Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture

K under conditionsof strong interactions controlled via an interspecies Feshbach resonance. We study the expansionof the mixture after release from the trap and, in a narrow magnetic field range, we observe twophenomena related to hydrodynamic behavior. The common inversion of the aspect ratio is foundto be accompanied by a collective effect where both species stick together and expand jointly despiteof their widely different masses. Our work constitutes a major experimental step for a controlledinvestigation of the many-body physics of this novel strongly interacting quantum system.