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0MOTION OF A TRACER DRIVEN BY QUANTUM VORTICES ON THE FREE SURFACE OF SUPERFLUID HELIUM
Boltnev R.E., Vasiliev M.M., Petrov O.F.
Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, Russia,
boltnev@gmail.com
The motion of linear quantized vortex terminating at the free surface He-II can be visualized either by a charged particle expelled from the bulk helium by electric field [1], or by a tracer with positive buoyancy [2]. We report an experimental study of motion of a tracer particle driven by quantum vortices on the free surface of superfluid helium (He-II) at T = 1.45 K [3]. We used tracers consisted of 8-10 hollow microspheres with the density less than the liquid helium density. The upper end of a linear vortex trapped the tracer and they moved together along the He-II surface. Asymmetric shape of the tracer as well as its large size < 0.2 mm (Fig. 1) allowed us to observe its rotational motion along with the translational one (the mean values ~ 10 revolutions per second and ~ 7 mm/s, respectively). The tracer mean-square displacement and the energy dissipation within a vortex bundle were measured through the observation of translational and rotational motion of the tracer. The rotational energy and angular momentum of the particle driven by a quantum vortex were determined as ~ 10-15 J and ~ 2.610-17 kgm2/s, correspondingly, in good accordance with the energy and total angular momentum of the quantum vortex estimated as ~ 6.110-14 J and ~ 2.610-15 kgm2/s, respectively. The translational motion of the tracers at short times (< 25 ms) related to anomalous diffusion mode typical for active particles, while for longer times it corresponded to normal diffusion mode. The interaction of a particle with quantum vortices revealed a very efficient energy transfer from a vortex tangle to a particle due to coupling of the normal and superfluid components of He-II [3].
Figure 1. A tracer on the free surface of He-II and its model: a) Glass cell for observation of motion of the tracer (shown as a red oval) driven by quantum vortices along the He-II surface; b) Image of a tracer in scattered light; c) Simplified model of a tracer used for estimations of its mass and inertial momentum: 6 glass microspheres surround the central one. The microspheres' diameter is equal to 60 ^m.
The work has been carried out under the support of the Russian Science Foundation (Project # 20-12-00372).
References
1. Moroshkin P., Leiderer P., Kono K., Inui S., and Tsubota M. // Phys. Rev. Lett. - 2019. - V. 122. - P. 174502.
2. Levchenko A.A., Lebedeva E.V., Mezhov-Deglin L.P., Pelmenev A. A. // Low Temp. Phys. -2019. - V. 45. - P. 469-475.
3. Boltnev R.E., Vasiliev M.M., Petrov O.F. // Sci. Rep. - 2023. - V. 13. - P. 22538.
