Scientists have discovered the first known piezoelectric liquids, which are able to convert mechanical force to electric charge, and vice versa. The generally environmentally friendly nature of these materials suggests they may find many applications beyond standard piezoelectric compounds, such as novel, electrically controlled optics and hydraulics. However, much remains unknown about how they work, and therefore what they may be capable of.
Piezoelectricity was first discovered in 1880. The effect has since found a wide range of applications, including cellphone speakers, inkjet printers, ultrasound imaging, sonar equipment, pressure sensors, acoustic guitar pickups, and diesel fuel injectors.
Until now, all known piezoelectric materials were solid. Now scientists have for the first time discovered piezoelectric liquids. They detailed their findings in a study online 9 March in the Journal of Physical Chemistry Letters.
“Electrically controlled optics are feasible, just based on what we know now.”
—Gary Blanchard, Michigan State University
The researchers experimented with ionic liquids. These fluids are salts—compounds that are each made of both a positively charged cation and a negatively charged anion—that are liquid at unusually low temperatures. In comparison, table salt melts at roughly 800 ºC.
“They are often relatively viscous—think about them like motor oil, or maple syrup,” says Gary Blanchard, one of the authors of the study and a professor of chemistry at Michigan State University, in East Lansing.
Blanchard says the team was conducting standard experiments designed to better understand the basic properties of liquid-state salts (also known as ionic liquids). The team found that two different room-temperature ionic liquids each generated electricity when a piston squeezed them within a cylinder. The strength of the effect the researchers observed was directly proportional to the force applied.
“It shocked the hell out of us to see that,” Blanchard says. “Nobody had ever seen the piezoelectric effect in liquids before.”
What can piezoelectric liquids do?
Blanchard and his colleagues found that the optical properties of these ionic liquids could alter dramatically in response to electric current. For instance, when the researchers placed these fluids in a lens-shaped container, they found that an electric charge could modify how much the liquids bent light, “changing the focal length of the lens,” Blanchard says.
It remains uncertain what applications piezoelectric liquids might have. At the very least, the changeable optical properties of these fluids suggest “electrically controlled optics are feasible, just based on what we know now,” Blanchard says.
If electricity does cause piezoelectric liquids to change in dimension just as it does piezoelectric solids, “one could imagine a new field of piezo-hydraulics,” Blanchard adds.
“One would hardly ever think to look for a piezoelectric response from a liquid. The fact that we found one in a liquid was therefore a real surprise.”
—Gary Blanchard, Michigan State University
Furthermore, many piezoelectric solids may pose environmental hazards. For instance, the most commonly used piezoelectric ceramic, PZT, contains lead. In contrast, room-temperature ionic liquids are generally significantly more recyclable and environmentally benign than many common piezoelectric materials, the researchers say.
In addition, creating piezoelectric components of the desired shapes and sizes can prove difficult. In contrast, piezoelectric liquids could offer a wider range of design opportunities, Blanchard says.
When it comes to understanding how piezoelectricity happens, previous research has found the effect occurs in solids when a mechanical force deforms their structures, which shifts electric charges within them. Conversely, an electric charge applied to these materials distorts their structures.
“Both those things require substantial organization within a material,” Blanchard says. The basic assumption with liquids is that there is no persistent order in those materials. As a consequence, one would hardly ever think to look for a piezoelectric response from a liquid. The fact that we found one in a liquid was therefore a real surprise.”
The researchers suspect that applying mechanical force against ionic liquids may cause electric charges to separate within these fluids, generating an electric current. However, “we’re still in the middle of trying to figure out the fundamental mechanisms underlying how piezoelectricity can occur in liquids,” Blanchard says. “We’ve come across an effect that defies a simple theoretical explanation.”
The piezoelectric effect seen in these room-temperature ionic liquids was roughly an order of magnitude smaller than that seen in quartz, a widely used piezoelectric material. However, “we have no idea if there are other ionic liquids that might have a bigger effect,” Blanchard says.
Much remains unknown about piezoelectric liquids, such as whether there are ways to modify these fluids to improve the strength or speed of their piezoelectric effects. It is also uncertain how electric charge moves within these fluids—through the slow diffusion of electrically charged ions across space, or the more rapid exchange of electric charge between molecules, much like how electricity moves in wires.
“We’re in uncharted territory,” Blanchard says. “Once we can better understand the mechanisms behind how this family of materials does what it does, we would then have a much better handle on what it might be able to do and what applications it might be useful for.”
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.