高浓度磁流变液的多分散性效应
Effect of polydispersity in concentrated magnetorheological fluids
摘要：
磁流变液（MRF）是一种智能材料，由于其在机械和机电一体化系统中的广泛应用而受到越来越多的关注。作为主要的流变特征，MRF必须在没有磁场的情况下呈现低粘度（0.1 - 1.0 Pa.s），并且在磁化时呈现高屈服应力（50 - 100 kPa），以便优化磁流变效应。这些性能又直接受颗粒的组成、体积分数、尺寸和尺寸分布（多分散性）的影响，后者是改善这些主要性能的重要部分。在这种情况下，本工作的目的是分析，通过实验和模拟，多分散性的最大包装分数的影响，屈服应力下的字段（开状态），并在没有字段（关状态）的集中MRF（φ = 48.5体积%）的塑性粘度。制备了三种羰基铁粉在聚α烯烃油中的共混物。这些共混物具有相同的模式，但不同的多分散性指数，范围从0.46至1.44。单独的模拟表明，随着多分散指数在该范围内增加，无规紧密堆积分数从约68%增加到80%。通态屈服应力从30 +/- 0.5 kPa提高到42 +/- 2 kPa（B ~ 0.57 T），关态塑性粘度从4.8 Pa.s降低到0.5 Pa. s。如文献中所熟知的，加宽尺寸分布增加了填充效率并降低了浓缩分散体的粘度，但除此之外，它被证明是增加浓缩MRF的磁流变效应的可行方式。提出了Brouwers模型作为解释磁场下屈服应力增加的可能假设，该模型考虑了对数正态分布的颗粒悬浮液中的空隙率。
英文摘要：
Magnetorheological fluids (MRF) are smart materials of increasing interest due to their great versatility in mechanical and mechatronic systems. As main rheological features, MRFs must present low viscosity in the absence of a magnetic field (0.1 - 1.0 Pa.s) and high yield stress (50 - 100 kPa) when magnetized, in order to optimize the magnetorheological effect. Such properties, in turn, are directly influenced by the composition, volume fraction, size, and size distribution (polydispersity) of the particles, the latter being an important piece in the improvement of these main properties. In this context, the present work aims to analyze, through experiments and simulations, the influence of polydispersity on the maximum packing fraction, on the yield stress under field (on-state), and on the plastic viscosity in the absence of field (off-state) of concentrated MRF (phi = 48.5 vol.%). Three blends of carbonyl iron powder in polyalphaolefin oil were prepared. These blends have the same mode, but different polydispersity indexes, ranging from 0.46 to 1.44. Separate simulations show that the random close packing fraction increases from about 68% to 80% as the polydispersity index increases over this range. The on-state yield stress, in turn, is raised from 30 +/- 0.5 kPa to 42 +/- 2 kPa (B ~ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8 Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the literature, increases packing efficiency and reduces the viscosity of concentrated dispersions, but beyond that, it proved to be a viable way to increase the magnetorheological effect of concentrated MRF. The Brouwers model, which considers the void fraction in suspensions of particles with lognormal distribution, was proposed as a possible hypothesis to explain the increase in yield stress under magnetic field.
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