Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, USA.
Soft Matter. 2019 Feb 13;15(7):1666-1675. doi: 10.1039/c8sm02192a.
Hydrogels of semiflexible biopolymers such as collagen have been shown to contract axially under shear strain, in contrast to the axial dilation observed for most elastic materials. Recent work has shown that this behavior can be understood in terms of the porous, two-component nature and consequent time-dependent compressibility of hydrogels. The apparent normal stress measured by a torsional rheometer reflects only the tensile contribution of the axial component σzz on long (compressible) timescales, crossing over to the first normal stress difference, N1 = σxx - σzz at short (incompressible) times. While the behavior of N1 is well understood for isotropic viscoelastic materials undergoing affine shear deformation, biopolymer networks are often anisotropic and deform nonaffinely. Here, we numerically study the normal stresses that arise under shear in subisostatic, athermal semiflexible polymer networks. We show that such systems exhibit strong deviations from affine behavior and that these anomalies are controlled by a rigidity transition as a function of strain.
水凝胶的半刚性生物聚合物如胶原已被证明在剪切应变下轴向收缩，与大多数弹性材料观察到的轴向扩张形成对比。最近的工作表明，这种行为可以用多孔、双组分性质和由此产生的水凝胶的时变压缩性来理解。扭转流变仪测量的表观法向应力仅反映了轴向分量 σzz 在长（可压缩）时间尺度上的拉伸贡献，在短（不可压缩）时间尺度上过渡到第一法向差 N1 = σxx - σzz。虽然各向同性粘弹性材料在经历仿射剪切变形时对 N1 的行为有很好的理解，但生物聚合物网络通常是各向异性的，并且非仿射变形。在这里，我们通过数值研究了在亚等静压、非热半刚性聚合物网络中剪切下产生的法向应力。我们表明，此类系统表现出与仿射行为的强烈偏离，并且这些异常受作为应变函数的刚性转变控制。
