Semiconducting nanoparticles can become trapped along structures called quantized vortices in superfluid helium-4, allowing them to act as “tracers” in studies of vortex dynamics. This finding, from researchers at Osaka University and Osaka Metropolitan University in Japan, could improve our understanding of quantum fluids and materials, including superconductors, while also shedding more light on turbulence.
When helium-4 is cooled to about 2 K, it transforms from a liquid into a quantum state of matter with zero viscosity. In this supercooled state, the material can, in principle, flow forever without losing any kinetic energy, which gives it several curious properties. It can climb up the walls of a container, for one, and it also supports the existence of excitations known as vortices. These structures, which are created by turbulence, look like tiny cyclones and occur over large scales in the superfluid. Importantly, they are quantized, meaning that each vortex carries a fixed amount of angular momentum.
The quantized nature of the vortices means that a system that is initially chaotic will become more ordered and structured as increasing amounts of energy are supplied to it. This result is somewhat counterintuitive, yet vortices of this type have been observed in systems ranging from soap films to atmospheric flow on planets, with the best-known example being Jupiter’s Great Red Spot. Visualizing them in experiments has, however, proved difficult.
Nanoparticles as tracers
Researchers led by Yosuke Minowa of Osaka University’s Graduate School of Engineering Science have now succeeded in doing just that by using silicon nanoparticles as tracers, with the vortices revealing themselves through the behaviour of nanoparticles trapped along their cores. The researchers also used their technique to study vortex reconnection, which is a process in which vortices coalesce and exchange parts of their structures.
Minowa and colleagues prepared their silicon nanoparticles using a technique called laser ablation, which involves directing a high-energy laser pulse onto the surface of a piece of solid silicon material located within the superfluid helium. In this fashion, they were able to suddenly melt, vaporize and cluster the material. “This drastic process leads to the immediate ejection of the melted vaporized/clustered materials,” explains Minowa. “The ejected particles are then quickly cooled and we end up with many nanoparticles distributed in the superfluid helium.”
Optical tweezers hold nanoparticles in superfluid helium
The researchers observed that the nanoparticles clustered along a curved line, confirming that they were trapped inside the vortices. They also compared the patterns they observed with theoretically expected vortex dynamics and saw an excellent agreement between the two.
“We have developed a new tool for studying quantized vortex properties that will help us better understand the science of turbulence,” Minowa tells Physics World. “Our technique could also be applied to different materials and different sizes of nanoparticles to investigate the details of the nanoparticle-vortices interactions.”
Minowa says he and his colleagues are now planning to manipulate quantized vortices using optical forces. They detail their present work in Science Advances.