(a) Schematic of polycrystalline ice in the experimental cell. (b) An ice-water interface imaged through crossed polarizers, highlighting individual grains with different crystal orientations. (c) Schematic of the cryoaspiration process in a pre-melted layer. (d) Grain boundaries (thin lines) in a brightfield micrograph of the ice in the cell. (e) Stresses under the ice in (d). The image is taken 10 min after the start of the experiment. Credit: Physical Examination Letters (2023). DOI: 10.1103/PhysRevLett.131.208201
A combined team of materials scientists and engineers from the Swiss Federal Institute of Technology and Yale University have shown, via laboratory experiments, how water-filled channels crisscrossing multicrystalline ice can lead to fractures in materials such as cement and asphalt. In their article published in the journal Physical Examination LettersThe group describes experiments they conducted with transparent objects, water and silicone, to show how liquid channels in ice can lead to fractures in porous materials.
Water, unlike other liquids, expands when it freezes. This is due to the unique shape of water molecules and the angles that form between them when water freezes. Such expansion is often blamed on damage to materials such as roads and driveways, but as the researchers point out, this damage is due to the growth of ice crystals, not the expansion of water. So the team studied crystal growth to determine how it causes damage.
Noting that in the real world, most of this damage occurs in opaque materials, such as concrete and asphalt, making it very difficult to study the process taking place, the researchers took another approach. They created an environment in which all materials would behave the same but would also be transparent.
The team started with two glass slides separated by spacers. They then created a single small pore using a light-curing glue, just a few millimeters long and wide. Next, they coated the inside of the lower part of the pore with a thin layer of silicone, which they speckled with fluorescent particles before letting it harden. Then they filled the pores with water.
Once their device was built, they then cooled just one end of the pore they created while heating the other end. And then they observed the action using a microscope. They found that when the water in the chilled end froze, the silicone began to deform and as it did so the ice crystal that had formed in the pore grew larger and in doing so put pressure on the layer of silicone.
Closer examination of the silicone layer showed that a film of water persisted between the ice and the silicone, serving as a source of new water for continued expansion, leading to the type of damage seen in materials such than cement and asphalt.
More information:
Dominic Gerber et al, Polycrystallinity enhances stress buildup around ice, Physical Examination Letters (2023). DOI: 10.1103/PhysRevLett.131.208201
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