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Boffins claim discovery of the first piezoelectric liquid

Move over, magic crystals – electric syrup is here

A research team from Michigan State University (MSU) has discovered a liquid they say "defies a simple theoretical explanation" because it has piezoelectric characteristics.

While much about the allegedly piezoelectric liquid remains a mystery – even to the boffins who discovered it – such materials could be used to create electrically controlled optics or even "a new field of piezo-hydraulics," according to MSU professor Gary Blanchard, one of the two authors on the study [PDF].

Piezoelectricity isn't anything new. We've known about the phenomenon – by which certain materials release electricity when placed under pressure – since around 1880. Piezoelectric materials are also subject to an inverse piezoelectric effect, which sees their dimensions change when electricity is applied.

Quartz crystals and other commonly-used piezoelectric materials, like lead zirconate titanate (PZT) and barium titanate, can be found in many consumer products. Quartz wrist watches, amplifiers, electric transformers and even cigarette lighters make use of the materials.

But all use solid piezoelectric substances because – at least until Blanchard's team's discovery – no piezoelectric liquid had been observed. 

Enter, magic sauce

Blanchard and his team discovered their piezoelectric liquid when working with a pair of ionic liquid salts that hold their liquid state at room temperature. Blanchard described the materials as being viscous "like motor oil, or maple syrup" – the best of which can (in this MSU alumnus and Michigander reporter's opinion) be found in the Mitten State.

When the materials were put in a cylinder and compressed with a piston, both generated electricity proportional to the force applied – which Blanchard said "shocked the hell out of us." (One assumes metaphorically.) The team also found an effect in the liquid they call the "induced free charge density gradient," which they said fulfils the inverse piezoelectric effect required of piezoelectric materials.

Prior to this discovery, it was believed that piezoelectric materials were only crystalline, and lacked inversion symmetry. Such crystals are highly ordered, and as a consequence researchers haven't bothered to look for such materials among liquids, Blanchard said.

For that reason, Blanchard and coauthor Iqbal Hossain are still trying to figure out how, exactly, piezoelectricity can occur in liquids. They're also trying to sort out whether other materials may be more conductive, as their tests resulted in a piezoelectric effect "an order of magnitude smaller than that of quartz." 

To test the potential for practical applications including optics, Blanchard and Hossain placed their liquids in a lens-shaped container. By zapping it with electricity, Blanchard said they were able to change the focal length of the lens.

Beyond those possibilities, the pair said in their paper that the discovery will likely "require some modification [of current solid-state piezoelectric theory] to account for [these] experimental observations." 

Liquids like the ones tested by Blanchard and Hossain are also more readily recyclable and environmentally friendly than solid state piezoelectric materials, many of which contain lead. But let's not get ahead of ourselves: as Blanchard puts it "we're in uncharted territory." ®

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