Waves on the surface of an oscillating liquid
When a bowl of a liquid (here milk) is placed on a vibrating plate, the
inertial forces create the same situation as if the gravity were changing
periodically. This results in parametric oscillations that have the form of
different vibrational modes of the liquid surface. It is interesting that the
frequency of these waves can be different than the frequency at which the bowl
oscillates. For example, in the second picture the frequencies of the bowl and
the waves were 60 and 30 Hz, respectively. The photographs were taken using a
stroboscope. It is remarkable that a rectangular structure of waves can emerge
in a circular bowl. The second row shows different phases of a mode with
frequency 13 Hz and the subsequent pairs of pictures show various modes of the
liquid in the bowl or a lid of a film container. The last four images show
modes of a drop with a diameter of about 5 mm.