Stare long enough at your nearest ice-laden drinking glass and you’ll notice that, unsurprisingly, the ice melts away in due time. From your last soda stint to glacier melts that unfortunately contribute to rising sea levels, the ubiquitous physical phenomenon is just as deceptively complex as it is unavoidable.
Researchers from New York University (NYU) seem to corroborate on this, as they’ve just demonstrated just how strange the liquid that surrounds our planet truly is. In fact, their research, which was published in the journal Physical Review Letters, showed that while ice melts just as expected when heated to certain temperatures above its melting point, it’s the way that it melts that has scientists interested.
Leif Ristroph, experimental physicist and NYU Courant Institute of Mathematical Sciences associate professor, details their work: “The shapes and patterning of ice are sensitive indicators of the environmental conditions at which it melted, allowing us to ‘read’ the shape to infer factors such as the ambient water temperature.”
To help with their work, Ristroph and the team used “clear ice,” or ice that’s free of bubbles and other impurities, to see how they melt in pristine detail. These specially-made pieces of ice were submerged in water tanks, which themselves were placed inside a temperature-controlled “cold room” for the study.
Ristroph continued: “We focused on the cold temperatures—[0-10 °C] (32-50 °F)—at which ice in natural waters typically melts, and we found a surprising variety of shapes that formed.”
When the water is fixed at temperatures on or below 5 °C (41 °F), the ice that’s melting takes on the shape of spikes that point downward, akin to icicles. When the temperatures are a bit higher at about 7 °C (44.6 °F), the spike seems to invert and instead points upward. When temperatures are in between the two, around 6 °C (42.8 °F), the ice takes on a wavy and rippled pattern on its surface as it melts—a feature they call “scallops,” which can also be seen on naturally-occurring ice structures, according to the NYU press release.
“Melting causes gradients in the temperature of the water near the ice, which causes the liquid at different places to have different densities,” explains first author and NYU graduate student Scott Weady. “This generates flows due to gravity—with heavier liquid sinking and lighter fluid rising—and such flows along the surface lead to different rates of melting at different locations and thus changes in shape.”
“The strange bit of physics is that liquid water has a highly unusual dependence of density on temperature, in particular a maximum of density at about 4 degrees C,” Weady continued. “This ‘density anomaly’ makes water unique in comparison to other fluids.”
To be specific, the researchers noticed that this very same property is at work in their findings. At the first range of temperatures close to 4 °C (41 °F), water manages to flow upward, creating the downward-facing spike. The reverse can be said of those at temperatures at 7 °C (44.6 °F) and higher, creating upward-facing spikes instead. The two flows compete at the temperatures between, creating the vortices that carve the pits that we see as waves and ripples.
Ristroph added: “Our findings help to explain some characteristic shapes of ice seen in nature, specifically the so-called pinnacle morphology of icebergs that consists of sharp spikes or spires and the so-called scallops that consist of wavy patterns of pits.”
To Ristroph and the team, their findings carry with it certain implications for the understanding of larger systems that depend on melting ice, like Earth’s changing climate. Ristroph added that understanding the physics and math of what happens to the ice at smaller scales enables us to understand “key components of larger-scale climate models.”
References
- Nield, D. (2022, February 2). Physicists reveal the weird shapes ice takes as it melts at different temperatures. ScienceAlert. https://www.sciencealert.com/the-shape-of-melting-ice-depends-on-what-temperature-it-is
- NYU Web Communications. (2022, January 31). Scientists uncover how the shape of melting ice depends on water temperature. New York University. http://www.nyu.edu/content/nyu/en/about/news-publications/news/2022/january/scientists-uncover-how-the-shape-of-melting-ice-depends-on-water
- Weady, S., Tong, J., Zidovska, A., & Ristroph, L. (2022). Anomalous convective flows carve pinnacles and scallops in melting ice. Physical Review Letters, 128(4), 044502. https://doi.org/10.1103/PhysRevLett.128.044502
