New Insights into Ice's Quasi-Liquid Surface Layer

doi:10.5281/zenodo.11656256

Published June 14, 2024 | Version v1

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New Insights into Ice's Quasi-Liquid Surface Layer

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For over 160 years, scientists have debated the peculiarities of ice's exterior. Frozen water is covered by a layer of molecules that behave like a liquid. A recent experiment visualizes the surface of ice, offering clues about the origins of this quasi-liquid layer.

This melty coating, known as "premelting," appears even at temperatures well below freezing, acting as a lubricant that explains ice's slipperiness in frigid conditions. Since British scientist Michael Faraday first pondered the idea of a liquid-like coating in the 1850s, the unusual surface of ice has remained poorly understood.

In a new study, scientists used atomic force microscopy to measure the locations of atoms on ice's surface. At temperatures around –150° Celsius, the surface of ice consists of not just one kind of ice, but two, report physicist Ying Jiang of Peking University and colleagues on May 22 in Nature. Jiang notes, “Ice is not so perfect,” as the team found structural defects that seem to initiate the premelting.

Ice varies in types based on molecular arrangements. Under normal conditions, water molecules form layers of hexagons stacked atop one another, known as hexagonal ice (ice Ih), which Jiang's team studied. However, their atomic force microscope images revealed that the surface consisted of regions of both ice Ih and cubic ice (ice Ic), where hexagons in each layer are shifted to resemble the carbon atom arrangement in diamonds.

Chemist Yuki Nagata of the Max Planck Institute for Polymer Research in Mainz, Germany, praised the study, saying, “I was super impressed.… The picture is so beautiful.” Identifying molecule locations is challenging, but the team succeeded admirably.

At the borders between the two ice types, the researchers found defects due to misalignment of the two patterns. When the temperature increased slightly, these disordered regions expanded, marking the initial stages of premelting. Further temperature increases amplified the disorder at these borders. Typically, ice has crinkled layers with some water molecules positioned lower or higher, but the team discovered regions where molecules lined up in a plane, serving as "seeds" of further disorder.

At even higher temperatures, computer simulations suggest this disorder would expand to cover the entire ice surface, creating a fully liquid-like layer.

Determining the structure of ice's surface required pinpointing individual hydrogen atoms—effectively just single protons—a challenging task with atomic force microscopy. The researchers enhanced their technique by attaching a carbon monoxide molecule to the probe tip. By measuring the force between the tip and the ice surface as it scanned, the team deduced the proton locations.

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