  Complex fluids such as emulsions, colloidal gels, polymer or surfactant
solutions are all characterized by the existence of a "microstructure" which
may couple to an external flow on timescales that are easily probed in
experiments. Such a coupling between flow and microstructure usually leads to
instabilities under relatively weak shear flows that correspond to vanishingly
small Reynolds numbers. Wormlike micellar surfactant solutions appear as model
systems to study two examples of such instabilities, namely shear banding and
elastic instabilities. Focusing on a semidilute sample we show that two
dimensional ultrafast ultrasonic imaging allows for a thorough investigation of
unstable shear-banded micellar flows. In steady state, radial and azimuthal
velocity components are recovered and unveil the original structure of the
vortical flow within an elastically unstable high shear rate band. Furthermore
thanks to an unprecedented frame rate of up to 20,000 fps, transients and fast
dynamics can be resolved, which paves the way for a better understanding of
elastic turbulence.
