Wang Jing, Burton Justin C
Emory University, Department of Physics, 400 Dowman Drive, Atlanta, Georgia 30322, USA.
Phys Rev Lett. 2025 Apr 11;134(14):148203. doi: 10.1103/PhysRevLett.134.148203.
Hydrogels are 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. Previous studies have shown that low-modulus hydrogels and semiflexible biopolymer networks 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. Poroelastic relaxation 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, which 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.
水凝胶是一种膨胀的聚合物网络，其中弹性变形与纳米级流体流动相互关联。因此，水凝胶能够承受较大应变，并表现出非线性的超弹性特性。先前的研究表明，低模量水凝胶和半柔性生物聚合物网络在比多孔弹性弛豫时间尺度长得多的时间尺度上受到剪切时普遍会收缩。通过流变学和摩擦学测量，我们发现模量约为10 - 100 kPa的刚性聚丙烯酰胺和聚丙烯酸水凝胶在受到剪切时只会膨胀（扩张）。使用应变控制压缩来研究多孔弹性弛豫，结果表明体积扩散常数约为10⁻⁹ m²/s。在剪切过程中，我们观察到法向应力增加，且与剪切应变呈二次方关系，这种情况会持续数小时。此外，我们表明这种剪胀行为在摩擦滑动过程中表现为膨胀，使水凝胶吸收流体。我们认为这种固有的超弹性剪胀是所有刚性水凝胶的一个重要特征，并且可能解释诸如软骨等生物组织中的再水化和机械恢复活力现象。
