Direct observations of field-induced assemblies in magnetite ferrofluids

Abstract

Evolution of microstructures in magnetite-based ferrofluids with weak dipolar moments (particle size 10 nm) is studied with an emphasis on examining the effects of particle concentration (/) and magnetic field strength (H) on the structures. Nanoparticles are dispersed in water at three different concentrations, / ¼ 0.15%, 0.48%, and 0.59% (w/v) [g/ml%] and exposed to uniform magnetic fields in the range of H ¼ 0.05-0.42 T. Cryogenic transmission electron microscopy is employed to provide in-situ observations of the field-induced assemblies in such systems. As the magnetic field increases, the Brownian colloids are observed to form randomly distributed chains aligned in the field direction, followed by head-to-tail chain aggregation and then lateral aggregation of chains termed as zippering. By increasing the field in low concentration samples, the number of chains increases, though their length does not change dramatically. Increasing concentration increases the length of the linear particle assemblies in the presence of a fixed external magnetic field. Thickening of the chains due to zippering is observed at relatively high fields. Through a systematic variation of concentration and magnetic field strength, this study shows that both magnetic field strength and change in concentration can strongly influence formation of microstructures even in weak dipolar systems. Additionally, the results of two commonly used support films on electron microscopy grids, continuous carbon and holey carbon films, are compared. Holey carbon film allows us to create local regions of high concentrations that further assist the development of field-induced assemblies. The experimental observations provide a validation of the zippering effect and can be utilized in the development of models for thermophysical properties such as thermal conductivity. V C 2015 AIP Publishing LLC.

Magnetic-Field-Assisted Assembly of Anisotropic Superstructures by Iron Oxide Nanoparticles and Their Enhanced Magnetism

“…The magnetic-field-assisted assembly of IONPs is a particle deposition process, which depends on the balance between evaporation-driven assembly and field-driven assembly [ 31 , 32 ]. During the assembly process, convection of the organic solvent due to evaporation drives the IONPs to transport and deposit at the three-phase contact line.…”

“…At the first stage of the assembly process, a gradient magnetic field (typical values magnitude 200 Oe, gradient 60 Oe/cm) was applied in plane, and the dominant forces exerted on IONPs are capillary force induced by solvent evaporation and magnetic force caused by external field and dipolar interactions, while effects of the thermal fluctuations and the local interactions (such as van der Waals attraction and steric repulsion) are negligible [ 15 , 21 ]. Under the magnetic field, each IONP behaves as a magnetic dipole with magnetic moment m = μ 0 MV , where μ 0 is the permeability of free space and M and V are the domain magnetization and volume of the IONP, respectively [ 32 ]. Magnetic dipole-dipole interaction between two magnetically aligned IONPs is given by F dip = 3 m 2 (1 − 3 cos 2 α )/ d 4 , where α is the angle between their magnetic moment and the line connecting their centers and d is the inter-particle distance [ 25 , 34 ].…”

Magneto-viscosity of hydrothermal synthesized Cu-Zn ferrite ferrofluids

“…Apart from chain formation perpendicular to the flow, the lateral aggregation of chains (zippering) occurs in the ferrofluids which refers to the high shear thickening at higher magnetic fields. 9 These plots are fitted with following power law equation: η = Kγ n−1 Here K is the consistency coefficient and the exponent n is power law index. The n-value is in the range of 0.15 to 0.26 and 0.005 to 0.12 at different applied magnetic fields for CZF1 and CZF2.…”

Effective in-field thermal conductivity of ferrofluids

“…10 Moreover, interactions between magnetic nanoparticles when coupled to the applied magnetic field lead to the formation of heterogeneous ordered structures along the field. 11 Due to the formation of field-assisted microstructures, effective thermal conductivity of the medium changes while certain thermodynamic properties such as effective heat capacity remain unaffected. 12 Early studies on thermophysical properties of magnetic fluids did not present magnetic field dependency of ferrofluids' thermal conductivity, 13 focusing primarily on investigations of heat transfer in ferrofluids by thermodiffusion and magnetoconvection or thermomagnetic convection phenomena.…”