Mohapatra Dillip Kumar, Camp Philip J, Philip John
Smart Materials Section, Corrosion Science and Technology Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, HBNI Kalpakkam-603102 India
School of Chemistry, University of Edinburgh David Brewster Road Edinburgh EH9 3FJ Scotland UK
Nanoscale Adv. 2021 Apr 24;3(12):3573-3592. doi: 10.1039/d1na00131k. eCollection 2021 Jun 15.
We probe the influence of particle size polydispersity on field-induced structures and structural transitions in magnetic fluids (ferrofluids) using phase contrast optical microscopy, light scattering and Brownian dynamics simulations. Three different ferrofluids containing superparamagnetic nanoparticles of different polydispersity indices (PDIs) are used. In a ferrofluid with a high PDI (∼0.79), thin chains, thick chains, and sheets are formed on increasing the in-plane magnetic field, whereas isotropic bubbles, and hexagonal and lamellar/stripe structures are formed on increasing the out-of-plane magnetic field over the same range. In contrast, no field-induced aggregates are seen in the sample with low polydispersity under the above conditions. In a polydisperse sample, bubbles are formed at a very low magnetic field strength of 30 G. Insights into the structural evolution with increasing magnetic field strength are obtained by carrying out Brownian dynamics simulations. The crossovers from isotropic, through hexagonal columnar, to lamellar/stripe structures observed with increasing field strength in the high-polydispersity sample indicate the prominent roles of large, more strongly interacting particles in structural transitions in ferrofluids. Based on the observed microstructures, a phase diagram is constructed. Our work opens up new opportunities to develop optical devices and access diverse structures by tuning size polydispersity.
我们使用相衬光学显微镜、光散射和布朗动力学模拟，探究了颗粒尺寸多分散性对磁流体（铁磁流体）中场诱导结构和结构转变的影响。我们使用了三种不同的含有不同多分散指数（PDIs）超顺磁性纳米颗粒的铁磁流体。在多分散指数较高（约0.79）的铁磁流体中，增加面内磁场时会形成细链、粗链和薄片，而在相同磁场范围内增加面外磁场时会形成各向同性气泡以及六边形和层状/条纹结构。相比之下，在上述条件下，低多分散性的样品中未观察到场诱导聚集体。在多分散样品中，在30 G的非常低的磁场强度下会形成气泡。通过进行布朗动力学模拟，我们获得了随着磁场强度增加结构演变的见解。在高多分散性样品中观察到的随着场强增加从各向同性到六边形柱状再到层状/条纹结构的转变，表明了较大、相互作用更强的颗粒在铁磁流体结构转变中的突出作用。基于观察到的微观结构，构建了相图。我们的工作为开发光学器件以及通过调整尺寸多分散性获得多种结构开辟了新机会。
