During the sweltering heat of summer, the simple act of making ice cubes may appear trivial. However, at the molecular level, the process of how freezing occurs is still not fully understood. Recently published on September 25 in Nature Communications, groundbreaking molecular-scale imaging has unveiled that the ice formed during freezing is unexpectedly flexible, as detailed by researchers.
The conversion of liquid water to ice represents a fundamental transition on Earth and beyond. Understanding the freezing mechanism and the stability of ice is crucial for atmospheric science, transportation safety, and the preservation of biological materials. To shed light on what enhances or reduces the stability of ice, materials scientist Jingshan Du and his team studied how ice reacts to structural imperfections and the minute air bubbles that can become trapped within its crystalline lattice.
Observing ice at the nanoscale presents significant challenges. The fragile chemical bonds among water molecules are susceptible to damage from the energy output of atomic-scale imaging techniques, such as X-rays and electron beams. According to Du from the Pacific Northwest National Laboratory in Richland, Washington, “To obtain atomic-level signals, substantial energy must be imparted to the sample, complicating the stabilization of ice under imaging conditions.”
To address these challenges, the research team employed an innovative technique that involved encapsulating liquid water between two carbon membranes within a cryogenic chamber. By meticulously cooling the chamber with liquid nitrogen to -180°C, they succeeded in creating a thin film of ice just a few hundred nanometers thick. The encapsulated ice was then transferred to a vacuum environment suitable for imaging, where the researchers captured rapid sequential images using a transmission electron microscope.
What they observed was remarkable.
As the ice froze, nanoscale bubbles became trapped, while others formed, migrated, merged, and dissolved—all within the solid ice matrix. “What’s intriguing is that throughout this entire process, the ice remains a singular solid crystal,” Du explains. Furthermore, investigations revealed that instead of presenting a smooth outline, the bubbles displayed a zigzag structure comprised of flat surfaces on the atomic scale. “This comes as no surprise, as bubbles, given time, develop facets for stabilization,” he adds.
Measurements established that the entrapment of gas bubbles did not impose strain on the ice crystal, which could lead to fractures. Rather, the crystalline structure adapted remarkably well to these imperfections, in contrast to metals or ceramics, indicating that “ice is quite accommodating to the bubbles,” Du notes. The inherent flexibility and malleability of the chemical bonds in water, even in solid form, were attributed to this unique tolerance for defects. Computer simulations corroborated these findings, demonstrating that ice retains its integrity despite the presence of these bubbles.
“Our hope is that this newfound understanding can help inform strategies to prevent ice accumulation, as well as elucidate how it occurs,” Du asserts. Comprehending the dynamics of ice formation, growth, and recrystallization is vital for engineering applications aimed at mitigating ice stabilization on aircraft wings, roadways, and other surfaces. This knowledge is also essential in the field of cryopreservation of tissues, where the formation of crystals can cause damage to cells and membranes. Furthermore, these findings may provide insights into glacier dynamics, where the presence of tiny bubbles influences large-scale melting and movement. “Our results indicate that ice remains stable even with inclusions of bubbles,” Du concludes.
Jungwon Park, a chemist at Seoul National University focusing on nanoscale material dynamics, expressed excitement over witnessing some of the earliest molecular-scale images of ice crystals, especially through a novel method that protects ice from the harsh conditions of high-vacuum imaging. His colleague, fellow chemist Minyoung Lee, emphasized that these findings open up “new avenues and vast opportunities” for examining effects at the liquid-solid interface in crystallization processes.
Although Du admits, “We are not yet observing the process of water transforming into ice,” he acknowledges that “this marks the first step towards that goal.”
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