Over 140 years after piezoelectricity was discovered, the first piezoelectric liquids have now been found by chance. Not only could they outperform solid piezoelectric materials in some existing uses, but they may also open up entirely new applications we’re yet to imagine.
Some crystals and other materials accumulate electric charge when pushed, a phenomenon known as piezoelectricity. Even proteins and DNA display this trait. Piezoelectric materials get a burst of publicity every few years, for example, shoes that generate electricity from steps. Showy and fun as such projects are, any claims to being a useful replacement for fossil fuels have not come close to being realized. On the other hand, piezoelectric materials are the basis of cigarette and gas stove lighters and are widely used in sensing devices.
Ever since their discovery, however, all piezoelectric materials have been solid. Now, a paper has announced the finding of two liquids that display the same behavior, along with a unique twist.
The discovery came out of experiments by Professor Gary Blanchard and graduate student Md Iqbal Hossain of Michigan State University on room-temperature ionic liquids, a type of liquid salt. Sodium chloride melts at 801° C (1,474° F), and most other familiar salts stay solid to similarly high temperatures – molten salt storage of solar thermal energy requires a lot of heat to get started. Ionic liquids consist of much larger ions than those in familiar salts, and these fail to pack together into neat lattices and therefore need far less energy to melt.
Although the first ionic liquids were found almost as long ago as the piezoelectric effect, until recently, they were considered just a curiosity. As the need to store solar and wind energy has become acute, ionic liquids have attracted renewed attention. An initially highly reactive class of liquids has also been made safer.
The authors were using a piston to squeeze two commercially available ionic liquids with names only an organic chemist could love – 1-butyl-3-methyl imidazolium bis(trifluoromethyl-sulfonyl)imide and 1-hexyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide – when they discovered the process generated electricity. “It shocked the hell out of us to see that,” Blanchard said in a statement. “Nobody had ever seen the piezoelectric effect in liquids before.”
Piezoelectric materials also change shape when exposed to electric fields, known as the converse piezoelectric effect. The liquids have what may be an equivalent behavior. The team found that passing an electric current through either liquid produced a dramatic change in optical properties.
When the fluids are placed in a container shaped like a lens, they bend light differently depending on electric charge, which Blanchard said, changes "the focal length of the lens.”
This creates the potential to make telescopes or cameras with widely flexible optical properties, just by adjusting a small current.
There are almost certainly abundant other applications no one has yet thought of, but the first uses could be to replace piezoelectric solids in cases where these have proven hard to manufacture.
One advantage of ionic liquids over solids is that the ones on which the discovery was made are recyclable and not considered hazardous to the environment. This contrasts with piezoelectric solids, many of which contain heavy metals such as lead. On the other hand, the effect Blanchard observed is weak – about a tenth of that in quartz – so other liquids may be needed.
“We’re still in the middle of trying to figure out the fundamental mechanisms underlying how piezoelectricity can occur in liquids,” Blanchard said. The working hypothesis is that the pressure makes positive and negative charges separate.
The discovery is published in The Journal of Physical Chemistry Letters.