In a study whose results appear to fly in the face of common understanding, it appears that water needs a little bit of heat to initiate ice formation on graphene, or single layers of carbon arranged in a hexagonal honeycomb lattice. The study, published in Nature Communications, found this out by focusing on the movement of individual water molecules placed on a cold graphene surface using helium spin-echo, a technology developed at the University of Cambridge that involves firing a beam of helium atoms at water molecules then tracking the helium atoms’ scattering once they collide with the forming ice. The technology functions much like “a radar trap for molecules, on an atomic scale,” first author Anton Tamtögl, a postdoctoral researcher at the Institute of Experimental Physics at Graz University of Technology in Austria, said to LiveScience, or how radar traps track how fast vehicles go when on the freeway.
The moment of ice formation targeted by the study is called nucleation, or where water molecules first coalesce and form into ice crystals. This process, however, is extremely fast, occurring within timescales of fractions of a billionth of a second. This often requires researchers to “cool down” the molecules by using liquid nitrogen, lowering the temperature to around -250 °C (-418 °F), to slow the molecules down. If the study necessitates viewing the molecules at slightly higher temperatures, such as this study, they would need to use helium spin-echo. The graphene surface in this study was cooled to only around -173 to -143 °C (-279 to -225 °F).
Upon examination, the researchers observed that the water molecules seemed to repel each other on the graphene surface prior to ice formation, much like how similar poles on a pair of magnets do. The hydrogen atoms on the molecules, arranged like “mouse ears” relative to the oxygen atom, orient themselves downward towards the graphene surface. The molecules actually appear to cluster together, but create some sort of repulsion between them due to their similar orientations, keeping a few molecules’ worth of space between each molecule. To form ice, the molecules must overcome this repulsion and break the uniformity of their orientation, creating a “barrier” that requires energy to overcome before starting nucleation.
Tamtögl believes that there’s “still much more to be learned” from water molecules and ice formation. “Water is such a ubiquitous molecule, right? But it appears there’s still so much we don’t understand in detail, even though it’s a simple molecule.” Further understanding of the formation of ice could lead to improvements in reducing ice-induced damage to structures and engineering components like turbines and communications towers, as well as allowing more detailed analysis on existing ice on our planet and beyond, like in glaciers and comets.
Bibliography
- Lanese, N. (2021, May 26). New study turns our understanding of ice upside down. LiveScience. Retrieved August 20, 2021, from https://www.livescience.com/ice-formation-requires-some-heat.html
- Tamtögl, A., Bahn, E., Sacchi, M. et al. Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene. Nat Commun 12, 3120 (2021). https://doi.org/10.1038/s41467-021-23226-5
