metamitya ·
Significance
Plasmas at atmospheric pressure conditions are ubiquitous, in natural form, such as the familiar lightning, or produced through artificially created electromagnetic or electrostatic fields for industrial and scientific applications. One distinctive feature of these cold-type plasmas is their lack of a topologically defined shape, concurrent with spatial unsteadiness and nonuniformity. Here, we report the observation of a coherent and stable toroidal plasma that spontaneously forms under extreme hydrodynamic shear, without external electromagnetic action. The confined and chamberless nature of this plasmoid has potential implications for the investigation of plasma–matter interactions, in the development of plasma-based deposition techniques for the microelectronics industry, in the emerging field of plasma medicine, or as a model for energy-storing self-maintained plasmoids.
Keywords: water jet, hydrodynamic shear, tribo-electricity, luminescence, toroidal plasmoid
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.
In the epic poem The Lusiads (1), the great Portuguese explorer Vasco da Gama narrates how, during his discovery voyage to the Indies, he “saw, and clearly saw, the living light” at the tip of his caravel’s masts. The recount describes from experimental evidence what is known as Saint Elmo’s fire to seafarers facing stormy weather. This luminous phenomenon is, with the more common lightning (2), perhaps the only other atmospheric pressure plasma found in nature.
Atmospheric pressure plasmas are a nonthermal nonequilibrium form of plasma that, as a technology, requires no chamber or complex vacuum machinery and has a considerable scientific and industrial potential. Applications in science and engineering span from analytical chemistry, material processing, environmental science, energy, medicine, biology, and low-temperature plasma physics to microelectronics and display technology (3–6). Unlike the natural counterparts, these plasmas are obtained by artificially created electromagnetic fields and tend to be nonuniform and unstable (7) without any defining morphology. In this context, miniaturizing and confining plasmas to microscales and in microcavities is considered a viable and promising approach (4, 5, 7, 8).
In air a high potential electric field applied between two electrodes can cause the ionization of the gas elements. The phenomenon manifests as a blue–pink luminescent cloud and is commonly known as an electrical corona. The physics and conditions for onset of this atmospheric pressure plasma are well understood (9). According to Paschen law, electric fields of are sufficient to trigger a corona in dry air.
Here we report on the observation of a naturally formed, stable, unconstrained but topologically confined microscale toroidal plasma in air, at room temperature and atmospheric pressure. We demonstrate the creation of an atmospheric pressure microplasma without external electromagnetic action, through focused hydrodynamic shear. By spectroscopic, electrical, physico-chemical, and radio-frequency experimental evidence, we postulate a scenario for its formation. Our study suggests that energetic free electrons are produced by tribocharging from intense hydrodynamic shear in a wall-impinging water jet. Electron collisions with water molecules trigger the formation of charge carriers, allowing the free electrons to reach the water–gas interface and pass into the gas phase, where a plasma forms through the excitation and ionization of the gas elements. When the surrounding gas is air, the plasma cloud shapes into a topologically coherent structure characterized by a radio-frequency signature compatible with plasma resonance frequencies.
Results
Discovery.
Self-generation of a toroidal microplasma was observed in the flow field of a water microjet in air impinging upon a dielectric surface. The experimental setup consisted of a ruby nozzle with a orifice diameter and a nonelectri…