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It's not often we pause in the middle of our day and think about the nature of viscosity, unless perhaps we're on our way to Oil Can Henry's for an oil change on the 'ol family truckster. But thanks to some fluid-friendly physicists, now we've got a reason!
According to Princeton University's official definition, "Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction." Got that?
Now that we're all in agreement as to what constitutes resistant flow, imagine a free-moving substance, call it a "perfect fluid" that operates with the least possible amount of friction allowed by the immutable laws of quantum mechanics, and you'll grasp what scientists at MIT have just created. Far from just a friction-less environment, the results of the experiments might aid physicists in their pursuits to investigate the viscosity in cores of neutron stars, the plasma of the ancient universe, and additional forceful interacting fluids.
To create this universal magic, scientists made a recording of sound waves delivered through a controlled gas of basic particles called fermions. This rising scale, known as a glissando, can be heard in the clip below, which demonstrates the distinct frequencies that the gas resonates like a guitar string when sound waves are injected.
By analyzing thousands of sound waves shooting through this gas to calculate its “sound diffusion,” or how fast sound dissipates, the team was able to determine the material’s viscosity, or internal friction. The results of their efforts were published this week in the online journal, Science.
“It’s quite difficult to listen to a neutron star,” says study co-author Martin Zwierlein, the Thomas A. Frank 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. 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.”
So low was the value of the sound diffusion that it could only be measured by a molecular-level of friction. It confirmed that this strongly interacting fermion gas demonstrates properties of a perfect fluid, and is considered universal in nature. This marks the first instance of scientists measuring sound diffusion in a perfect fluid.
Fermions are elementary particles like electrons, protons, and neutrons, and are regarded as the building blocks of all matter. Normally content to exist as loners, fermions display characteristics of low viscosity when aroused and made to strongly interact. To manufacture this unflawed fluid, the researchers employed a system of lasers to entrap a gas of lithium-6 atoms, which are recognized as fermions.
By configuring lasers to form an optical box around the fermion gas, scientists were able to tune them to cause fermions to ricochet back into the box when they collided with the edges of the optical enclosure.
“All these snapshots together give us a sonogram, and it’s a bit like what’s done when taking an ultrasound at the doctor’s office,” Zwierlein added. “The quality of the resonances tells me about the fluid’s viscosity, or sound diffusivity. If a fluid has low viscosity, it can build up a very strong sound wave and be very loud, if hit at just the right frequency. If it’s a very viscous fluid, then it doesn’t have any good resonances.”
Zwierlein and his colleagues are confident that these new tools can be harnessed to estimate quantum friction within cosmic matter like neutron stars, and also allow for a greater understanding of how diverse materials can be manipulated to portray perfect, superconducting flow.
“This work connects directly to resistance in materials,” Zwierlein notes. “Having figured out what’s the lowest resistance you could have from a gas tells us what can happen with electrons in materials, and how one might make materials where electrons could flow in a perfect way. That’s exciting.”