2 research outputs found
Nanoscopic Interfacial Hydrogel Viscoelasticity Revealed from Comparison of Macroscopic and Microscopic Rheology
Deviations between
macrorheological and particle-based microrheological
measurements are often considered to be 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 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 an interfacial shell around tracers with viscoelastic
properties that deviate from bulk, consistently explains our macrorheological
and microrheological measurements. The extracted shell diameter is
comparable to the PEO end-to-end distance, indicating the importance
of dangling chain ends. Our methodology reveals the nanoscopic interfacial
rheology of hydrogels and is applicable to different kinds of viscoelastic
fluids and particles
Impact of Nanostructuring on the Phase Behavior of Insertion Materials: The Hydrogenation Kinetics of a Magnesium Thin Film
Nanostructuring is widely applied
in both battery and hydrogen
materials to improve the performance of these materials as energy
carriers. Nanostructuring changes the diffusion length as well as
the thermodynamics of materials. We studied the impact of nanostructuring
on the hydrogenation in a model system consisting of a thin film of
magnesium sandwiched between two titanium layers and capped with palladium.
While we verified optically the coexistence of the metallic α-MgD<sub><i>x</i></sub> and the insulating β-MgD<sub>2–<i>y</i></sub> phase, neutron reflectometry shows significant deviations
from the thermodynamic solubility limits in bulk magnesium during
the phase transformation. This suggests that the kinetics of the phase
transformation in nanostructured battery and hydrogen storage systems
is enhanced not only as a result of the reduced length scale but also
due to the increased solubility in the parent phases