(Top) Phase contrast optical microscopy images of field-induced microstructures in a polydisperse system at different out-of-plane magnetic field strengths. (Bottom) Scattered light intensity pattern of the structures.

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The field induced anisotropic structure formation in magnetic fluids (popularly known as ferrofluids) is exploited in several applications such as optoelectronic devices, sensors, heat transfer, and biomedicine. We study the role of surface charge screening on critical magnetic fields associated with field induced structural formation in a charged magnetic nanofluid of hydrodynamic diameter ∼200 nm, containing superparamagnetic iron-oxide nanoparticles of diameter ∼10 nm. Three distinct critical magnetic fields are identified from the drastic changes in transmitted forward scattering light intensity. The first critical field occurs at the commencement of small aggregate formation, the second one on completion of linear aggregation process before the commencement of lateral coalescence of individual chains, and the third one occurs when the densely packed columnar solidlike structures are formed through zippering of individual chains. During the structural transitions, the transmitted light spot is transformed into a diffused ring, with distinct speckle characteristics, due to scattering from self-assembled linear aggregates. The speckle pattern was fully reversible, and the aggregation rate was found to increase linearly with increasing surfactant concentration. The experimentally observed critical fields were in good agreement with theoretical predictions at lower surfactant concentrations. These results provide better insights into the field induced structure formation useful in designing magnetic fluidic based optical devices such as tunable filters and optical switches.

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