Ice worms, they are called, strangely thin string-shaped air bubbles in freezing ice, which can grow to centimeters in length. They often appear in bands of parallel strands in natural ice. “They can only arise in a very precisely determined equilibrium,” says Jochem Meijer. Meijer received his PhD last year from the University of Twente on research into the literal textbook example of a phase transition: water freezes at 0 degrees Celsius, or: it transforms from a liquid to a solid form.
But as is often the case, such a natural phenomenon is in reality more complicated than in the books: messier, more multifaceted and therefore more interesting. Meijer investigated freezing water with air bubbles, oil droplets and small spheres of different materials, from plastic to metal.
The funny thing is: what exactly happens changes very much when you change the speed of the front
“Jochem had a lot of freedom in what he could do and found very nice things,” says Detlef Lohse, professor of fluid physics and Meijer’s supervisor, in his office on the Twente campus. In between the interviews, he jokingly and quickly switching gears holds an audience as head of an extensive research group. There is a coming and going of PhD students, postdocs and researchers at companies who have something to discuss with Lohse.
“We have been investigating phase transitions with multiple components and multiple phases in recent years,” says Lohse. For example, the people of Twente previously investigated dancing evaporating droplets on a hot plate Leidenfrost effectthe dissolution of droplets in water, and the melting of sea ice. Lohse: “And now there is freezing.”
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The fact that his research group was able to dedicate a PhD student, Meijer, to such a purely curiosity-driven topic was thanks to the Italian-Swiss Balzan Prize, which Lohse won in 2018. Lohse was able to spend 750,000 Swiss francs (then about 670,000 euros), including on freezing water.
A glass with a channel
That amount is not immediately apparent from Meijer’s test setup, which easily fits on a table in a small laboratory space. “It’s quite small-scale, one table topexperiment,” says Meijer. The heart of the installation is a glass containing a channel a few centimeters long, a few millimeters wide, and 0.2 millimeters high. This contains water, containing dissolved gas, oil droplets and other spheres or bubbles, typically about 0.1 millimeters in diameter.
The glass is placed on two copper blocks of different temperatures, for example 3 and -5 degrees Celsius. Meijer: “There is a hollow space under the blocks, and cooling oil runs through it, which can cool down to minus 20 degrees.” There is a space of a few millimeters between the blocks, Meijer shows: “Somewhere in between the water temperature becomes exactly 0 degrees. So that’s where the freezing front is.”
A camera films that front from above, visible as a dark line. “In the meantime, we slowly slide the glass under the camera. The front remains in the same place relative to the camera, while the bubbles slide towards it.”
This sliding does not happen very quickly; typical speeds at which the freezing front advances are millimeters per hour. A single experiment can easily take a few hours. “There have been periods when I was working here for months,” says Meijer.
“The funny thing is: what exactly happens changes very much if you change the speed of the front,” says Meijer. In this way, if the front advances slowly enough, small oil droplets are pushed out in front of the front. “As the freezing front and the droplet come closer together, a very thin layer of water will eventually separate the droplet and the ice. That very thin layer of liquid water, maybe 100 nanometers [een tienduizendste millimeter, ofwel enkele honderden watermoleculen]you always have. There is never a direct border between oil and ice.” As the front advances, the pressure in that thin layer of water increases, pushing the oil droplet away.
Even ice cream, good Italian ice cream, should not be put in the freezer
But if the speed of the advancing freezing front is increased, the drop can no longer keep up and is swallowed up by the freezing water. Meijer noted that the oil droplets take on a stereotypical teardrop shape, with a sharp point at the end where the front passes last. “The droplet enters the ice and appears to be compressed by the ice. The drop bulges out on the side of the liquid water. Ultimately, that gives it that pointed shape.”
Part of the effect can be explained by the fact that water expands when it freezes. Meijer: “But that was not enough to fully explain the distortion. It is also due to the extra pressure of that extremely thin layer of liquid water between the oil and the ice.” All molecular interactions that lead to the droplet shape have not yet been fully elucidated, but it is certain that the frozen droplet is still surrounded by a thin layer of liquid water.
Film from Jochem Meijer’s dissertation
The advancing front itself also showed unexpected phenomena. If the ice front advances toward an air or oil droplet at 15 millimeters per hour, the front bulges slightly as the two approach each other. That makes sense, because oil or air conducts heat less well than water. So the drop stops the incoming heat from the water, and the cold gets a little more space.
Indented front
But if the front advances a little faster, Meijer discovered, the front surprisingly bulges in exactly where it approaches the drop. “At first I thought: something went wrong,” says Meijer, “so I tried again, but the front continued to dent.” Ultimately, it turned out that a current can start around the drop, a result of the surface tension of the water. “It is higher at low temperatures, so the front of the oil droplet pulls harder on the water. This creates a current, which carries heat with it. That heat actually pushes the freezing front back a bit.”
The discovery led to a publication in the physics journal Physical Review Letters. Lohse and Meijer do not immediately see applications of this specific effect. Lohse: “But freezing plays a major role in the food industry: if you freeze and thaw certain foods, they lose taste and texture. Even ice cream, good Italian ice cream, should not be put in the freezer. This has to do with fat and oil particles that undergo changes.”
Freezing also plays a role in metalworking and, for example, the purification of silicon for computer chips: crystallization is a way to remove impurities. Some knowledge of freezing is already commonplace, for example among natural ice skaters, who know that ice with bubbles is weaker than slowly frozen ‘black ice’ without bubbles. And in climate science, the freezing and melting of seawater and polar ice plays a major, yet poorly understood role. Lohse: “But we are fundamentally interested: how does it work? We are systematically trying to get a grip on these types of phenomena.”
This worked with the ice worms, the elongated gas bubbles that can sometimes be seen in natural ice. Lohse explains it with an example close to home: “When you make ice cubes in your freezer, you often see that there is a whitish cloud inside. Those are air bubbles. There is always some air dissolved in water, but that gas can never be dissolved in ice.”
As the freezing front advances, the gas molecules are pushed forward, which can cause the water just in front of the front to become supersaturated: there is more gas dissolved in it than is stable. A gas bubble can then form at a condensation core, for example a small contamination in the water. Gas from the surrounding water flows in, and the bubble grows.
A precise balance
Lohse: “The question is which is faster: if the ice front advances quickly, the bubble will quickly be enclosed. But if the ice front is slow, the gas molecules will spread over the water, and there will no longer be supersaturation.’
Only with a precise balance between these two effects will the bubble not be overtaken by the advancing front, and will be continuously replenished with new gas, creating the string-shaped ice worm. In one article not yet publishedwhich can already be read on the preprint website Arxiv, Twente and French researchers mathematically describe the precise shape of the ice worm.
“The ice worms are therefore always perpendicular to the freezing front,” says Meijer. This is how ice worms record the past: from their direction you can see how the freezing front once advanced.