Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA.
Soft Matter. 2018 Apr 18;14(15):2923-2937. doi: 10.1039/c7sm02135f.
A discrete element method (DEM) model is developed to simulate the dynamics of wet, flexible fibers. The angles of repose of dry and wet fibers are simulated, and the simulation results are in good agreement with experimental results, validating the wet, flexible fiber model. To study wet fiber flow behavior, the model is used to simulate shear flows of wet fibers in a periodic domain under Lees-Edwards boundary conditions. Significant agglomeration is observed in dilute shear flows of wet fibers. The size of the largest agglomerate in the flow is found to depend on a Bond number, which is proportional to liquid surface tension and inversely proportional to the square of the shear strain rate. This Bond number reflects the relative importance of the liquid-bridge force to the particle's inertial force, with a larger Bond number leading to a larger agglomerate. As the fiber aspect ratio (AR) increases, the size of the largest agglomerate increases, while the coordination number in the largest agglomerate initially decreases and then increases when the AR is greater than four. A larger agglomerate with a larger coordination number is more likely to form for more flexible fibers with a smaller bond elastic modulus due to better connectivity between the more flexible fibers. Liquid viscous force resists pulling of liquid bridges and separation of contacting fibers, and therefore it facilitates larger agglomerate formation. The effect of liquid viscous force is more significant at larger shear strain rates. The solid-phase shear stress is increased due to the presence of liquid bridges in moderately dense flows. As the solid volume fraction increases, the effect of fiber-fiber friction coefficient increases sharply. When the solid volume fraction approaches the maximum packing density, the fiber-fiber friction coefficient can be a more dominant factor than the liquid bridge force in determining the solid-phase shear stress.
本文建立了一个离散单元法（DEM）模型来模拟湿软纤维的动力学行为。模拟了干纤维和湿纤维的休止角，模拟结果与实验结果吻合良好，验证了湿软纤维模型的有效性。为了研究湿纤维的流动行为，采用该模型模拟了 Lees-Edwards 边界条件下周期性区域内湿纤维的剪切流动。在稀相剪切流动中，观察到湿纤维发生明显团聚。发现流场中最大团聚体的尺寸取决于一个邦德数（Bond number），该邦德数与液体表面张力成正比，与剪切应变速率的平方成反比。该邦德数反映了液体桥力与颗粒惯性力的相对重要性，邦德数越大，团聚体越大。随着纤维纵横比（aspect ratio，AR）的增加，最大团聚体的尺寸增大，而最大团聚体中的配位数在 AR 大于 4 时先减小后增大。由于更柔软的纤维之间的连接更好，具有较小的键弹性模量的纤维更容易形成具有更大配位数的更大团聚体。液体粘性力抵抗液体桥的拉伸和接触纤维的分离，因此有利于更大团聚体的形成。在较大的剪切应变速率下，液体粘性力的影响更为显著。在中等密度流动中，由于存在液体桥，固相剪切应力增大。随着固相体积分数的增加，纤维-纤维摩擦系数的影响急剧增加。当固相体积分数接近最大堆积密度时，纤维-纤维摩擦系数可以成为决定固相剪切应力的比液体桥力更为重要的因素。
