How water waves are formed, and what shapes they take is something that can easily be taken for granted. Waves are just waves, right? Actually, they aren’t, because in addition to variations in size and velocity, waves can also differ in shape. New research into the shapes waves take has documented for the first time a completely different wave behavior called an odd standing wave.
Researchers led by Jean Rajchenbach, Alphonse Leroux, and Didier Clamond of the University of Nice-Sophia Antipolis in France discovered two new types of standing water waves, called Faraday waves. The two-dimensional even wave, is new for the type of experiment conducted, but is very similar to another phenomenon witnessed in nature. But, the researchers believe the odd standing wave is something completely new.
For their study, which was published in Physical Review Letters, the researchers used a Hele-Shaw cell. This is a container consisting of two vertically positioned parallel glass plates that are separated by a gap of 1.5mm. The container was filled with water, about 5cm deep. The Hele-Shaw cell was mounted on a shaker that caused the cell and the water inside to vibrate. Faraday waves were formed on the surface of the water when the vibration frequency got too high and the surface tension became unstable.
During the experiment, the researchers controlled for the frequency and amplitude of the vibrations and recorded the waves that formed on the water’s surface with a high speed camera. The two  wave shapes that were observed were different because one had even symmetry and the other had odd symmetry. If the even symmetry wave was sliced in half vertically, it would be a mirror image on both sides, whereas the odd symmetry wave was lower on one side and not the same shape on either end.
The researchers believe that the different waves formed because of changes in the positioning of the external probe that was used to disturb the water’s surface. The even symmetry wave is similar to a type of wave called a three-dimensional axisymmetric oscillon, which has previously been observed on the surface of a layer of vibrating bronze beads. But, there are no previous examples of a wave with the same shape as the odd symmetry wave.
According to Rajchenbach, prior to this research there were two main classes of solitary water waves, propagative solitons and envelop solitons. The waves observed in Rajchenbach’s experiment don’t fit into either of these categories. The new wave shapes are likely caused by the overlap of flat and wavy regions of the water, which are caused by the vibration of the liquid.
Further studies of the type of shaking-induced instabilities that formed the new waves may have an important role in a variety of fields including nonlinear optics, chemistry, biology, and even studies of large sea waves. In an interview with Physorg.com, Rajchenbach said:
“The main interest of our work obviously applies to sea nonlinear waves, and strengthens our knowledge concerning the formation of ocean waves of large amplitudes (giant ‘rogue’ waves or ‘tsunamis’).”
The term tsunami has become mainstream, following devastating natural disasters in Indonesia in 2004 and in Japan in 2011. Tsunami’s are caused by an undersea earthquake that causes a giant wave to rise up in the ocean. While Rajchenbach’s study wasn’t based on ocean waves, better understanding of how vibrations cause waves to form could be useful for researchers studying waves like Tsunamis.
(via Physorg.com, photo via Jean Rajchenbach, American Physical Society)
