Viota J L, Delgado A V, Arias J L, Durán J D G
Department of Applied Physics, School of Sciences, University of Granada, 18071 Granada, Spain.
J Colloid Interface Sci. 2008 Aug;324(1-2):199-204. doi: 10.1016/j.jcis.2008.05.029. Epub 2008 May 24.
In this paper we describe the magnetorheological (MR) behavior of aqueous suspensions consisting of magnetite particles stabilized by poly(acrylic acid) polymers (PAA). A previous work on the colloidal stability of the same systems for different pH values and polymer concentrations demonstrated that the addition of PAA polymers has a very significant effect on the stability. In the present contribution, we study the MR effect of the suspensions stabilized by two different commercial polymers, as a function of pH, magnetic field strength and magnetite volume fraction. All the results are discussed in terms of the interfacial properties of the systems. It is demonstrated that for a given concentration of micrometer particles, the rheological response strongly depends on pH, on the volume fraction of magnetite particles, on the type of polymer added for increasing the stability and on the magnetic field strength. Changing the polymer used provokes clear rheological differences for the same sample conditions (field strength, volume fraction and pH). This is suggested to be due to the hydrophobic/hydrophilic balance of the polymer affecting the magnetic field ability to form magnetic structures by aggregation of the magnetized particles. The results are compared to the predictions of the so-called standard chain model, based on the assumption that the MR effect is the result of the balance between the magnetic interactions (tending to establish some degree of order in the suspension by formation of particle chains in the direction of the field) and hydrodynamic ones (tending to destroy the formed structures by viscous stress on the chains). It is found that the behavior of the yield stress does not agree well with the predictions of the model when the relative proportion of both particle and polymer confers optimum stability to the dispersions. This is likely due to the fact that the presence of the stabilizing polyelectrolyte provokes that the magnetic field is not as effective in structuring the suspension as deduced from the chain model.
在本文中，我们描述了由聚（丙烯酸）聚合物（PAA）稳定的磁铁矿颗粒组成的水悬浮液的磁流变（MR）行为。先前关于相同体系在不同pH值和聚合物浓度下的胶体稳定性的研究表明，添加PAA聚合物对稳定性有非常显著的影响。在本论文中，我们研究了由两种不同商业聚合物稳定的悬浮液的MR效应，该效应是pH值、磁场强度和磁铁矿体积分数的函数。所有结果均根据体系的界面性质进行讨论。结果表明，对于给定浓度的微米级颗粒，流变响应强烈依赖于pH值、磁铁矿颗粒的体积分数、为提高稳定性而添加的聚合物类型以及磁场强度。在相同的样品条件（场强、体积分数和pH值）下，更换使用的聚合物会引发明显的流变差异。这被认为是由于聚合物的疏水/亲水平衡影响了磁场通过磁化颗粒聚集形成磁性结构的能力。将结果与所谓的标准链模型的预测进行了比较，该模型基于以下假设：MR效应是磁相互作用（倾向于通过在磁场方向形成颗粒链在悬浮液中建立一定程度的有序性）和流体动力学相互作用（倾向于通过对链的粘性应力破坏形成的结构）之间平衡的结果。结果发现，当颗粒和聚合物的相对比例赋予分散体最佳稳定性时，屈服应力的行为与模型预测不太相符。这可能是由于稳定聚电解质的存在导致磁场在构建悬浮液方面不如从链模型推导的那样有效。
