinging on a polished quartz surface; the jet diameter is . The transition of the impinging round jet to an expanding radial jet can be seen through the sharp jet annulus. 85 μ m Inset schematic shows the jet in the context of the experimental setup with the nozzle and the wafer support plate. ( B) Optical access beneath the experimental setup was used to examine the visual and spectral content of the radiating light through high-resolution microscopy. This photograph, taken with a photographic sensitivity of 640 (ISO standard) and a 1/5-s exposure time, shows the luminescent structure as observed through the wafer surface with a 50× microscope objective. Water was impinging on a polished quartz surface with a velocity exceeding . The central dark disk corresponds to the radial cross-section of the jet. ( 200 m ⋅ s − 1 C) Photographs of the luminescent structures taken with a 10× objective, on a polished quartz surface for a jet velocity of ( 295 m ⋅ s − 1 Left) and on a fine-ground LiNbO 3 surface for a jet velocity of ( 212 m ⋅ s − 1 Right). Streamers, resembling those of common plasma balls, are seen in C, Right as they stretch radially from the impinging site. Note that color levels of images have been enhanced by for better display. 33 %
Fig. 2.
Optical emission spectra. (
A…
Fig. 2.
Optical emission spectra. (
A ) The color of the rings changes noticeably…
Fig. 2.
Optical emission spectra. (
A) The color of the rings changes noticeably when the gas is changed from air to helium. This photograph of the luminescent spot in helium atmosphere was taken through the impinging surface of a polished quartz wafer with a 10× microscope objective. Jet velocity was . ( 212 m ⋅ s − 1 B) Luminescence spectra in air and helium on a polished quartz wafer. In air, we recognize the second positive system of N 2. In helium, we notice the emission bands of the •OH radical in the region – 305 (visible only on SiO 310 nm 2, for LiNbO 3 optical transmittance starts at ) and the first three spectral lines of the Balmer series. ∼ 320 nm
Fig. 3.
Potential and electric fields. We…
Fig. 3.
Potential and electric fields. We surveyed the region surrounding the toroidal plasma, using…
Fig. 3.
Potential and electric fields. We surveyed the region surrounding the toroidal plasma, using a probe at a fixed potential paired with a floating probe placed at various points in the plasma vicinity (
SI Materials and Methods). The potential of the interior of the plasma was determined to be consistent enough to act as a reference potential. The color plot shows the isocontour map of the potential field V , after interpolation of the experimental values. The electric field V , represented here as white arrowed lines, was calculated by spatial derivation of the interpolated potential field E . Within V from the jet boundary where no data are available, the electric field is extrapolated to a radial distance of 75 μ m (dashed black arrowed lines). 50 μ m
Fig. 4.
RF spectra. Shown are power…
Fig. 4.
RF spectra. Shown are power spectra of the RF signals, on SiO
2…
Fig. 4.
RF spectra. Shown are power spectra of the RF signals, on SiO
2 and on LiNbO 3, concurrent with luminescence in air (blue line) and helium (red line). Jet velocity was . 255 m ⋅ s − 1
Fig. 5.
Computational fluid dynamics simulation of…
Fig. 5.
Computational fluid dynamics simulation of th…