Don’t mess with ice. When it’s stressed, ice can get seriously sparky.
Scientists have discovered that ordinary ice—the same substance found in iced coffee or the frosty sprinkle on mountaintops—is imbued with remarkable electromechanical properties. Ice is flexoelectric, so when it’s bent, stretched, or twisted, it can generate electricity, according to a Nature Physics paper published August 27. What’s more, ice’s peculiar electric properties appear to change with temperature, leading researchers to wonder what else it’s hiding.
The paper changes “how we view ice: from a passive material to an active material that may be at play for both fundamentals and applications,”
Xin Wen, study lead author and a nanophysicist at Institut Catala de Nanociencia i Nanotecnologia in Spain, told Gizmodo in an email.
A cold case in molecular chemistry
An unsolved mystery in molecular chemistry is why the structure of ice prevents it from being piezoelectric. By piezoelectricity, scientists refer to the generation of an electric charge when mechanical stress changes a solid’s overall polarity, or electric dipole moment.
The water molecules that make up an ice crystal are polarized. But when these individual molecules organize into a hexagonal crystal, the geometric arrangement randomly orients the dipoles of these water molecules. As a result, the final system can’t generate any piezoelectricity.
However, it’s well known that ice can naturally generate electricity, an example being how lightning strikes emerge from the collisions between charged ice particles. Because ice doesn’t appear to be piezoelectric, scientists were confused as to how the ice particles became charged in the first place.
“Despite the ongoing interest and large body of knowledge on ice, new phases and anomalous properties continue to be discovered,” the researchers noted in the paper, adding that this unsatisfactory knowledge gap suggests “our understanding of this ubiquitous material is incomplete.”
A shockingly simple solution
Fortunately, science likes to compartmentalize seemingly fundamental concepts. Electricity is no exception, so the researchers decided to investigate different “types” of electricity.
Geometry posed the biggest obstacle to understanding ice’s observed electric behavior, so the team opted for flexoelectricity, which can “exist in materials of any symmetry,” they explained.
For the experiment, they placed a slab of ice between two electrodes while simultaneously confirming that any electricity produced wasn’t piezoelectric. To their excitement, bending the ice slab created an electric charge, and at all temperatures, too. What they didn’t expect, however, was a thin ferroelectric layer that formed at the ice slab surface below -171.4 degrees Fahrenheit (-113 degrees Celsius).
“This means that the ice surface can develop a natural electric polarization, which can be reversed when an external electric field is applied—similar to how the poles of a magnet can be flipped,” Wen explained in a statement.
Surprisingly, “ice may have not just one way to generate electricity but two: ferroelectricity at very low temperatures and flexoelectricity at higher temperatures all the way to 0 [degrees C],” Wen added.
The utility of stressed-out ice
The finding is both useful and informative, the researchers said. First, the “flip” between flexoelectricity and ferroelectricity puts ice “on par with electroceramic materials such as titanium dioxide, which are currently used in advanced technologies like sensors and capacitors,” they noted.
Perhaps more apparent is the finding’s connection to natural phenomena, namely thunderstorms. According to the paper, the electric potential generated from flexoelectricity in the experiment closely matched that of the energy produced by colliding ice particles. At the very least, it would make sense for flexoelectricity to be partly involved in how ice particles interact inside thunderclouds.
“With this new knowledge of ice, we will revisit ice-related processes in nature to find if there is any other profound consequence of ice flexoelectricity that has been overlooked all the way,” Wen told Gizmodo.
Both conclusions will need further scrutiny, the researchers admitted. Nevertheless, the findings offer illuminating new insight into something as common as ice—and demonstrate how much there’s still to be learned about our world.