School of Vehicle and Mobility, Tsinghua University, Beijing 100084, China.
General Research and Development Institute, China FAW Corporation Limited, Changchun 130000, China.
Rev Sci Instrum. 2023 Jan 1;94(1):015103. doi: 10.1063/5.0129497.
Magnetorheological fluid (MRF) is a widely used smart material that suffers from sedimentation. Since sedimentation is unavoidable, it is crucial to study and improve the redispersibility of MRFs. However, previous redispersibility testing methods have problems, such as complicated operation and low precision. Simultaneously, a simple and effective method is urgently needed for high-precision modeling of MRF sedimentation to test the rheological properties of settled MRFs at different depths. After systematically analyzing the redispersion problem, this paper proposes decoupling the energy required for redispersing settled MRFs into two parts, which are related to different factors. These two parts are the energy required to separate the agglomerated particles (related to the MRF formula) and that to redisperse the settled MRF uniformly vertically against gravity (related to the solid concentration and packing limit). The energy that separates the agglomerated particles is proportional to the shear stress of slowly shearing the corresponding agglomerated samples, i.e., the yield stress. Thus, this paper proposes a simple microdamage quasi-static indentation method to measure the yield stresses of settled MRFs at different depths to characterize the redispersibility of the corresponding MRFs. Herein, this method is applied to study the mechanisms of the influences of surfactants, thixotropic agents, and their networks on the redispersibility of MRFs. The results indicate that a well-dispersed plate-like thixotropic agent network can effectively improve redispersibility, while surfactants with poor compatibility degrade redispersibility. In summary, this redispersibility test method will greatly facilitate studies of MRFs, such as optimizing the formulas and establishing sedimentation models.
磁流变液（MRF）是一种广泛使用的智能材料，但存在沉降问题。由于沉降是不可避免的，因此研究和提高 MRF 的再分散性至关重要。然而，以前的再分散性测试方法存在操作复杂、精度低等问题。同时，迫切需要一种简单有效的方法来对 MRF 沉降进行高精度建模，以测试不同深度下沉降 MRF 的流变性能。本文在系统分析再分散问题的基础上，提出将再分散沉降 MRF 所需的能量解耦为两部分，这两部分与不同的因素有关。这两部分分别是分离团聚颗粒所需的能量（与 MRF 配方有关）和垂直于重力均匀再分散沉降 MRF 所需的能量（与固体浓度和堆积极限有关）。分离团聚颗粒所需的能量与缓慢剪切相应团聚样品所需的剪切应力（即屈服应力）成正比。因此，本文提出了一种简单的微损伤准静态压痕法，用于测量不同深度沉降 MRF 的屈服应力，以表征相应 MRF 的再分散性。在此基础上，该方法用于研究表面活性剂、触变剂及其网络对 MRF 再分散性的影响机制。结果表明，分散良好的板状触变剂网络可以有效提高再分散性，而相容性差的表面活性剂会降低再分散性。总之，这种再分散性测试方法将极大地促进 MRF 的研究，如优化配方和建立沉降模型。
