由于具有易于产生、易于控制和通用性等特点,磁场和电场一直是应用较多的用来辅助纳米颗粒进行自组装的外场。外场与胶体间的相互作用是非常复杂的，就外场而言，如果某些参数的复杂性增加的话会对组装的结果产生极大的影响。目前静磁场已经大量的被用来诱导纳米颗粒进行一维结构的组装，但是使用交变磁场还未见报道。我们利用大功率高频交变磁场组装了预先合成好的Fe3O4纳米颗粒，结果发现纳米颗粒聚集为纤维状的微米线，并且纤维的形态与表面电荷有密切关系。我们认为这是由于交变磁场兼具磁偶极作用和静电作用两种效应引起的。
A layer of pre-synthesized Fe3O4 aqueous suspension is subjected to an alternating-current magnetic field which is normal to the substrate. Ultra-long mesoscopic fibers of nanoparticles aggregates, coupled with a transition from quasi-1D to amorphous self-assembly, are observed with the field intensity increasing. Colloidal samples with different surface charges are experimentalized and the surface-controlled morphological diversity is found. Based on the experiments, a conceivable mechanism is proposed involving electrostatic interaction and magnetic moments interaction
Figure . a) Scanning electron microscopy (SEM) image of Fe3O4 nanoparticle aggregates after solvent evaporation in the absence of an alternating-current magnetic field. Inset: Magnified image. b) SEM image of pattern formation of electrostatically stabilized Fe3O4 nanoparticlesunder an alternating-current magnetic field. The excited current is 170 A, which is the minimum of our equipment. For subsequent magnetic property measurement, the concentration is higher than 28 mgmL-1, at approximately 0.5 mgmL-1. c) Formation of microfibers from an electrostatically stabilized colloidal suspension under an alternating-current magnetic field. 1) Breakdown of electric double layer leads to aggregate formation; alternating magnetic field destroys the electric double layers. 2) There exist parallel or antiparallel magnetic moments inside clusters; dipolar attraction leads to the formation of meso-fibers.
Jianfei Sun, Yu Zhang, Zhongping Chen, Jie Zhou, Ning Gu.Fibrous Aggregation of Magnetite Nanoparticles Induced by a Time-Varied Magnetic Field , Angew.Chem.Int.Ed. 2007, 46,4767-4770
