Yielding and rearrangements in disordered emulsions

We use diffusing-wave spectroscopy to measure the motion of droplets in concentrated emulsions subjected to a periodic shear strain. The strain gives rise to periodic echoes in the correlation function which decay with increasing strain amplitude. For a given strain amplitude, the decay of the echoes implies that a finite fraction of the emulsion droplets never rearranges under periodic strain while the remaining fraction of droplets repeatedly rearranges. Yielding occurs when about 4-5% of the droplets rearrange.