Kavli Affiliate: Jing Wang
| First 5 Authors: Jing Wang, Justin C. Burton, , ,
| Summary:
Hydrogels are biphasic, swollen polymer networks where elastic deformation is
coupled to nanoscale fluid flow. As a consequence, hydrogels can withstand
large strains and exhibit nonlinear, hyperelastic properties. For low-modulus
hydrogel and semiflexible biopolymer networks, previous studies have shown that
these materials universally contract when sheared on timescales much longer
than the poroelastic relaxation timescale. Using rheological and tribological
measurements, we find that stiff polyacrylamide and polyacrylic acid hydrogels,
with moduli of order ~10-100 kPa, exclusively swell (dilate) when sheared. The
poroelastic relaxation process was examined using strain-controlled
compression, indicating a volumetric diffusion constant of order 10^-9 m^2/s.
Upon shearing, we observed an increase in normal stress that varied
quadratically with shear strain, and persisted for hours. Moreover, we show
that this dilatant behavior manifests as swelling during tribological sliding,
imbibing the hydrogel with fluid. We suggest that this inherent, hyperelastic
dilatancy is an important feature in all stiff hydrogels, and may explain
rehydration and mechanical rejuvenation in biological tissues such as
cartilage.
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