Superfluids are one of the 20th century’s most fascinating and important discoveries. Their study is the basis of Nobel Prize-winning work, and they hold promise for better electricity transmission and may even help answer the secrets of the universe with their mysterious “quantum fluid” properties.
Wei Guo, an associate professor of mechanical engineering at the FAMU-FSU College of Engineering, is the principal investigator for a new grant funded by the National Science Foundation to investigate the dynamics of superfluids. Superfluids exist at low enough temperatures that quantum mechanics — which deals with physics on the scale of atoms or subatomic particles — govern their behavior. The research will advance our understanding of quantum turbulence using three-dimensional flow visualization technologies.
The grant, which starts in summer 2021, is funded at $521,000 for three years.
Previous research from Takeshi Egami of Oak Ridge National Laboratory showed that when helium gas is cooled to extreme temperatures, it becomes a liquid and behaves oddly — it can flow without friction as a superfluid. Absence of friction means no loss of kinetic energy in the fluid.
Guo and his team of graduate students and post-doctoral researchers will use multidimensional flow visualization to further study the superfluid and its unique properties.
“We will cool helium to its superfluid state,” Guo said. “Then we will develop and use advanced 3D flow visualization systems to study phenomenon related to turbulence in this quantum fluid.”
The helium superfluid, known as He II, is of interest to scientists because it allows them to study the quantum world with visual reference. It also has a superior ability to cool scientific instruments such as particle colliders, superconducting magnets and satellites. One condition that can impair its efficiency is turbulence. When the heat transfer is relatively strong, turbulence can appear spontaneously, and the researchers want to understand this better.
Vortex tubes are another phenomenon found in He II that the researchers will explore. When superfluid helium is stirred, vortex tubes — like tiny tornadoes with hollow cores — can appear. Studying their dynamics may help us understand various physical systems that cannot be studied in a laboratory, such as superfluid neutron stars and even cosmic strings created in the early universe.
“We know that the most effective method to study vortex motions is to image the vortices,” Guo said. “We aim to implement 3D imaging flow visualization technologies to solve several challenging problems in the quantum turbulence field.”
Guo’s team will work on two projects. In the first project, the researchers plan to construct a stereoscopic flow visualization system that allows them to obtain 3D velocity field information in He II. The scientists will create a thin line of molecular tracers using laser ionization in liquid helium. These tracers can be excited with a laser pulse to produce glow. Using two cameras placed in the perpendicular direction, they can capture images to construct a 3D profile of the tracer line.
“The 3D image will help us to understand how turbulence affects the heat transfer in He II,” doctoral student Toshiaki Kanai said. “We will be applying this in our study of heat-induced thermal counterflow in He II.”
In the second project, the researchers plan to use small micron-sized frozen hydrogen particles to identify the vortex tubes in He II. They will illuminate the particles on the vortices to take images of the vortex lines with the camera. The vortex lines look like electric cables with many small lightbulbs attached.
“Seeing these little bulbs on vortices will allow us to determine where the vortex lines are and how fast they move around,” said post-doctoral research Yuan Tang.
The researchers will stack images of this phenomenon together to generate the 3D profile of the vortices. This method will allow them to track the motion of vortices, which will yield valuable information about their velocity and orientation statistics.
Guo’s team, with prior NSF support, developed powerful flow visualization techniques used in their past quantum turbulence research. They successfully applied these techniques to visualize vortex tubes in quantum fluids and want to go further by implementing three-dimensional processes to the equation.
Kanai will work on the stereoscopic molecular tagging velocimetry experiment. Tang will develop the scanning particle tracking velocimetry system. The group is collaborating with Professor William Vinen from the University of Birmingham in the United Kingdom. Vinen is a renowned expert in quantum turbulence, and he will work with data analysis and result interpretation in both research activities.
The research will be conducted in the National High Magnetic Field Laboratory at Florida State University. The team also plans to conduct educational outreach activities and conduct workshops and conferences to engage junior researchers.