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Linking plasma formation in grapes to microwave resonances of aqueous dimers
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2019 Feb 19, 7:27pm 872 views 1 comment
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anonymous
2019 Feb 19, 7:29pm
Non-technical version. Scientists Produce Rigorous Study of Why Grapes Spark in the Microwave
A paper published Monday (shown above this comment) in a well-known science journal begins with the following sentence: “It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma.” A universally acknowledged truth indeed... but what causes this microwave marvel?
If you’re not familiar, putting grapes into a microwave to make sparks has become a popular YouTube trick. This new research from Canadian scientists shows that worthwhile advances can come from anywhere, even by studying something sort of silly.
“This is a regime that hasn’t been significantly studied before,” one of the paper’s authors, Pablo Bianucci from Concordia University in Montreal, told Gizmodo.
The trick usually shows two grape halves connected by a thin sliver of skin. After a few seconds of being microwaved, they begin to spark. Though various explanations exist online, researchers wanted to study the phenomenon more rigorously.
The researcher imaged both sliced grapes and hydrogel beads—made from a material that absorbs lots of water—as they sparked in the microwave. They realized quickly that the grape skin wasn’t required in order to get the sparks, as evidenced by the sparking in the hydrogel beads, held together only by their weight and the shape of the dish they sat in, according to the research published in the Proceedings of the National Academy of Sciences.
The specific geometry of two touching water-filled circular objects in an electromagnetic field creates resonances concentrated at the point where the spheres or half-spheres intersect. This becomes a very small hotspot with a high energy density, enough to create plasma out of the ions in the region where the objects touch.
Is the research worth publishing in a journal as high-profile as PNAS? The paper’s editor, University of Illinois chemistry professor Catherine Murphy, certainly thought so. “The fact that they were rigorous enough to pass through the process of peer review is a testament that they’re doing a good job on the technical end,” she told Gizmodo.
But the paper is far more than a gimmick, Murphy said. This sort of research on directed energy could find important use in other directed-energy systems, such as explosives or high-intensity laser pulses. Additionally, the paper presents a way to image electric fields in these sorts of physical setups, and could lead to advances in photonics more generally.
Plenty of quirky things could yield useful scientific results if you apply rigorous thinking and conduct a well-thought-out experiment, as demonstrated by this work. Also, so we’re clear: If you accidentally burn your house down microwaving grapes, it’s not my fault.
https://www.msn.com/en-us/news/technology/scientists-produce-rigorous-study-of-why-grapes-spark-in-the-microwave/ar-BBTOOif
A paper published Monday (shown above this comment) in a well-known science journal begins with the following sentence: “It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma.” A universally acknowledged truth indeed... but what causes this microwave marvel?
If you’re not familiar, putting grapes into a microwave to make sparks has become a popular YouTube trick. This new research from Canadian scientists shows that worthwhile advances can come from anywhere, even by studying something sort of silly.
“This is a regime that hasn’t been significantly studied before,” one of the paper’s authors, Pablo Bianucci from Concordia University in Montreal, told Gizmodo.
The trick usually shows two grape halves connected by a thin sliver of skin. After a few seconds of being microwaved, they begin to spark. Though various explanations exist online, researchers wanted to study the phenomenon more rigorously.
The researcher imaged both sliced grapes and hydrogel beads—made from a material that absorbs lots of water—as they sparked in the microwave. They realized quickly that the grape skin wasn’t required in order to get the sparks, as evidenced by the sparking in the hydrogel beads, held together only by their weight and the shape of the dish they sat in, according to the research published in the Proceedings of the National Academy of Sciences.
The specific geometry of two touching water-filled circular objects in an electromagnetic field creates resonances concentrated at the point where the spheres or half-spheres intersect. This becomes a very small hotspot with a high energy density, enough to create plasma out of the ions in the region where the objects touch.
Is the research worth publishing in a journal as high-profile as PNAS? The paper’s editor, University of Illinois chemistry professor Catherine Murphy, certainly thought so. “The fact that they were rigorous enough to pass through the process of peer review is a testament that they’re doing a good job on the technical end,” she told Gizmodo.
But the paper is far more than a gimmick, Murphy said. This sort of research on directed energy could find important use in other directed-energy systems, such as explosives or high-intensity laser pulses. Additionally, the paper presents a way to image electric fields in these sorts of physical setups, and could lead to advances in photonics more generally.
Plenty of quirky things could yield useful scientific results if you apply rigorous thinking and conduct a well-thought-out experiment, as demonstrated by this work. Also, so we’re clear: If you accidentally burn your house down microwaving grapes, it’s not my fault.
https://www.msn.com/en-us/news/technology/scientists-produce-rigorous-study-of-why-grapes-spark-in-the-microwave/ar-BBTOOif
In a popular parlor trick, plasma is created by irradiating grape hemispheres in a household microwave oven. This work ties the source of the plasma to microwave photonic hotspots at the junction of aqueous dielectric spherical dimers. We use a combination of thermal-imaging techniques and computer simulations to show that grape-sized fruit and hydrogel beads form resonant cavities that concentrate electromagnetic fields to extreme subwavelength regions. This is enabled by the large dielectric susceptibility of water at microwave frequencies. Furthermore, the absorptive properties of water are key to washing out complex internal modes and for allowing the evanescent hotspot build-up. Our approach to microwave resonances in high-dielectric materials opens a sandbox for nanocluster photonics research.
Abstract
The sparking of cut grape hemispheres in a household microwave oven has been a poorly explained Internet parlor trick for over two decades. By expanding this phenomenon to whole spherical dimers of various grape-sized fruit and hydrogel water beads, we demonstrate that the formation of plasma is due to electromagnetic hotspots arising from the cooperative interaction of Mie resonances in the individual spheres. The large dielectric constant of water at the relevant gigahertz frequencies can be used to form systems that mimic surface plasmon resonances that are typically reserved for nanoscale metallic objects. The absorptive properties of water furthermore act to homogenize higher-mode profiles and to preferentially select evanescent field concentrations such as the axial hotspot. Thus, beyond providing an explanation for a popular-science phenomenon, we outline a method to experimentally model subwavelength field patterns using thermal imaging in macroscopic dielectric systems.
It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma. This parlor trick has become a mainstay of science-fair projects and popular-science blogs (1), as well as online videos for over two decades (a YouTube search for “grape plasma microwave” will show numerous results for the phenomenon). The phenomenon is invariably demonstrated with a grape, cut in half with a thin line of skin left to bridge the two hemispheres and irradiated in a household microwave oven for a few seconds, sparking a plasma from the skin bridge (Fig. 1A). Numerous online videos that demonstrate this effect in an identical arrangement have garnered millions of views. While no formal literature exists to offer a physical explanation for this phenomenon, several popular-science sources online presume that the pair of hemispheres act as a short dipole antenna of sorts (2), with the conductivity of the wet and ion-rich skin bridge being a key component
While an explanation based on surface conductivity is a priori plausible, we present evidence that the effect has a bulk optical origin. Specifically, that the effect is a result of aqueous dielectric objects displaying morphology-dependent resonances (MDRs) at microwave frequencies. MDRs are synonymous with Mie resonances, which describe the near-field effects of resonant interactions of light with wavelength-scale objects (3, 4). The objects can be conductive or dielectric and absorptive or transparent, depending on the complex dielectric permittivity of the material. Research into pairs of conducting particles at nanoscales and microscales has shown a ubiquity of hotspots at the point of contact (5). Such surface plasmon resonances (SPRs) are localized to the surface (6, 7) and have been used to probe or excite molecular species that are too small to resolve by traditional optical methods (8, 9). The fact that nonabsorbing, nonconductive dielectrics can form MDR hotspots has garnered considerable recent attention (10⇓⇓⇓–14).
In this article, we present methods for studying Mie resonances in absorbing dielectrics in the microwave regime. With thermographic studies, we offer a low-tech method for experimentally measuring internal and evanescent near-field electromagnetic concentrations with subwavelength resolution. We combine these methods with finite-element simulations to show progressions from isolated resonances to coupled-resonator supermodes in aqueous dimers. The hotspots formed represent superfocusing on the order of
λ0/100. . With these tools, we provide a detailed description and explanation of plasma formation from fruit dimers in a microwave oven, as well as opening a sandbox for the study of nanocluster photonics using absorbing dielectrics.
More including The Formation of Plasma from Aqueous Dimers, Internal Field Characterization, The Effects of Absorption, Evanescent Hotspot Imaging with Thermal Paper, Surface Geometry and Hollow Quail Eggs, Summary:
https://www.pnas.org/content/early/2019/02/13/1818350116
Also here in color: https://www.pnas.org/content/pnas/early/2019/02/13/1818350116.full.pdf
#SciTech #microwavephotonics #dielectricresonators #plasmaionization #hydrogels #morphology-dependentresonances #Grapes #Microwave