Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture

We report on the expansion of an ultracold Fermi-Fermi mixture of $^6$Li and $^{40}$K under conditions of strong interactions controlled via an interspecies Feshbach resonance. We study the expansion of the mixture after release from the trap and, in a narrow magnetic field range, we observe two phenomena related to hydrodynamic behavior. The common inversion of the aspect ratio is found to be accompanied by a collective effect where both species stick together and expand jointly despite of their widely different masses. Our work constitutes a major experimental step for a controlled investigation of the many-body physics of this novel strongly interacting quantum system.