Scientists control Kelvin waves for first time

In a new study published in Nature Physics, researchers have developed the first controlled method for exciting and observing Kelvin waves in superfluid helium-4.

First described by Lord Kelvin in 1880, Kelvin waves are helical (spiral-shaped) waves that travel along the vortex lines, playing a vital role in how energy dissipates in quantum systems. However, they are difficult to study experimentally.

Creating a controlled setting to observe them has been the biggest challenge that the researchers overcame. Phys.org spoke to the first author of the study, Associate Prof. Yosuke Minowa from Kyoto University.

The discovery came about through serendipity. “We applied an electric field to a nanoparticle decorating a quantized vortex, hoping to translate the entire structure. Instead, we observed a clear wavy motion of the vortex core, namely Kelvin wave excitation. This unexpected result prompted us to shift our focus toward studying the excitation of Kelvin waves in-depth,” said Prof. Minowa.

The heart of the experiment lies in the properties of superfluids.

Superfluids

Superfluids are a state of matter in which certain fluids begin to display quantum effects at a macroscopic scale at extremely low temperatures. In this case, no viscosity.

This means that superfluids can flow without any friction or loss of energy. The most common example of a superfluid is helium-4, which demonstrates this behavior when cooled below 2.17 Kelvin (-270.98 degrees Celsius or -455.764 degrees Fahrenheit).

Helium is the only material that doesn’t freeze at these low temperatures, allowing superfluidity to be observed. In this state, the fluid can flow upward against gravity and escape containers by climbing up walls.

Superfluidity is explained by Bose-Einstein condensation, where a large fraction of the atoms enter the same quantum state and begin behaving as a single quantum entity. Since superfluids have no viscosity, any energy provided to the system cannot dissipate as heat like in traditional fluids. The solution is Kelvin waves.

Kelvin waves

For the formation of Kelvin waves, the researchers start with a vortex line, a thin line around which the superfluid helium rotates, similar to a tornado. Then, a disturbance is created or introduced into the system. Since the system is quantum in nature, the vortex line’s rotation is quantized (it can only rotate at specific strengths).

Instead of moving straight, the disturbance creates a helical motion with the vortex line wiggling and twisting. The superfluid still rotates but around this now-spiraling vortex line. This is the lowest-energy way for the vortex line to respond to the disturbance, similar to a guitar string making waves when plucked.

“In previous studies, Kelvin wave-like oscillations were observed only accidentally. We developed a novel method to manipulate an ideal vortex in superfluid helium, providing a new way to study the behavior of these quantized vortices,” said Prof. Minowa.

Nanoparticles for visualization

The researchers’ approach involved creating silicon nanoparticles in superfluid helium-4 at 1.4 Kelvin. To do so, they placed a silicon wafer in the helium, with the laser directly hitting it.

This not only created the silicon nanoparticles, but also created local, drastic flows in the fluid which amplified the remnant vortices in the superfluid, causing some of the nanoparticles to become trapped in the core of these vortices.

Now that the vortex filaments could be seen, the researchers applied a time-varying electric field to the system. This created forced oscillations of the nanoparticles, propagating as a helical wave along the vortex.

The researchers tested different excitation frequencies (0.8 to 3.0 Hertz) to analyze wave behavior. A dual-camera setup was used to reconstruct the wave motion in three dimensions using spline curve fitting techniques.

“By utilizing nanoparticles to decorate quantized vortices, our study introduces a versatile tool for manipulating and observing quantum fluid behavior. This approach could inspire similar techniques in other quantum fluid systems, expanding the toolbox for experimental studies,” noted Prof. Minowa.

Successful excitation of Kelvin waves

The researchers successfully demonstrated the controlled excitation of Kelvin waves in superfluid helium-4. Their 3D reconstruction of the waves helped to confirm the helical nature of the waves.

According to Prof. Minowa, one of the biggest challenges was proving that the observed phenomenon was indeed a Kelvin wave.

Prof. Minowa said, “To address this, we collected key information such as the dispersion relation, phase velocity, and three-dimensional dynamics. The three-dimensional image reconstruction played a critical role in confirming the helical nature of the Kelvin waves. By visualizing the vortex’s three-dimensional dynamics, we obtained direct and concrete evidence that the observed oscillations were indeed Kelvin waves.”

The images also helped to confirm the handedness (or direction of rotation) of Kelvin waves, something that had never been experimentally done before. The excited waves had a left-handed helical structure.

To validate their experimental observations, the researchers developed a vortex filament model to simulate Kelvin wave excitation.

The simulations confirmed that forcing a charged nanoparticle led to the generation of helical waves in both directions, matching experimental results.

The researchers have provided a novel and controlled method to excite and study Kelvin waves, which play a key role in the energy transfer and dissipation processes in superfluids and other similar quantum systems.

Prof. Minowa concluded, “We have introduced a new tool to study Kelvin waves in superfluid helium, paving the way for numerous experimental investigations. Future research could explore the nonlinearity and decay processes of Kelvin waves, as well as the mechanical properties and characterization of quantized vortices.”

© 2025 Science X Network