Classical turbulence observed in a superfluid

Stirred coffee, like any normal fluid, can swirl around with an arbitrary speed and angular momentum. That’s not the case for superfluids. They can rotate only by winding themselves around vortices whose circulation is quantized. Their flow is also completely inviscid. That relative simplicity has turned superfluids such as helium and Bose–Einstein condensates into model systems for studying turbulence (see the article by Joe Vinen and Russell Donnelly, Physics Today, April 2007, page 43 ). BECs are particularly appealing because their vortices are visible. A condensate’s wavefunction goes to zero at the center of each one, so when the BEC is illuminated with light, the vortices show up as dark, density-depleted holes. Researchers led by Yong-il Shin (Seoul National University) have now exploited that shadow-imaging technique to investigate the transition between laminar and turbulent flows in an oblate BEC of sodium-23 atoms. Importantly, their experiments were designed to mimic a classic phenomenon in fluid dynamics: the wake behind a moving obstacle. The obstacle they chose was a thin cylindrical laser beam whose potential repelled the atoms as it was moved from left to right. Using a mirror to control the beam’s speed in a series of trials, the researchers captured a narrow velocity regime, in which the vortices shed from the obstacle resemble a space- and time-periodic pattern of alternating eddies known as a von Kármán street. For more than a century, the street has been associated with classical systems, like wind blowing past a tall building or clouds flowing over an island (left panel). Although the BEC analogue was predicted in 2010, this first experimental demonstration (right panel) gives researchers new confidence that the physics of quantum turbulence is relevant to classical turbulence. (W. J. Kwon et al., Phys. Rev. Lett., in press.)