Order emerges from chaos in 2D vortices
Many-body systems usually become more disordered as energy is added to them. But in 1949, theoretical chemist Lars Onsager predicted that fluids ought to follow the opposite behavior, provided they’re restricted to two dimensions. That is, initially turbulent 2D flows should give rise to low-entropy flows if enough energy is pumped into them. The counterintuitive behavior has been seen in systems as diverse as shallow rivers, soap bubbles, and cyclones.
Onsager’s theory only makes predictions for superfluids, which can rotate only by winding themselves in vortices whose circulation is quantized. That constraint makes liquid helium and Bose–Einstein condensates (BECs) model systems for studying turbulence. BECs are especially appealing because their atoms interact so weakly that the low-density region around a vortex can be as large as a micron. And when the magnetic field used to confine the BEC is turned off, the gas expands to a few times its original size within milliseconds. Illuminated with visible light, the vortices show up as dark, density-depleted holes. (See Physics Today, January 2017, page 19 .)
Two Australian groups—one led by Tyler Neely , Matthew Davis , and Halina Rubinsztein-Dunlop (all at the University of Queensland), the other led by Kristian Helmerson and Shaun Johnstone (both at Monash University in Victoria)—have now experimentally confirmed the evolution of an initially turbulent rubidium-atom BEC into higher-energy, long-lived, large-scale vortices.
The Queensland team pulled off the achievement by generating two large clusters of vortices. They used two laser-induced “paddle potentials” (black lines in the optical-density image in panel a) to stir (white arrows) the BEC into a higher-energy flow, whose injected vortices (holes in panel b) become organized into two persistent, macroscopic clusters within half a second. Numerical simulations show that the vortices in the clusters (red and blue in panel c) circulate in opposite directions. The Monash team, by contrast, generated vortex distributions at a range of temperatures in a similarly stirred system and observed the vortices’ subsequent temporal evolution.
In both studies, the vortices were observed to order themselves into clusters—a nonequilibrium development of Onsager’s equilibrium treatment, in which the conservation of energy leads to the creation of giant quantized vortices at negative temperature. The temperature is negative when the system’s phase-space volume decreases with increasing energy. (See the article by Gregory Falkovich and Katepalli Sreenivasan, Physics Today, April 2006, page 43 .)
(G. Gauthier et al., Science 364, 1264, 2019 ; S. P. Johnstone et al., Science 364, 1267, 2019 .)
