  Like many quantum fluids, superfluid helium-4 (He II) can be considered as a
mixture of two miscible fluid components: an inviscid superfluid and a viscous
normal fluid consisting of thermal quasiparticles [1]. A mutual friction
between the two fluids can emerge due to quasiparticles scattering off
quantized vortex lines in the superfluid [2]. This quantum dissipation
mechanism is the key for understanding various fascinating behaviors of the
two-fluid system [3,4]. However, due to the lack of experimental data for
guidance, modeling the mutual friction between individual vortices and the
normal fluid remains an unsettled topic despite decades of research [5-10].
Here we report an experiment where we visualize the motion of quantized vortex
rings in He II by decorating them with solidified deuterium tracer particles.
By examining how the rings spontaneously shrink and accelerate, we provide
unequivocal evidences showing that only a recent theory [9] which accounts for
the coupled motion of the two fluids with a self-consistent local friction can
reproduce the observed ring dynamics. Our work eliminates long-standing
ambiguities in our theoretical description of the vortex dynamics in He II,
which will have a far-reaching impact since similar mutual friction concept has
been adopted for a wide variety of quantum two-fluid systems, including atomic
Bose-Einstein condensates (BECs) [11,12], superfluid neutron stars [13-15], and
gravity-mapped holographic superfluid [16,17].
