In a groundbreaking experiment, physicists observed for the first time a classic liquid phenomenon in quantum gases, capillary instability. By cooling a mixture of potassium and rubidium atoms to near absolute zero, the researchers created tiny self-binding droplets that behave like liquids while still in the gas phase. When stretched, these quantum droplets split into smaller droplets, simulating the process by which a stream of water breaks down into droplets.
In the Quantum Mixing Laboratory of the National Institute of Optics (CNR-INO), researchers from the CNR, the University of Florence and the European Laboratory for Nonlinear Spectroscopy (LENS) observed a well-known fluid phenomenon – capillary instability – in an unusual medium – ultradilute quantum gases.
This discovery provides new insights into the behavior of matter under extreme conditions and may lead to new ways to control quantum fluids. The study, published in Physical Review Letters, was also carried out with the participation of scientists from the Universities of Bologna, Padua and the Basque Country (UPV/EHU).
In classical physics, surface tension arises from the cohesive force between the molecules of a liquid, which minimizes the surface area of the liquid. This effect is responsible for everyday phenomena such as raindrops and soap bubbles. Surface tension can also lead to capillary instability (also known as Prato-Rayleigh instability), where the fine stream breaks down into droplets. Understanding this process is critical for fields such as industrial design, biomedicine, and nanotechnology.
The numerical simulation of the experiment reproduces the fragmentation dynamics of the quantum droplet. Image source: CNR-INO
When atomic gases are cooled to near absolute zero, their behavior begins to follow the rules of quantum mechanics. Under certain conditions, these ultra-cold gases can operate like liquids, even if they are still strictly in the gas phase. In recent years, scientists have mastered how to precisely modulate atom-to-atom interactions to create self-bound liquid-like droplets. These droplets are stabilized by quantum effects and have some of the same properties as classical droplets.
The experimental team, led by Cnr-Inno researcher Alessia Burchianti, used imaging and optically controlled techniques to study the dynamic evolution of individual quantum droplets produced by a mixture of ultra-cold potassium and rubidium atoms. The droplets are released in the optical waveguide and elongate to form filaments that break down into smaller droplets when the critical length is exceeded. The number of sub-droplets is proportional to the length of the filament when it breaks.
"By combining experiments and numerical simulations, we were able to describe the fragmentation dynamics of quantum droplets in terms of capillary instability. Prato-Rayleigh instability is a common phenomenon in classical liquids and has also been observed in superfluid helium, but not in atomic gases. Chiara Fort, a UNIFI researcher involved in the study, said. The measurements carried out in our lab provide insight into this particular liquid phase and open the way for the creation of quantum droplet arrays that can be used in quantum technology in the future. Luca Cavicchioli, lead author and researcher at Cnr-Ino, added.
編譯自/ScitechDaily