For the first time, researchers have successfully controlled and observed Kelvin waves in superfluid helium-4, marking a significant step in understanding energy dissipation in quantum systems. The study has provided a controlled method to excite these helical waves, which had previously only been observed in unpredictable conditions. The research opens new possibilities for studying quantised vortices and their role in energy transfer at the quantum level.
Controlled Excitation of Kelvin Waves
According to the study published in Nature Physics, also available on arXiv, Kelvin waves—first described by Lord Kelvin in 1880—are helical disturbances that travel along vortex lines in superfluid systems. These waves play a crucial role in energy dissipation within quantum fluids but have remained difficult to study due to the challenges of controlled excitation.
Associate Professor Yosuke Minowa from Kyoto University, the lead author of the study, told Phys.org that the breakthrough occurred unexpectedly. An electric field was applied to a nanoparticle decorating a quantised vortex with the intention of moving the structure. Instead, the vortex core exhibited a distinct wavy motion, leading researchers to shift their focus toward controlled Kelvin wave excitation.
Superfluid Properties and Quantum Vortex Behaviour
Superfluid helium-4, which exhibits quantum effects at macroscopic scales when cooled below 2.17 Kelvin, has no viscosity, allowing it to flow without friction. This unique state prevents energy from dissipating as heat, leading to the formation of Kelvin waves when disturbances occur in the vortex lines of the fluid. The research team demonstrated that these waves, rather than traditional fluid turbulence, provide an essential mechanism for energy transfer in superfluid systems.
Nanoparticles Used for Wave Visualisation
To track the motion of Kelvin waves, the researchers introduced silicon nanoparticles into superfluid helium-4 at 1.4 Kelvin by directing a laser at a silicon wafer submerged in the fluid. Some nanoparticles became trapped within vortex cores, making them visible under controlled conditions. A time-varying electric field was then applied, forcing oscillations in the trapped particles and generating a helical wave along the vortex.
Experiments were conducted across different excitation frequencies ranging from 0.8 to 3.0 Hertz. A dual-camera system allowed for three-dimensional reconstruction of the wave’s motion, confirming its helical nature.
Experimental Confirmation and Future Research
Prof. Minowa explained to Phys.org that proving the observed phenomenon was indeed a Kelvin wave required an in-depth analysis of dispersion relations, phase velocity, and three-dimensional dynamics. By reconstructing the vortex’s motion in 3D, the researchers provided direct evidence of the wave’s handedness, confirming its left-handed helical structure—something never experimentally demonstrated before.
To validate their findings, the team developed a vortex filament model, which simulated Kelvin wave excitation under similar conditions. These simulations confirmed that forced oscillations of a charged nanoparticle generated helical waves in both directions, aligning with experimental results.
The study introduces a new approach for studying Kelvin waves in superfluid helium, offering insights into the mechanics of quantised vortices. Future research may explore the nonlinearity and decay processes of Kelvin waves, potentially revealing further details about quantum fluid dynamics.
