  We study theoretically the velocity cross-correlations of a viscous fluid
confined in a slit between two viscoelastic media. We analyze the effect of
these correlations on the motions of particles suspended in the fluid. The
compliance of the confining boundaries gives rise to a long-ranged pair
correlation, decaying only as $1/r$ with the interparticle distance $r$. We
show how this long-ranged effect may be used to extract the viscoelastic
properties of the confining media without embedding tracer particles in them.
We discuss the remarkable robustness of such a potential technique with respect
to details of the confinement, and its expected statistical advantages over
standard two-point microrheology.
