Eggert Matthew D, Kumar Satish
Department of Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Ave. SE, Minneapolis, MN 55455, USA.
J Colloid Interface Sci. 2004 Oct 1;278(1):234-42. doi: 10.1016/j.jcis.2004.05.043.
We perform a set of experiments to study the nonlinear nature of an instability that arises in low-Reynolds-number flow past polymer gels. A layer of a viscous liquid is placed on a polydimethylsiloxane (PDMS) gel in a parallel-plate rheometer which is operated in stress-controlled mode. As the shear stress on the top plate increases, the apparent viscosity stays relatively constant until a transition stress where it sharply increases. If the stress is held at a level slightly above the transition stress, the apparent viscosity oscillates with time. If the stress is increased to a value above the transition stress and then decreased back to zero, the apparent viscosity shows hysteretic behavior. If the stress is instead decreased to a constant value and held there, the apparent viscosity is different from its pretransition value and exhibits sustained oscillations. This can happen even if the stress is held at values below the transition stress. Our observations suggest that the instability studied here is subcritical and leads to a flow that is oscillatory and far from viscometric. The phenomena reported here may be useful in applications such as microfluidics, membrane separations, and polymer processing. They may also provide insight into the rheological behavior of complex fluids that undergo flow-induced gelation.
我们进行了一系列实验，以研究在低雷诺数下聚合物凝胶流动中出现的一种不稳定性的非线性特性。在以应力控制模式运行的平行板流变仪中，将一层粘性液体放置在聚二甲基硅氧烷（PDMS）凝胶上。随着顶板上的剪切应力增加，表观粘度保持相对恒定，直到达到一个转变应力，此时表观粘度会急剧增加。如果应力保持在略高于转变应力的水平，表观粘度会随时间振荡。如果应力增加到高于转变应力的值，然后再降至零，表观粘度会表现出滞后行为。相反，如果应力降至一个恒定值并保持在那里，表观粘度与其转变前的值不同，并表现出持续振荡。即使应力保持在低于转变应力的值，这种情况也可能发生。我们的观察结果表明，这里研究的不稳定性是亚临界的，并导致一种振荡且远离粘度测量的流动。这里报道的现象可能在微流体、膜分离和聚合物加工等应用中有用。它们还可能为经历流动诱导凝胶化的复杂流体的流变行为提供见解。
