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0VISUALIZATION OF THE INTERACTION OF MICRON-SIZED PARTICLES
WITH VORTICES IN SUPERFLUID 4He DOWN TO 140mK
Golov A. I.1, Goodwin C. O.1, Doyle M. J.1, Hay, J. A.1, Skachko I.1, Guo, W.23
1 University of Manchester, Manchester, UK, andrei.golov@manchester.ac.uk 2National High Magnetic Field Laboratory, Tallahassee, USA 3Florida State University, Tallahasse, USA
In recent decades a great deal of experimental information about the microscopic dynamics of vortex tangles in superfluid helium has been obtained by observing the motion of micron-sized particles through helium. Yet, due to experimental limitations, these have only been performed at temperatures Tabove ~1.4 K, i.e. with a strong damping of the dynamics of vortex lines by their interaction ('mutual friction') with the normal component. Our goal was to extend this type of experiment to temperatures below ~0.5 K where the effect of the normal component can be neglected, and the intrinsic dynamics of undamped vortex lines and their tangles take over.
We constructed a vibrationally-isolated rotating refrigerator that can cool, down to T = 0.14 K, an optical cell containing superfluid helium with in-situ dispersed polymer fluorescent particles of diameters in the range of 1-6 p,m. Particles' positions, within a remotely adjustable illuminating light sheet, were monitored by an intensified camera at the rate of up to 990 fps. When necessary, the apparatus could rotate continuously at angular velocity of up to ~1 rad s-1.
The turbulence was generated by a burst of particles injected from the cell's floor by ultrasound. Two types of particle trajectories were observed: erratic random-walk like and nearly straight, suggesting that particles could be either trapped by chaotically moving vortex lines or move untrapped. The erratic trajectories have not been seen below 0.7 K, likely due to the shortening of the lifetime of the trapped state with decreasing temperature.
Particle velocity distributions revealed a coexistence of a Gaussian function with power-law tails at all temperatures investigated, 0.14 K - 1.4 K. This is similar to the the distributions observed previously at higher temperatures. There, the Gaussian part is believed to be due to either the interaction of untrapped particles with the viscous turbulent normal fluid or effect of multiple vortex reconnections on the trajectories of trapped particles; and the power-law tails are likely due to the effect of either singular events of vortex reconnections on the trapped particles or the nature of the attractive force between the particles and singular vortices. Yet, as there is no viscosity in superfluid helium at T ~ 0.14 K, the Gaussian function for untrapped particles should be coming from a different mechanism - perhaps from the fluctuations of the flow acceleration within the vortex tangle. Similarly, the power-law tails could not be due to the effects of reconnections on the trapped particles. Possible interpretations will be discussed.
