Bubbles Trapped in Ice express
In a glass of soda, bubbles merrily rise to the top. But if you freeze the water, some bubbles end up trapped in the ice. How do they get there? After all, ice grows, pushing gas aside, but it doesn't always manage to.
Scientists took carbonated water (water with dissolved carbon dioxide) and froze it at different rates: from 1 to 20 micrometers per second. That's like the ice front crawling forward at speeds ranging from the thickness of a human hair per minute to the thickness of a hair in three seconds. They changed the temperature smoothly, with a constant gradient of 15 degrees per millimeter.
Under a microscope with fluorescent illumination, they could see gas accumulating into tiny bubbles just ahead of the advancing ice. It turned out that each bubble has its own "thinking time": how many seconds the gas needs to gather into a bubble. If the ice creeps slowly, the bubbles have time to grow and escape. If it's fast, the ice catches up and seals them in.
This isn't just a fun experiment with soda. In metallurgy, gas bubbles inside a casting are a defect that weakens the part. In ice, on the other hand, bubbles preserve ancient air — scientists use them to learn what the climate was like thousands of years ago. Now we can more accurately predict where and how many bubbles will form during solidification.
Original abstract
as written by the authors arXiv:2601.15045
The formation and entrapment of gas bubbles during solidification significantly influence the microstructure and mechanical properties of materials, from metallic alloys to ice. While gas segregation at the solidification front is well-documented, the real-time dynamics of bubble nucleation, growth, and engulfment-and their dependence on solidification velocity-remain poorly understood. In this study, we use in situ cryo-confocal fluorescence microscopy to investigate the coupled gas-bubble dynamics at the solidification front of carbonated water, systematically varying the solidification velocity ($$V = 1-20 \mu m/s$$) while maintaining a constant thermal gradient ($$G = 15 K/mm$$). Our experiments reveal that bubble nucleation is governed by a characteristic nucleation time, which emerges from the interplay between gas diffusion ahead of the front, nucleation kinetics, and bubble growth, all competing with the advancing solidification front. These results allow us to estimate the critical gas concentration for bubbles nucleation in carbonated water. These results offer a detailed understanding of the mechanisms controlling bubble nucleation and entrapment during solidification at constant thermal gradient. They contribute to the development of strategies to control bubble formation in industrial processes where the presence of bubbles can either be detrimental or intentionally harnessed.
Carbonated water contains about 3–4 times more dissolved carbon dioxide than regular tap water. When it freezes, this gas is pushed out by the ice and gathers into bubbles, the same effect that makes ice from soda cloudy.
In Kurt Vonnegut's novel "Cat's Cradle," there is ice-nine — a fictional form of ice that freezes everything around it. Our experiment with bubbles in freezing water is a step toward understanding how to control the structure of ice and other materials.