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Deviations between macrorheological and particle-based microrheologicalmeasurements are often considered a nuisance and neglected. We study aqueouspoly(ethylene oxide) (PEO) hydrogels for varying PEO concentrations and chainlengths that contain microscopic tracer particles and show that thesedeviations in fact reveal the nanoscopic viscoelastic properties of theparticle-hydrogel interface. Based on the transient Stokes equation, we firstdemonstrate that the deviations are not due to finite particle radius,compressibility or surface-slip effects. Small-angle neutron scattering rulesout hydrogel heterogeneities. Instead, we show that a generalizedStokes-Einstein relation, accounting for a nanoscopic interfacial shell aroundtracers 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 thenanoscopic interfacial rheology of hydrogels and is generally applicable todifferent kinds of viscoelastic fluids and particles.

Nanoscopic Interfacial Hydrogel Viscoelasticity Revealed from Comparison of Macroscopic and Microscopic Rheology