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Science Technology

Making a Liquid Invisibility Cloak 93

Researchers at Fudan University in Shanghai, China are proposing a method which could lead to the first soft, tunable metamaterial, the key ingredient in building an invisibility device. "The fluid proposed by Ji-Ping Huang of Fudan University in Shanghai, China, and colleagues, contains magnetite balls 10 nanometers in diameter, coated with a 5-nanometer-thick layer of silver, possibly with polymer chains attached to keep them from clumping. In the absence of a magnetic field, such nanoparticles would simply float around in the water, but if a field were introduced, the particles would self-assemble into chains whose lengths depend on the strength of the field, and which can also attract one another to form thicker columns. The chains and columns would lie along the direction of the magnetic field. If they were oriented vertically in a pool of water, light striking the surface would refract negatively – bent in way that no natural material can manage."
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Making a Liquid Invisibility Cloak

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  • by stevenj ( 9583 ) <stevenj.alum@mit@edu> on Friday January 08, 2010 @03:00PM (#30698168) Homepage

    All metamaterials are not created equal. A metamaterial is an electromagnetic medium created by a composite of tiny (very subwavelength) constituent structures, put together in such away that longer wavelengths see an "average" material with properties very different from those of the constituents. Usually, the goal is to use resonant effects in the microscopic constituents to make a material that is effectively very different from naturally occuring EM media. But this can be done for many different purposes.

    A negative-refractive metamaterial is designed to have an effective "negative" index of refraction, which makes Snell's law (refraction) bend backwards, and can potentially be used for flat-lens near-field imaging, subwavelength imaging (again only in the near field), etcetera. The main practical difficulty here is that the most interesting applications of negative-index materials are in the visible or infrared regime, but negative-index metamaterials rely on metallic constitutents and metals become very lossy at those wavelengths.

    Recent "invisibility" cloak proposals are based on the observation that there is a one-to-one mapping between transforming space to "curve around" the object being cloaked and keeping space the same and transforming the materials. So, if you can make materials with certain properties, they could effectively cloak an object by causing all the light rays to curve around the object just as if space were curved. Although this is mathematically quite beautiful, there are many practical obstacles to making this a reality. The proposal is to make the required materials via metamaterials, but these are NOT negative-index metamaterials. The required materials theoretically tend to require some singularities (points where the index blows up or vanishes), and trying to approximate that in practice inevitably involves losses which spoil the cloaking. In general, the bigger the object to be cloaked compared to the wavelength, the smaller the losses have to be, and the narrower the bandwidth is going to be. When you work out the numbers, you see that this is why all the experimental demonstrations of cloaking have only "cloaked" (reduced the scattering crosssection, but not to zero) objects that were a wavelength or two in diameter. Cloaking macroscopic objects at visible wavelengths is a fantasy because the material requirements are insane. The only remotely practical prospects seem to be cloaking objects on the ground (which makes things technically easier because the coordinate transformations are nonsingular) to long-wavelength radiation, e.g. cloaking something against radio waves.

  • by theIsovist ( 1348209 ) on Friday January 08, 2010 @03:01PM (#30698190)
    you're missing the point. this is more of a lens. It redirects light around the object. now, how fluidly it does this has yet to be seen, but any light that originates behind the object will be bent in a way that it never strikes the object. I'm not sure how this would look in real life, but given a mathmatically perfect lens, the object would bend all light around it so that it comes out exactly on the other side. In that case, as far as our sense of sight is concerned, the object would not exist
  • by stephanruby ( 542433 ) on Friday January 08, 2010 @03:19PM (#30698442)

    Very cool link, there was just a typo in your url.

    The correct url is http://www.moillusions.com/2007/12/julian-beevers-new-3d-sidewalk.html [moillusions.com]

  • Re:Star Wars (Score:3, Informative)

    by jfengel ( 409917 ) on Friday January 08, 2010 @03:19PM (#30698452) Homepage Journal

    The big question though is this something new?

    Well, besides not being fictional, the big idea is that the thing (potentially) has a negative index of refraction, something not even the fictional lenses do.

    Negative refraction is useful in making invisibility shields, by directing light completely around object surrounded by it.

    This doesn't go nearly that far; it's a step towards a new way of constructing metamaterials with negative indexes. That's important; the "invisibility" stuff is just press-release science because invisibility is far more interesting than magnetically-controlled metamaterials.

You know, Callahan's is a peaceable bar, but if you ask that dog what his favorite formatter is, and he says "roff! roff!", well, I'll just have to...