Even well below freezing, the ice surface begins to melt as temperatures rise

Physics is full of mysteries. To find a few worth exploring, look no further than an ice cube. At room temperature, of course, the cube will melt before your eyes. But even well below freezing, ice can move in barely perceptible ways that scientists are still trying to understand. Using imaging tools at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, researchers detected a phenomenon known as premelting at temperatures much lower than previously observed.

Their findings are published in the journal Proceedings of the National Academy of Sciences.

Premelting is why a patch of ice can be slippery even on clear, freezing days. Although the place is frozen, part of the surface is wet, an idea put forward by Michael Faraday in the mid-1800s. The idea of ​​a pre-melted liquid layer on the ice raises other long-standing questions about the how water transforms from liquid to solid to vapor – and how, under certain conditions, it can be all three at once.

In a recent study, scientists examined ice crystals formed below minus 200 degrees Fahrenheit. The team used Argonne’s Center for Nanoscale Materials (CNM), a DOE Office of Science user facility, to grow and observe the ice nanocrystals, which measured just 10 millionths of a meter in diameter .

In addition to what the study reveals about the nature of water at subzero temperatures, it demonstrates a method for examining sensitive samples in molecular detail: low-dose, high-resolution transmission electron microscopy (TEM). TEM directs a flow of electrons, which are subatomic particles, toward an object. A detector creates an image by sensing how electrons scatter across the object.

“Some materials are sensitive to beams. When you use an electron beam to image them, they can be changed or destroyed,” said Jianguo Wen, an Argonne materials scientist and lead author of the paper. Electrolytes, which exchange charged particles in batteries, are an example of a material sensitive to electron beams. » Being able to study them in detail without disturbing their structure could help the development of better batteries.

But to start, researchers are experimenting with the low-dose TEM technique on frozen water. After all, water is cheap and plentiful. Additionally, Wen said: “Ice is very difficult to image, because it is very unstable under the high-energy electron beam. If we can successfully demonstrate this technique on ice, imaging other beam-sensitive materials will be child’s play. “

The low-dose technique combines the aberration-corrected TEM of the CNM with a specialized direct electron detection camera. The system is extremely efficient at capturing information from each electron that hits a sample. It is therefore possible to obtain a high-resolution image using fewer electrons, thereby inflicting less damage on the target than a conventional TEM approach.

The low level of electron exposure makes it possible to capture something as delicate as an ice crystal in situ or in its environment. The research team used liquid nitrogen to grow ice crystals on carbon nanotubes at 130 degrees Kelvin, or minus 226 degrees Fahrenheit.

Previous studies have observed premelting near the triple point of water. At the triple point, the temperature is just above freezing and the pressure is low enough that ice, liquid, and water vapor can exist at the same time. At temperatures and pressures below the triple point, ice sublimates directly into water vapor.

The “rules” of water behavior are often clearly summarized in a simple phase diagram that maps the different states of water under different combinations of temperature and pressure.

“But the real world is much more complex than this simple phase diagram,” said Tao Zhou, an Argonne materials scientist and the paper’s other corresponding author. “We showed that prefusion can occur very far down the curve, although we can’t explain why.”

In a video captured during the experiment, two separate nanocrystals can be seen dissolving into each other as the ice is heated under constant pressure to 150 degrees Kelvin, or minus 190 degrees Fahrenheit. Although still well below freezing, the ice formed a quasi-liquid layer. This ultraviscous water is not accounted for among the simple lines of the phase diagram, where water passes directly from ice to steam.

The study raises intriguing questions that could be explored in future work. What is the exact nature of the liquid layer observed by the researchers? What would happen if pressure and temperature increased? And does this technique open the way to a glimpse of “no man’s land”, the state in which supercooled water suddenly crystallizes from liquid to ice? The centuries-old scientific investigation into the many states of water continues.

Co-authors with Wen and Zhou are Lei Yu, Thomas Gage, Suvo Banik, Arnab Neogi, Henry Chan, Xiao-Min Lin, Martin Holt, and Ilke Arslan of Argonne; Yulin Lin and Aiwen Lei of Wuhan University; and Nathan Rosenmann of the University of Illinois at Chicago.