treatment ensures that particles do not aggregate at the
surface. This is illustrated in Fig. 2(a) with particles homogeneously
distributed at the surface of the wave field.
It has recently been shown that the Faraday wave properties
are consistent with those of an ensemble of bounded
oscillating solitons, or oscillons [10]. Figure 2(b) shows
examples of the oscillons’ trajectories to be compared with
a few particle tracks measured by using the same tracking
code. The horizontal motion of oscillons is random in time
but confined to a disordered lattice as seen from the black
trajectories shown in Fig. 2(b) [19]. The particles also
wander erratically, but their trajectories show substantially
larger excursions in comparison to oscillons. A particle
usually visits several oscillon sites which randomize its
trajectory. Although the particle and oscillon trajectories
are characterized by completely different length scales, the
oscillon mobility is an essential feature of Faraday flows.
In the case of a perfect oscillonic crystal, in which the
relative position of the nodes is frozen, the horizontal
mobility of particles is zero, showing that the horizontal
particle transport is strongly correlated to the oscillon
motion. Such crystals are obtained by adding minute
amounts of bovine serum to water [10].
Though the oscillon lattice in Fig. 2(a) seems disordered,
the wave number spectrum of the ripples is very
narrow, as seen in Fig. 2(c). This is consistent with the fact
that oscillon motion is restricted in space to about half of
the lattice characteristic wavelength [19]. To generate 2D
turbulence, one needs to inject energy into the horizontal
fluid motion at some intermediate range of scales in a
localized wave number domain. The horizontal mobility
of oscillons is the manifestation of a momentum transfer
along the water surface; this transfer is restricted to a
narrow k domain.
Figure 2(d) shows frequency power spectra of velocity
of particles moving along their Lagrangian trajectories.
The spectra show peaks close to the Faraday frequency
(30 Hz, the first subharmonic of the excitation frequency)
and a broad low-frequency band whose energy grows with
the increase in vertical acceleration. Since energy is
injected at f ¼ 30 Hz, the growth and the broadening of
the low-frequency band indicates a nonlinear energy transfer
to slower temporal (larger spatial) scales. This transfer,
as will be shown below, is due to the inverse energy
cascade.
Figure 3(a) shows many trajectories of particles on a
densely seeded fluid surface; the presence of multiple
length scales in this Faraday flow is visible. In the following,
particle image velocimetry is used to characterize the
Eulerian velocity field [20].
Figure 3(b) shows wave number spectra of the horizontal
kinetic energy measured at f0 ¼ 60 Hz at three vertical
acceleration levels, a ¼ 0:7, 1.2, and 1.6 g, corresponding
to supercriticalities in the range ¼ ½0:17; 1:7 (ath ¼
0:6 g at 60 Hz). The spectrum is close to the