Behind the Rotating Flattop: when vortex meets a deformable surface

We study experimentally a two-fluid system, driven by a rotating upper boundary, inside a stationary cylinder. For a range of aspect ratios, the interface displays various changes with driving rates, with the most striking being the formation of a plateau. Direct imaging and flow visualization allow us to identify the interplay between the morphology of our two-fluid interface and the vortex loops reported previously in literatures, in a way that we can rigorously define the transition by the switch of topology in the flow structure rather than just the shape of the free surface. Further extensions of the parameter space show a wealth of phenomena involving various instabilities on the interface that call for further understanding.