  We have directly observed short-time stress propagation in viscoelastic
fluids using two optically trapped particles and a fast interferometric
particle-tracking technique. We have done this both by recording correlations
in the thermal motion of the particles and by measuring the response of one
particle to the actively oscillated second particle. Both methods detect the
vortex-like flow patterns associated with stress propagation in fluids. This
inertial vortex flow propagates diffusively for simple liquids, while for
viscoelastic solutions the pattern spreads super-diffusively, dependent on the
shear modulus of the medium.
