  Deviations between macrorheological and particle-based microrheological
measurements are often considered a nuisance and neglected. We study aqueous
poly(ethylene oxide) (PEO) hydrogels for varying PEO concentrations and chain
lengths that contain microscopic tracer particles and show that these
deviations in fact reveal the nanoscopic viscoelastic properties of the
particle-hydrogel interface. Based on the transient Stokes equation, we first
demonstrate that the deviations are not due to finite particle radius,
compressibility or surface-slip effects. Small-angle neutron scattering rules
out hydrogel heterogeneities. Instead, we show that a generalized
Stokes-Einstein relation, accounting for a nanoscopic interfacial shell around
tracers with viscoelastic properties that significantly deviate from bulk,
consistently explains our macrorheological and microrheological measurements.
The extracted shell diameter is comparable with the PEO end-to-end distance,
indicating the importance of dangling chain ends. Our methodology reveals the
nanoscopic interfacial rheology of hydrogels and is generally applicable to
different kinds of viscoelastic fluids and particles.
