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arXiv:2411.19807 · Science Advances

The Rayleigh-Taylor instability in a binary quantum fluid

Yanda Geng, Junheng Tao, M. Zhao, Shouvik Mukherjee, Stephen Eckel, Gretchen K. Campbell, Ian B. Spielman

Published Aug 27, 2025• 5 citations

Abstract

Instabilities, where small fluctuations seed the formation of large-scale structures, govern dynamics in a variety of fluid systems. The Rayleigh-Taylor instability (RTI), present from tabletop to astronomical scales, is an iconic example characterized by mushroom-shaped incursions appearing when immiscible fluids are forced together. Despite its ubiquity, RTI experiments are challenging; here, we report the observation of the RTI in an immiscible binary superfluid consisting of a two-component Bose-Einstein condensate. We force these components together to initiate the instability, and observe the growth of mushroom-like structures. The interface can also be stabilized, allowing us to spectroscopically measure the "ripplon" interface modes. Last, we use matter-wave interferometry to transform the superfluid velocity field at the interface into a vortex chain. These results-in agreement with our theory-demonstrate the close connection between the RTI in classical and quantum fluids.

DOI: 10.1126/sciadv.adw9752

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Instabilities, where small fluctuations seed the formation of large-scale structures, govern dynamics in a variety of fluid systems. The Rayleigh-Taylor instability (RTI), present from tabletop to astronomical scales, is an iconic example characterized by mushroom-shaped incursions appearing when immiscible fluids are forced together. Despite its ubiquity, RTI experiments are challenging; here, we report the observation of the RTI in an immiscible binary superfluid consisting of a two-component Bose-Einstein condensate. We force these components together to initiate the instability, and observe the growth of mushroom-like structures. The interface can also be stabilized, allowing us to spectroscopically measure the "ripplon" interface modes. Last, we use matter-wave interferometry to transform the superfluid velocity field at the interface into a vortex chain. These results-in agreement with our theory-demonstrate the close connection between the RTI in classical and quantum fluids.

Three-component Bose-Einstein condensates (BECs) offer a rich platform for exploring inquisitive quantum phenomena. We investigate the Rayleigh-Taylor instability at the two interfaces in a phase-separated three-component BEC in the mean-field framework. The instability in the system can be instigated in various ways owing to the versatility of the three-component system and the subsequent dynamics for certain cases has been studied in detail. We also study the impact of immiscibility of the intermediate component on the onset of instability in the system. The rotational symmetry of the system breaks when the atom-atom interaction is tuned in such a way that the interface between the components becomes unstable giving rise to nonlinear patterns of mushroom shapes which grow exponentially with time. We identify these nonlinear patterns as the solutions of the angular Mathieu equation, representing the normal modes.

The Rayleigh-Taylor instability is a gravitational instability in two fluids where the heavier fluid is set over the lighter fluid. The instability occurs both in classical fluids and quantum fluids. We numerically study the Rayleigh-Taylor instability using coupled Gross-Pitaevskii equations for two-component Bose-Einstein condensates. We carry out numerical simulations that the heavier component is set in a torus initially which is surrounded by the lighter component. When the torus falls, the Rayleigh-Taylor instability develops and a sagging pattern appears. This instability is investigated for the two cases with and without a vortex ring inside the torus. We find that a vortex ring suppresses the instability when the radius of the torus is small.

We investigate three kinds of instabilities in binary immiscible homogeneous Bose-Einstein condensate, considering rubidium isotopes

^{85}\mathrm{Rb}

and

^{87}\mathrm{Rb}

confined in two-dimensional circular box. Rayleigh-Taylor and Kelvin-Helmholtz (KH) instability types are studied under strong perturbations. Without external perturbation, instabilities are also probed by immiscible-to-miscible quenching transition, under two different initial configurations. Our numerical simulations show that all such instability dynamics are dominated by large vortex production and sound-wave (phonon) propagation. For long-term propagation, vortex dynamics become dominant over sound waves in the KH instability, while sound-wave excitations predominate in the other cases. For all the dynamical simulations, the emergence of possible scaling laws are investigated for the compressible and incompressible parts of the kinetic energy spectra, in terms of the wave number

k

. The corresponding results are compared with the classical Kolmogorov scalings, ${k}^{\ensuremath{-}5/3}$ and ${k}^{\ensuremath{-}3}$, for turbulence, which are observed in the kinetic energy spectra at some specific time intervals.

Quantum turbulence (QT) generated by the Rayleigh-Taylor instability in binary immiscible ultracold

{}^{87}\text{Rb}

atoms at zero temperature is studied theoretically. We show that the quantum vortex tangle is qualitatively different from previously considered superfluids, which reveals deep relations between QT and classical turbulence. The present QT may be generated at arbitrarily small Mach numbers, which is a unique property not found in previously studied superfluids. By numerical solution of the coupled Gross-Pitaevskii equations we find that the Kolmogorov scaling law holds for the incompressible kinetic energy. We demonstrate that the phenomenon may be observed in the laboratory.