Researchers Illuminate How Ice Melts, One Nanosecond at a Time

Ice crystals melt into water within glaciers, on icy roads, and in cold drinks, but scientists know little about the fine details of this process. Now, researchers affiliated with the European Laboratory for Non-Linear Spectroscopy (LENS) have developed technology for monitoring thawing ice from one nanosecond after melting begins. The work will enable the researchers to study the formation of natural gas hydrates, ice-methane compounds, which form deposits on the sea floor.

DCO Extreme Physics and Chemistry Community members Margherita Citroni, Roberto Bini (both at Università di Firenze, Italy), Samuele Fanetti (LENS, Italy), and colleagues, developed ultrafast laser spectroscopy techniques to superheat an ice crystal under pressure and then detect the melting process at high temporal resolution. The study is the first to monitor melting and crystal formation at a time scale of nanoseconds to milliseconds. The researchers report their findings in a new paper in Proceedings of the National Academy of Sciences [1].

Previously, scientists used computer simulations to model the dynamics of melting ice during the picoseconds to nanoseconds after the process begins, but longer timescales become too complex to model. Ultrafast laser techniques account for the first tens of picoseconds, whereas photographic techniques capture the melting that occurs after milliseconds, but these experiments left a gap from nanoseconds to milliseconds that scientists had not yet explored.

In the current study, researchers shot a 15-picosecond pulse of infrared light at a crystal of water within a sapphire anvil cell held under high pressure or in a room pressure cell at low temperature. The laser’s energy superheated the crystal, seeding tiny “hotspots” where water molecules began to change from a solid to liquid. A nanosecond later, they turned on a second laser, which was scattered by the melting spots, called nucleation centers. The researchers then estimated the number and size of the droplets based on how much light they scattered and how much passed through the sample. They observed that, as melting proceeded, the nucleation centers grew and coalesced, then shrank and recrystallized as the water crystals cooled.

These experiments, which provide a high-resolution understanding of how ice melts, lay the foundation for future work on gas hydrates. “The main target of our study is to understand how gas hydrates form,” said Bini. “Previously we had the idea, but we didn’t have the technique. Now with this approach, we can give an answer.”

Gas hydrates form polyhedral structures, shaped much like a soccer ball, with multiple water molecules forming a cage around a molecule of methane. They are a significant carbon reservoir on the planet and a potential source of fuel.

Descriptions of seconds

millisecond

one thousandth of a second

4 milliseconds is how long it takes a fruit fly to flap its wing

microsecond

one millionth of a second

1 microsecond is the speed a strobe light flashes

nanosecond

one billionth of a second

1 nanosecond is how long it takes light to travel 1 foot in a vacuum

picosecond

one trillionth of a second

6 picoseconds is the average time it takes water molecules to make and break new hydrogen bonds

femtosecond

one quadrillionth of a second

10 - 100 femtoseconds is the time it takes for most chemical reactions to occur

Next, researchers will use a new ultrafast laser, which they built with DCO support, to examine how water and methane combine to make the polyhedral structure. Bini expects that the compound will form more slowly than the crystallization of ice, due to the complexity of the molecular cage and the fact that water and methane do not easily mix. The ultrafast laser spectroscopy project began soon after the initiation of the DCO, and the researchers look forward to applying these techniques, which they tested on ice, to answer their original questions about gas hydrates. “We had a very challenging idea and we had to build a lot of different small pieces to reach the end,” said Bini. “Now we are almost ready to probe the hydrate’s formation.”

From left to right: Researchers Samuele Fanetti, Naomi Falsini, Andrea Lapini, Margherita Citroni, Roberto Bini, and Paolo Foggi. Photo courtesy of Roberto Bini.

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