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.