Direct observation of particle rearrangement during cyclic stress hardening of magnetorheological gels
Direct observation of particle rearrangement during cyclic stress hardening of magnetorheological gels
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DOI：
10.1039/c2sm26587g
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发表时间：
2012-01-01
期刊：
影响因子：
3.4
通讯作者：
Mendes, Eduardo
中科院分区：
文献类型：
--
作者：
An, Hai-Ning;Picken, Stephen J.;Mendes, Eduardo
通过三嵌段共聚物中分散的软磁粒子的自组装，得到了具有磁性的物理磁响应凝胶。磁流变（MR）凝胶在外加磁场作用下进行简单剪切循环加载-卸载时，表现出应力硬化行为。应力硬化的特征在于最大应力随循环次数的增加而增加。大部分的应力增加是在第一和第二次循环中实现的，并且在随后的循环中仅发生较小的影响。利用自制的光学显微镜剪切池研究了颗粒体积分数较低的MR凝胶的循环行为。尽管颗粒体积分数的差异，我们能够与循环应力硬化的变化直接在微观尺度上观察到的颗粒-颗粒分布。虽然通过预结构化过程（原始样品对准）产生的各向异性粒子串仍然具有许多缺陷，但在磁场存在下的简单剪切的历史逐渐去除粒子串的缺陷，并帮助它们朝向拉链簇松弛。我们在这里展示了第一个直接观察到的例子剪切退火效应的MR凝胶。颗粒网络的剪切诱导重排是上述MR凝胶循环应力硬化行为的微观起源，并且在去除磁场后似乎是完全可逆的。结果表明，在团簇结构变得更加紧密的同时，也发生了一个缓慢的静止过程，这可能与溶剂向外流动有关。有趣的是，所有直接观察到的重排，包括弦断裂，在去除磁场后似乎都是完全可逆的。
Swollen physical magneto-responsive gels were obtained by self-assembly of triblock copolymers containing dispersed soft magnetic particles. When subject to simple shear cyclic loading-unloading cycles in the presence of a magnetic field, magnetorheological (MR) gels show a stress hardening behavior. The stress-hardening was characterized by the increase of the maximum stress against the number of cycles. Most of the stress increases are achieved in the first and second cycles and only minor effects take place in the subsequent ones. The cyclic behaviour of MR gels with dilute particle volume fraction is also studied with the help of a homemade shear cell mounted on an optical microscope. Despite the difference of particle volume fraction, we are able to relate cyclic stress hardening to the change in particle-particle distribution observed directly at the microscopic scale. While anisotropic particle strings created by a pre-structuring process (original sample alignment) still has many defects, the history of simple shear in the presence of the magnetic field gradually removes the defects of the particle strings and helps them to relax towards a zipped cluster. We show here the first directly observed example of the shear-annealing effect in MR gels. The shear induced rearrangement of the particle network serves as the microscopic origins of the above mentioned cyclic stress hardening behavior of MR gels and seems to be fully reversible after the removal of the magnetic field. Results show that a slow quiescent process also takes place and it is suggested to be related to solvent flow outwards of the clusters, while they get more compact. Interestingly, all directly observed rearrangements including string breaking seem to be fully reversible after removal of the magnetic field.