Surface flow, shapes and stability of rotating triangles on a water surface

We present an experimental study of polygons forming at the free surface of a water flow driven by a rotating bottom and confined to a stationary cylinder, as described in Jansson et al., Phys. Rev. Lett. {\bf 96}, 174502 (2006). In particular, we study the case of a triangular structure, either completely floating or with a dry center. For these structures, we present measurements of the surface flows, the surface shapes and the process of structure formation, and we analyze our results in terms of a collection of discrete vortices. We show that partial blocking of the surface flow destroys the triangular structure and reestablishes the circular symmetry.