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Toroidal plasmoid generation via extreme hydrodynamic shear

  1. Francisco J. Alves Pereirac,d,1
  1. aGraduate Aerospace Laboratories, California Institute of Technology, Pasadena, CA 91125;
  2. bSchool of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel;
  3. cIstituto Nazionale per Studi ed Esperienze di Architettura Navale, Consiglio Nazionale delle Ricerche (CNR-INSEAN), Rome 00128, Italy;
  4. dGraduate Aerospace Laboratories, California Institute of Technology, Pasadena, CA 91125
  1. Edited by Parviz Moin, Stanford University, Stanford, CA, and approved October 16, 2017 (received for review July 20, 2017)

  1. 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 <mml:math><mml:mrow><mml:mrow><mml:mpadded width="+5pt"><mml:mn>212</mml:mn></mml:mpadded><mml:mi mathvariant="normal">m</mml:mi></mml:mrow><mml:mo>?</mml:mo><mml:msup><mml:mi mathvariant="normal">s</mml:mi><mml:mrow><mml:mo>?</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>212m?s?1. (B) Luminescence spectra in air and helium on a polished quartz wafer. In air, we recognize the second positive system of N2. In helium, we notice the emission bands of the ?OH radical in the region <mml:math><mml:mn>305</mml:mn></mml:math>305<mml:math><mml:mrow><mml:mpadded width="+5pt"><mml:mn>310</mml:mn></mml:mpadded><mml:mtext>nm</mml:mtext></mml:mrow></mml:math>310nm (visible only on SiO2, for LiNbO3 optical transmittance starts at <mml:math><mml:mrow><mml:mo>~</mml:mo><mml:mrow><mml:mpadded width="+5pt"><mml:mn>320</mml:mn></mml:mpadded><mml:mtext>nm</mml:mtext></mml:mrow></mml:mrow></mml:math>~320nm) and the first three spectral lines of the Balmer series.

  2. 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 <mml:math><mml:mi>V</mml:mi></mml:math>V 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 <mml:math><mml:mi>V</mml:mi></mml:math>V, after interpolation of the experimental values. The electric field <mml:math><mml:mi>E</mml:mi></mml:math>E, represented here as white arrowed lines, was calculated by spatial derivation of the interpolated potential field <mml:math><mml:mi>V</mml:mi></mml:math>V. Within <mml:math><mml:mrow><mml:mpadded width="+5pt"><mml:mn>75</mml:mn></mml:mpadded><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>75μm from the jet boundary where no data are available, the electric field is extrapolated to a radial distance of <mml:math><mml:mrow><mml:mpadded width="+5pt"><mml:mn>50</mml:mn></mml:mpadded><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>50μm (dashed black arrowed lines).

  3. Fig. 4.

    RF spectra. Shown are power spectra of the RF signals, on SiO2 and on LiNbO3, concurrent with luminescence in air (blue line) and helium (red line). Jet velocity was <mml:math><mml:mrow><mml:mrow><mml:mpadded width="+5pt"><mml:mn>255</mml:mn></mml:mpadded><mml:mi mathvariant="normal">m</mml:mi></mml:mrow><mml:mo>?</mml:mo><mml:msup><mml:mi mathvariant="normal">s</mml:mi><mml:mrow><mml:mo>?</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>255m?s?1.

  4. Fig. 5.

    Computational fluid dynamics simulation of the impinging jet flow. The color plot represents an area of <mml:math><mml:mrow><mml:mrow><mml:mrow><mml:mpadded width="+5pt"><mml:mn>500</mml:mn></mml:mpadded><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow><mml:mo>×</mml:mo><mml:mpadded width="+5pt"><mml:mn>5</mml:mn></mml:mpadded></mml:mrow><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>500μm×5μm and shows the strain rate, with the yellow line outlining the jet interface between liquid water and gas phase. We note the concentrated region of intense strain rate, approximately at <mml:math><mml:mrow><mml:mpadded width="+5pt"><mml:mn>66</mml:mn></mml:mpadded><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>66μm from the jet axis and within <mml:math><mml:mrow><mml:mpadded width="+5pt"><mml:mn>1</mml:mn></mml:mpadded><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>1μm from the wall. The cyan curve reports the radial distribution of the wall shear stress (right vertical axis), with a peak of <mml:math><mml:mrow><mml:mo>~</mml:mo><mml:mrow><mml:mpadded width="+5pt"><mml:mn>0.5</mml:mn></mml:mpadded><mml:mtext>MPa</mml:mtext></mml:mrow></mml:mrow></mml:math>~0.5MPa.

  5. Fig. 6.

    The scenario for tribo-electric charging and plasma generation in air. (A) The schematic depicts an early instant of flow over the dielectric quartz wafer, creating a weak electrostatic field represented by the light blue lines. Also shown is the initial formation of negative charges (red symbols) at the wafer surface and of positive charges <mml:math><mml:msup><mml:mi mathvariant="normal">H</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math>H+ (green symbols) in water from <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O interaction with <mml:math><mml:msub><mml:mtext>SiO</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:math>SiO2. <mml:math><mml:msup><mml:mi mathvariant="normal">H</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math>H+ combines with <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O to form <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msup><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math>H3O+ ions which accumulate in the narrow region (the anode). (B) As the flow speed elevates, free electrons are produced through the tribo-electric effect in the high-shear region. Electron collisions dissociate <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O, thus creating <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msup><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math>H3O+ that induce electrical conductivity in water. This allows the electrons to reach the free surface (the cathode) and pass into the gas phase. Through collisions the gas molecules are excited and/or ionized, forming the plasma. Concurrent dissociation of <mml:math><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>N2 and <mml:math><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>O2 triggers the formation of nitrites and nitrates in liquid water, inducing a surplus of solvated cations <mml:math><mml:mrow><mml:mrow><mml:msup><mml:mi mathvariant="normal">H</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>/</mml:mo><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow><mml:msup><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math>H+/H3O+ and consequent water acidification. A secondary electron emission mechanism takes place at the air–water interface from <mml:math><mml:msubsup><mml:mi mathvariant="normal">N</mml:mi><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math>N2+ ion bombardment of <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O molecules, whereby more cations <mml:math><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msup><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math>H3O+ are introduced into water and hydrated electrons (blue symbols) are produced and released into the plasma. Electrons are represented traveling along the electric field lines toward theanode.

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