In the dog days of summer, popping a tray of water into the freezer to make ice cubes may seem mundane. But at the smallest scales, we still don’t know a lot about how freezing unfolds. Now, the first ever molecular-scale movies of ice reveal that the resulting crystal is surprisingly flexible, researchers report September 25 in Nature Communications.
The transformation of liquid water into ice is a fundamental process on Earth and beyond. The freezing process and the stability of ice are vital to atmospheric processes, transportation safety and the preservation of biological tissue. To better understand what stabilizes and what weakens ice, materials scientist Jingshan Du and his colleagues investigated how well ice tolerates structural imperfections and tiny bubbles trapped in its crystalline structure.
Watching ice at the nanoscale is incredibly hard. The weak chemical bonds between water molecules can be easily damaged by the energy sources used for atomic-scale imaging, such as X-rays and electron beams. “You need to put a lot of energy into the sample to get atomic-level signals,” says Du, of Pacific Northwest National Laboratory in Richland, Wash. “It’s really difficult to stabilize ice in the conditions you need for imaging.”
To overcome these issues, the researchers developed a technique that involved sandwiching liquid water between two protective carbon membranes inside a cryogenic cell. By slowly cooling the cell with liquid nitrogen to –180° Celsius, they created an encapsulated ice film less than a few hundred nanometers thick. The team then moved the protected crystal sandwich into a vacuum chamber, needed for imaging, and captured snapshots in rapid succession using a transmission electron microscope.
Then, they watched the magic unfold.
Nanoscale air bubbles became trapped during freezing; new bubbles also formed, moved, shrank, merged and dissolved — all within solid ice. “What’s fascinating is that, throughout the entire process, ice keeps being a single solid crystal,” Du says. Upon further examination, the researchers found that instead of a smooth curved surface, the bubbles had a zigzag pattern with repeated flat surfaces at the atomic level. “That’s what you’d expect if you give the bubbles enough time to settle down, as the curved bubbles develop facets to stabilize,” he explains.
Measurements confirmed that these trapped gas bubbles did not strain the ice crystal, which could cause fracturing. Instead, the structure adapted surprisingly well to these defects, unlike other materials such as metals or ceramics. “Ice is pretty happy with the bubbles,” Du says. The reason, he explains, is that water’s chemical bonds make it extremely flexible and malleable — even as a solid. Computer simulations confirmed ice’s unique tolerance for defects without compromising the crystal’s integrity.
“We hope this new insight can guide us in approaches to preventing ice buildup, and how it occurs,” Du says. Understanding the dynamics of how ice forms, grows and recrystallizes is important for developing engineering strategies that could inhibit crystals’ stabilization on airplane wings, roadways and other surfaces as well as during cryopreservation of tissues, where crystals could puncture cells and membranes. Finally, the results might help connect the dots in models of glacier behavior, where small-scale bubbles impact large-scale melting and movement. “What we found is that ice is not going to be less stable with bubbles in it,” Du says.
Jungwon Park, a chemist at Seoul National University who studies nanoscale material dynamics, says it’s exciting to see one of the earliest nano- to molecular-scale images of ice crystals, using a new method to shield the ice from the high-vacuum imaging environment. His colleague and fellow chemist Minyoung Lee note that the findings provide “new insight and vast opportunities” for investigating effects right at the liquid-solid interface in crystallization.
“We’re not watching water freeze into ice just yet,” Du says. “But this is the first step toward that.”