Thick layers of ice form and ensure blockages and buoyancy: icing in “supercooled” salt water threatens hard-shelled animals and technical equipment is also affected by so-called cryofouling. But in this case, too, nature has produced a sophisticated solution to the problem: researchers report that an Antarctic mussel species protects itself from freezing in the frosty underwater world with a sophisticated surface structure. Due to the concept, only weakly adhering ice crystals form, which are easily washed away by the current. This discovery could help in the long-term development of ice-free surfaces, the scientists say.
It is well known that ice formation can be very problematic: if aircraft wings ice up, solar cells become covered with frost or frosty coverings affect moving elements, there is a risk of performance losses or danger. The same applies to so-called supercooled water, which has temperatures just below the freezing point. Although it remains liquid due to certain factors, ice can form locally. This is particularly pronounced in Antarctic waters. Due to the high salt content, the freezing point there is around -1.9 degrees Celsius – but the water is often around 0.05 degrees Celsius colder. Therefore, the smallest disturbances such as grains of sand or surface structures in this supercooled water can trigger the formation of ice crystals.
Problematic cryofouling
This allows objects and living beings to freeze underwater. Cryofouling poses a significant problem for shipping in polar regions and the use of measuring devices and other technical units. But hard-shelled creatures in the frosty underwater world are also affected: ice crusts block them or tear them away due to increased water resistance or buoyancy. However, as the researchers led by Konrad Meister from the Max Planck Institute for Polymer Research in Mainz are now reporting, the Antarctic scallop Adamussium colbecki is surprisingly good at resisting this problem.
During an expedition in Antarctica, Meister was made aware of this clam by divers: “They reported that they had never seen ice on the surface of this type of clam,” says Meister. So the suspicion arose that in the course of evolution these creatures had created a structural feature that protected them from freezing. In order to follow the trail, Meister and his colleagues took a close look at the shells of Adamussium colbecki and also carried out freezing experiments with various mussel shells.
As they report, the microscopic investigations showed that while mussels in warmer regions have disordered or smooth shell surfaces, the Antarctic mussel species has a very regular fine structure: the researchers report that small ridges can be seen under the microscope that radiate out on the mussel shell. As their further investigations showed, these elevations ensure that water freezes preferentially at their peaks, but not in the “valleys”. If the freezing process then continues, a continuous layer of ice can form, but it is still only on the ridges.
Potential for bio-inspired surfaces
As a result, there is no form fit, as is the case with other structured surfaces, and there is therefore very little adhesion between the ice and the mussel shell, the researchers explain. Even the smallest underwater currents can wash away the layers and thus protect the mussel from freezing. Further experiments also document this effect in practice: The research team carried out icing experiments with Adamussium colbecki and a mussel from warmer ocean regions. This confirmed that much less force is required to remove layers of ice from the shell of the inhabitant of the supercooled Antarctic waters than with the warm-water mussel. Adamussium colbecki has thus produced a sophisticated adaptation to its extreme habitat. “It’s exciting how the evolution of this shell has obviously given it an advantage,” says Meister.
As the researchers emphasize, the nature patent is not only interesting from a biological point of view: In the long term, the discovery could help in the development of biologically inspired surfaces that hardly freeze. “New technological applications based on the principle of bionics are conceivable from the knowledge of the ice-free mussel shell. For example, non-icing surfaces could be extremely interesting for polar shipping,” says the scientist.
Source: Max Planck Institute for Polymer Research, technical article: Communications Biology, doi: 10.1038/s42003-022-03023-6