Rotating Polygonal Depression Soliton Clusters on the Surface of a Liquid Ring.

We report the first experimental observation of rotating depression  soliton sets on the inner surface of a  viscous liquid ring, carrying  background waves. These occur within a rotating shallow layer of oil inside a stationary cylindrical container.  The solitons are organized either in single, two, or regular polygonal (triangle and hexagon) clusters; they travel in unison at a higher speed than the  background traveling waves without changing their shapes. The  spectral power density  reveals  a possible  energy exchange  between the soliton clusters and the  background mixed radial-azimuthal modulations through  wave radiation.