  Peculiar dynamics of a free surface of the superfluid 4He has been observed
experimentally with a newly established technique utilizing a number of
electrically charged fine metal particles trapped electrically at the surface
by Moroshkin et al. They have reported that some portion of the particles
exhibit some irregular motions and suggested the existence of quantized
vortices interacting with the metal particles. We have conducted calculations
with the vortex filament model, which turns out to support the idea of the
vortex-particle interactions. The observed anomalous metal particle motions are
roughly categorized into two types; (1) circular motions with specific
frequencies, and (2) quasi-linear oscillations. The former ones seem to be
explained once we consider a vertical vortex filament whose edges are
terminated at the bottom and at a particle trapped at the surface. Although it
is not yet clear whether all the anomalous motions are due to the quantum
vortices, the vortices seem to play important roles for the motions.
