  Microscopic dynamics reveal the origin of the bulk rheological response in
complex fluids. In model systems particle motion can be tracked, but for
industrially relevant samples this is often impossible. Here we adapt
differential dynamic microscopy (DDM) to study flowing highly-concentrated
samples without particle resolution. By combining an investigation of
oscillatory flow, using a novel "echo-DDM" analysis, and steady shear, through
flow-DDM, we characterise the yielding of a silicone oil emulsion on both the
microscopic and bulk level. Through measuring the rate of shear-induced droplet
rearrangements and the flow velocity, the transition from a solid-like to
liquid-like state is shown to occur in two steps: with droplet mobilisation
marking the limit of linear visco-elasticity, followed by the development of
shear localisation and macroscopic yielding. Using this suite of techniques,
such insight could be developed for a wide variety of challenging complex
fluids.
