标题： 颗粒接触处的停滞点在三维多孔介质中产生弹性流动不稳定性 显示英文标题

标题： Stagnation points at grain contacts generate an elastic flow instability in 3D porous media

摘要： 许多环境、能源和工业过程涉及聚合物溶液在三维（3D）多孔介质中的流动，其中流体被限制在复杂的孔隙空间几何结构中导航。 当聚合物通过曲折的孔隙空间传输时，弹性应力会累积，导致在临界流量以上出现不稳定的流动波动。 孔隙空间几何结构如何影响这种弹性不稳定性的发展和特征？ 在此，我们通过直接成像微加工的三维有序多孔介质中聚合物溶液的流动来回答这个问题，这些介质具有由简单立方（SC）或体心长方（BC）阵列的球形颗粒组成的精确控制的几何结构。 在这两种情况下，我们发现流动不稳定性是在颗粒接触处产生的停滞点处形成的，而不是在单个颗粒的上游/下游极性表面处，这与单个颗粒周围的流动情况不同。 流动不稳定性特征强烈依赖于单元格几何结构：在SC填充中，不稳定性通过形成时间依赖的、波动的三维涡旋表现出来；而在BC填充中，则表现为流动路径线的持续波动和交叉。 尽管存在这种差异，我们发现随着流量增加，从稳定到不稳定流动的转变特征在不同几何结构中具有共性。 此外，对于这两种填充几何结构，我们的数据表明，由接触相关的停滞点产生的拉伸流动引起的聚合物应力是整个介质中宏观流动阻力的主要贡献因素。 总之，我们的工作强调了颗粒间接触的关键作用——这些接触通常被理想化为离散点而被忽视，但在大多数天然和工程介质中是固有的——在塑造多孔介质中的弹性不稳定性方面的作用。 显示英文摘要

摘要： Many environmental, energy, and industrial processes involve the flow of polymer solutions in three-dimensional (3D) porous media where fluid is confined to navigate through complex pore space geometries. As polymers are transported through the tortuous pore space, elastic stresses accumulate, leading to the onset of unsteady flow fluctuations above a threshold flow rate. How does pore space geometry influence the development and features of this elastic instability? Here, we address this question by directly imaging polymer solution flow in microfabricated 3D ordered porous media with precisely controlled geometries consisting of simple-cubic (SC) or body-centered cuboid (BC) arrays of spherical grains. In both cases, we find that the flow instability is generated at stagnation points arising at the contacts between grains rather than at the polar upstream/downstream grain surfaces, as is the case for flow around a single grain. The characteristics of the flow instability are strongly dependent on the unit cell geometry: in SC packings, the instability manifests through the formation of time-dependent, fluctuating 3D eddies, whereas in BC packings, it manifests as continual fluctuating 'wobbles' and crossing in the flow pathlines. Despite this difference, we find that characteristics of the transition from steady to unsteady flow with increasing flow rate have commonalities across geometries. Moreover, for both packing geometries, our data indicate that extensional flow-induced polymeric stresses generated by contact-associated stagnation points are the primary contributor to the macroscopic resistance to flow across the entire medium. Altogether, our work highlights the pivotal role of inter-grain contacts -- which are typically idealized as discrete points and therefore overlooked, but are inherent in most natural and engineered media -- in shaping elastic instabilities in porous media.