Inside stars and planets, heat is transported by matter that is constantly in motion. What exactly happens in these interiors cannot be observed directly, so physicists rely on calculations and laboratory experiments. One of the key states of such motion existed only in theory for decades. Now it has been reproduced for the first time using liquid metal.
Why This Matters
No instrument can look directly into the interior of a star or planet. What happens beneath the surface is understood through models, and those models are tested in laboratories using substances that can be heated and rotated.
The basic process is simple in principle. Heated matter rises, cooled matter sinks, and turbulent flows emerge. These flows are what sustain the Earth’s magnetic field and influence the dynamics of stars.
A State Independent of the Fluid’s Properties
Theoretical estimates indicate that in bodies with rapid axial rotation, such flows enter a limiting state known as the diffusion-free regime. Its essence is that large-scale motion is governed only by heating and rotation, while the properties of the liquid itself, such as viscosity or thermal conductivity, become almost insignificant.
This is what makes the diffusion-free regime valuable for astrophysics. If the flow in this state does not depend on the properties of a particular liquid, then models tested with it should also describe matter in the interiors of stars. Many models of the internal structure of planets and stars are based on this assumption, as Phys.org reports.
Decades Without an Answer
It had not been possible to test this assumption experimentally because of the container itself. Thermal boundary layers form near its walls and distort the motion so strongly that the required state remains hidden.
The researchers managed to overcome this obstacle by using a different type of flow. Instead of conventional steady convection, they used an oscillatory regime that occurs only in liquid metals. It is generated by a temperature difference within the body of the molten material itself rather than by processes near the walls, so the influence of the walls ceases to be dominant.
The Gallium Experiment
The researchers chose molten gallium for the experiments, while the entire apparatus rotated during the measurements. This combination created the conditions that models had described for a long time but that no one had previously observed in a laboratory.
Gallium is convenient for such work because it melts at around 30°C, meaning that it becomes liquid at a temperature only slightly above room temperature. It conducts heat tens of times better than water, while its viscosity is comparable to that of water. This particular combination makes oscillatory motion possible, something that cannot occur in ordinary non-metallic media.
“In our experiment, this theoretically expected state was demonstrated in the laboratory for the first time, which significantly strengthens our confidence in the models we use to describe processes in the interiors of stars and planets,” says Jewel Abbate of the University of California, Los Angeles, who carried out the work as part of her doctoral research. The results were published in the peer-reviewed journal Physical Review Letters.
Verification at Three Levels
The reliability of the result was checked using three independent indicators. Experimental data were compared with calculations for heat transfer, flow velocity, and temperature oscillations inside the molten material. All three quantities matched quantitatively, and independently, high-resolution computer simulations showed the same result.
“What is particularly convincing in this work is the quantitative agreement between theory, experiment, and numerical modeling. It confirms that the physical models describe the observed heat transport with very high accuracy,” summarizes Tobias Vogt of the Institute of Fluid Dynamics at the Helmholtz-Zentrum Dresden-Rossendorf. He participated in the measurements during two research visits to the University of California, Los Angeles.
