Physicists capture the sound of a ‘perfect’ fluid

Physicists capture the sound of a "perfect" fluid
MIT physicists have created a perfect fluid in the laboratory, and recorded the sound of that “perfect flow.” The way that sound travels through this fluid can be used to calculate the sound, and “quantum friction,” in neutron stars and other perfect fluids. Credit: Christine Daniloff, MIT

For some, the sound of a “perfect flow” might be the gentle lapping of a forest brook or perhaps the tinkling of water poured from a pitcher. For physicists, a perfect flow is more specific, referring to a fluid that flows with the smallest amount of friction, or viscosity, allowed by the laws of quantum mechanics. Such perfectly fluid behavior is rare in nature, but it is thought to occur in the cores of neutron stars and in the soupy plasma of the early universe.


Now MIT physicists have created a perfect fluid in the laboratory, and listened to how sound waves travel through it. The recording is a product of a glissando of sound waves that the team sent through a carefully controlled gas of elementary particles known as fermions. The pitches that can be heard are the particular frequencies at which the gas resonates like a plucked string.

The researchers analyzed thousands of sound waves traveling through this gas, to measure its “sound diffusion,” or how quickly sound dissipates in the gas, which is related directly to a material’s viscosity, or internal friction.

Surprisingly, they found that the fluid’s sound diffusion was so low as to be described by a “quantum” amount of friction, given by a constant of nature known as Planck’s constant, and the mass of the individual fermions in the fluid.

This fundamental value confirmed that the strongly interacting fermion gas behaves as a perfect fluid, and is universal in nature. The results, published today in the journal Science, demonstrate the first time that scientists have been able to measure sound diffusion in a perfect fluid.

Scientists can now use the fluid as a model of other, more complicated perfect flows, to estimate the viscosity of the plasma in the early universe, as well as the quantum friction within neutron stars—properties that would otherwise be impossible to calculate. Scientists might even be able to approximately predict the sounds they make.

“It’s quite difficult to listen to a neutron star,” says Martin Zwierlein, the Thomas A. Franck Professor of Physics at MIT. “But now you could mimic it in a lab using atoms, shake that atomic soup and listen to it, and know how a neutron star would sound.”

While a neutron star and the team’s gas differ widely in terms of their size and the speed at which sound travels through, from some rough calculations Zwierlein estimates that the star’s resonant frequencies would be similar to those of the gas, and even audible—”if you could get your ear close without being ripped apart by gravity,” he adds.

Zwierlein’s co-authors are lead author Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard Fletcher, and Julian Struck of the MIT-Harvard Center

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Remote control of heat nanosources motion and thermal-induced fluid flows by using light forces

Remote control of heat nanosources motion and thermal-induced fluid flows by using light forces
a, Multiple gold NPs (spheres of 200 nm radius) are confined by a ring-shaped laser trap (wavelength of 532 nm) and optically transported around it. These NPs rapidly assemble into a stable group of hot particles creating a confined heat source (G-NP) of temperature ~500 K. Free (not trapped) gold NPs acting as tracer particles are dragged toward the G-NP by the action of the thermal-induced water flow created around it (see Video S5 of the paper). The speed of the G-NP is controlled by the optical propulsion force which is proportional to the phase gradient strength tailored along the laser trap as displayed in b, corresponding to the transport state 1. This non-uniform propulsion force drives the G-NP reaching a maximum speed of 42 μm/s. b, Sketch of the switching of the phase gradient configuration (state 1 and 2) enabling a more sophisticated manipulation of the heat source: split and merge of the G-NP. (c), The opposite averaged propulsion forces in the split region (see state 3 at ~0 deg, shown in b) separate the NPs belonging to the original G-NP thus creating G-NP1 and G-NP2, as observed in the displayed sequence (see Video S6 of the paper). These two new heat sources are propelled by the time averaged propulsion force corresponding to state 3 in opposite directions toward the region where they finally merge into a joint G-NP again. Complex transport trajectories for G-NP delivery, for example in form of knot circuit (see Video S7 of the paper), can be created enabling spatial distribution of moving heat sources across a target network Credit: José A. Rodrigo, Mercedes Angulo and Tatiana Alieva

Today, optofluidics is one of the most representative applications of photonics for biological/chemical analysis. The ability of plasmonic structures (e.g., colloidal gold and silver nanoparticles, NPs) under illumination to release heat and induce fluid convection at the micro-scale has attracted much interest over the past two decades. Their size- and shape-dependent as well as wavelength-tunable optical and thermal properties have paved the way for relevant applications such as photothermal therapy/imaging, material processing, biosensing and thermal optofluidics to name a few. In-situ formation and motion control of plasmon-enhanced heat sources could pave the way for further harnessing of their functionalities, especially in optofluidics. However, this is a challenging multidisciplinary problem combining optics, thermodynamics and hydrodynamics.


In a recent paper published in Light Science & Applications, Professor Jose A. Rodrigo and co-workers from Complutense University of Madrid, Faculty of Physics, Department of Optics, Spain, have developed a technique for jointly controlling the formation and motion of heat sources (group of gold NPs) as well as of the associated thermal-induced fluid flows created around them. The scientists summarize the operational principle of their technique, “The technique applies a structured laser-beam trap to exert an optical propulsion force over the plasmonic NPs for their motion control, while the same laser simultaneously heats up them. Since both the shape of the laser trap and the optical propulsion forces are easily and

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