How Hair Ice Grows
(Below is a transcript of the video.)
In 2013, American Scientist featured an article on odd ice formations on plant stems, including these curling ribbons of ice.
One of the types of ice discussed in the article was hair ice—long, thin strands of ice that grow under quite specific conditions. The only problem is that a new study shows the theory put forth at the time—that gas pressure pushes the water out—isn’t correct.
We asked Christian Mätzler, a physicist at the University of Bern in Switzerland, and the lead author of the study, to provide an update on the hair ice formation mechanism.
"The earlier idea was that the water is pushed out by some gas pressure acting inside the wood. But when we thought about how gas pressure can pump out the water from the vessels inside the wood, it occurred to us that it’s not possible to grow hairs of several centimeters in length when the diameter of the branch is only about five or ten centimeters. So it seems to be as soon as the air gets at the surface of the wood, then a bubble must form and that’s it, the pressure is gone."
Dr. Mätzler explained that pressure cannot be the cause of the long hair ice forming, because in samples of wood where the fungus was killed off by heat, the same quantity of ice formed on the surface, but not in the same formation—just a crust instead of the hairs."
"And other the other hand, I got in contact with people who suggested ice segregation to be active here in this case. And it turned out that this is just the opposite, it’s not a pressure it’s a suction, the water is sucked out by a force that is caused by the ice."
Dr. Mätzler explains how ice forms during ice segregation.
"From the way the wood is cooled, it must first form on the surface. As soon as this ice exists there, it touches the wood, and the ice doesn’t like to touch the wood, it needs more energy than when there is some water in between, so a small water film acts as a medium between the wood."
In other words, as new ice forms, it pushes the older ice upward, creating the long, thin hairs. But what about the fungus? It turns out that its role comes in once the hair ice has formed, preventing the thin hairs from clumping together in a process called recrystallization.
"The ice tries to reduce the surface, because the surface costs energy, and therefore as more and more ice comes together, it tries to form larger crystals and reduce in this way the overall surface of the ice."
Dr, Mätzler explained that it's still an open question of what substance from the fungus is preventing recrystallization.
"I think the fungus must provide this material, but we still don’t know what it is. It could be the lignin particles which the fungus decomposes, but I’m not sure if it’s this or if it’s something else."
It could be that the fungus is simply the intermediary, decomposing the lignin in the wood into the key substance that mixes with the freezing water. However, Dr. Mätzler and his colleagues have provided a target to further investigate what this substance might be.
"When hair ice melts, it appears that some droplets form on a very tiny string, and because that lasts only for a short moment, it was not possible to see it in a microscope. And this micro-string could be the material on which the ice particles stick in order not to recrystallize. If this is the case then it would be worth looking at melting hair ice and then try to develop a method to separate the remaining water from this string and see if this is possible."
This post is published in The Long View