Liquid gallium experiment confirms key flow regime inside rapidly rotating stars and planets

Since we cannot look into the interiors of stars and planets, we rely on lab experiments to replicate the physical processes that occur there. Led by the University of California, Los Angeles (UCLA), an international research team has produced the first experimental verification of a theoretically predicted flow state deemed characteristic of the interiors of rapidly rotating celestial bodies. The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) also participated in the study, which has been published in Physical Review Letters. The results provide a robust experimental basis for testing theoretical models of the processes that occur inside these celestial bodies.

Inside stars and planets, heat is transported by convection: Hot material rises while cooler material sinks. This generates turbulent flows that drive Earth’s magnetic field and shape the dynamics of stars. According to theoretical predictions, these flows reach a special state inside rapidly rotating celestial bodies. The scientific community calls this ultimate state the diffusivity-free regime, in which large flows are determined almost exclusively by buoyancy and rotation while fluid properties such as viscosity or thermal conductivity become almost negligible. Many models of the interiors of stars and planets are based on the assumption that convection operates precisely within this ultimate regime.

A question that has been unanswered for decades

Yet until now, this had precisely been the issue: In classical lab experiments, thermal boundary layers form on the walls of the test vessels, affecting the flow to the point that its state remained hidden. For a long time, it was therefore unclear whether it was even possible to verify the theoretical predictions in an experimental setting.

The international research team achieved the decisive breakthrough with a rotating liquid-metal experiment using liquid gallium as the test medium and a special oscillating flow mode that occurs exclusively in liquid metals instead of the usual stationary convection. Unlike classical convection flows, this mode is not determined by the thermal boundary layers on the vessel walls but by the temperature gradient inside the liquid, making it possible to create the conditions that models have long predicted.

“Our experiment demonstrated this theoretically predicted state in the lab for the first time, which greatly strengthens our confidence in the models we use to describe processes inside stars and planets,” says Dr. Jewel Abbate of UCLA, who conducted this research as part of her Ph.D. studies.

A new pathway into the interior of stars

Dr. Tobias Vogt from HZDR’s Institute of Fluid Dynamics also participated in the experiments and measurements during two research stays at UCLA. To validate the experimental results, the researchers compared three independent metrics with the theoretical predictions: heat transport, flow velocity and temperature fluctuations within the fluid. All three matched the models quantitatively. High-resolution numerical simulations further confirmed the results.

“What makes our work particularly compelling is the fact that theory, experiment and numerical simulations now agree quantitatively, allowing us to confirm experimentally that the underlying physical models describe the observed heat transport very accurately. This gives us far greater confidence to apply these models to the interiors of planets and stars,” Vogt summarizes.

The results close a decades-old gap between theory and experiment, opening up new possibilities for investigating the dynamics inside stars and planets in lab experiments—for more reliable models of these fundamental processes in the universe.

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Gaby Clark

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Robert Egan

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