As mentioned in the main text, the reversal of the central jet in the nonlinear regime of the
wave generation using a cylindrical plunger is a robust effect observed in a broad range of the
plunger accelerations with cylinders of various lengths. The stability of the pattern can be
optimized by adjusting the excitation frequency, which affects the mode number of the cross
wave. The reversal of the jet direction appears to be independent of different driving
frequencies. Fig. S4 shows inward flow patterns produced by the cylindrical plunger (white
rectangle in the centres of the plots) at two frequencies, in the gravity and in the capillary
wave range.
The velocities of the floaters in both outward and the inward jets have been measured as a
function of the distance from the plunger. The velocities of the central jets are shown in
Fig. S5 for the regimes corresponding to Figs. 1(a-d) of the main text.
Figure S5 | Structure of the flow produced by the cylindrical plunger (130 mm long) as
described in the main text (Figs. 1b,d) for the cases of (a) outward, and (b) inward jets. (c)
Measured (normalized) jet velocity versus the distance away from the wave maker. The
excitation frequency f0 = 20 Hz.
The motion of floaters on the surface perturbed by capillary-gravity waves propagating away
from a wave maker seems inconsistent with the Stokes drift model. This should not be
surprising since the original model was developed for planar waves of very small amplitude.
However the floaters uniformly move in the direction of the wave propagation in the initial
stage of the flow development, before a return flow starts to develop. Surface particle streaks
(moving averaged over 5 wave periods) are illustrated in Fig. S6a, where they are filmed
shortly after the plunger is activated. During this time most floaters in front of a plunger
move in the direction of the wave propagation. However even then their velocities disagree
with the Stokes drift expectation, S kaU ω 2 ~ , where a is the wave amplitude, k and ω are the
wave number and frequency. Since the wave amplitude decays away from the plunger,
particle velocities should be the highest near the plunger and should decay with the distance.
This is not the case, as seen in Fig. S6c. At a later stage, after a stationary flow develops, this
discrepancy becomes more pronounced. We conclude that in all wave-driven flows described
here, the velocity field is not directly determined by the Stokes drift of the underlying wave
field.
The velocity maxima in Figs. S5 and S6 can be considered as separating near-field from farfield
flow pattern. In particular, Fig. S6 shows that initially the jet velocity is closer to the
wave maker (blue diamonds), while in the steady state (red squares), the velocity profile is
broader with the maximum velocity being further away from the plunger. The green triangles
in Fig. S6c and the solid line show that the squared wave intensity a
2
strongly decays as a
function of the distance from the plunger. This suggests that the flow velocity in not
determined by the local wave field, but results from a global flow pattern.
Figure S6 | Surface particle streaks (a) shortly after the plunger is turned on, and (b) in the
steady state, after the establishment of a stable quadrupole vortex flow. (c) Central jet
velocity as a function of the distance from the wave maker during the start-up phase
(diamonds), and in the steady state (squares). Triangles and the solid line show squared wave
amplitude a
2
in front of the cylindrical wave maker.
Surface flows produced by various wave makers
As discussed in the main text, stable flow patterns exhibiting inward and outward jets as well
as stationary vortices can be generated by appropriately shaping wave makers. Some
examples of such flows are shown in Fig. S7.
Figure S7 | Surface flow patterns produced by the wave makers of different shapes in the
linear regime: (a) elliptical, (c) triangular pyramid, and (d) square pyramid. Waterlines are
shown as solid white lines on the plungers. The patterns correspond to the weakly nonlinear
waves, below the modulation instability threshold. The excitation frequency in these
examples is f0 = 60 Hz.
Similarly to the cylindrical wave maker, pyramidal plungers produce outwards jets normal to
the sides of a triangle or a square, while the return flows are directed towards vertices. Above
the threshold of modulation instability pyramidal wave makers produce inward central jets
and reverse the flow direction. An example of the tractor beam driven by the triangular
pyramid is shown in Fig. S8.
Visualization of the surface flow produced by a triangular pyramid above the
threshold of modulation instability. The wave maker frequency f = 60 Hz. The central jet is directed inward (tractor beam mode). The tractor beam is sufficiently strong to attract small
objects, see video “Toy_boat_capture_1fps.mp4”. A corresponding fl