  Hydrodynamic interactions are important for diverse fluids especially those
with low Reynold's number such as microbial and particle-laden suspensions, and
proteins diffusing in membranes. Unfortunately, while far-field (asymptotic)
hydrodynamic interactions are fully understood in two- and three-dimensions,
near-field interactions are not, and thus our understanding of motions in dense
fluid suspensions is still lacking. In this contribution, we experimentally
explore the hydrodynamic correlations between particles in
quasi-two-dimensional colloidal fluids in the near-field. Surprisingly, the
measured displacement and relaxation of particle pairs in the body frame
exhibit direction-dependent dynamics that can be connected quantitatively to
the measured near-field hydrodynamic interactions. These findings, in turn,
suggest a mechanism for how and when hydrodynamics can lead to a breakdown of
the ubiquitous Stokes-Einstein relation (SER). We observe this breakdown, and
interestingly, we show that the direction-dependent breakdown of the SER is
ameliorated along directions where hydrodynamic correlations are smallest. In
total, the work uncovers significant ramifications of near-field hydrodynamics
on transport and dynamic restructuring of fluids in two-dimensions.
