Rheological Properties Of Ferrofluids Research Articles

Approaches on Ferrofluid Synthesis and Applications: Current Status and Future Perspectives.

Ferrofluids are colloidal suspensions of iron oxide nanoparticles (IONPs) within aqueous or nonaqueous liquids that exhibit strong magnetic properties. These magnetic properties allow ferrofluids to be manipulated and controlled when exposed to magnetic fields. This review aims to provide the current scope and research opportunities regarding the methods of synthesis of nanoparticles, surfactants, and carrier liquids for ferrofluid production, along with the rheology and applications of ferrofluids within the fields of medicine, water treatment, and mechanical engineering. A ferrofluid is composed of IONPs, a surfactant that coats the magnetic IONPs to prevent agglomeration, and a carrier liquid that suspends the IONPs. Coprecipitation and thermal decomposition are the main methods used for the synthesis of IONPs. Despite the fact that thermal decomposition provides precise control on the nanoparticle size, coprecipitation is the most used method, even when the oxidation of iron can occur. This oxidation alters the ratio of maghemite/magnetite, influencing the magnetic properties of ferrofluids. Strategies to overcome iron oxidation have been proposed, such as the use of an inert atmosphere, adjusting the Fe(II) and Fe(III) ratio to 1:2, and the exploration of other metals with the oxidation state +2. Surfactants and carrier liquids are chosen according to the ferrofluid application to ensure stability. Hence, a compatible carrier liquid (polar or nonpolar) is selected, and then, a surfactant, mainly a polymer, is embedded in the IONPs, providing a steric barrier. Due to the variety of surfactants and carrier liquids, the rheological properties of ferrofluids are an important response variable evaluated when synthesizing ferrofluids. There are many reported applications of ferrofluids, including biosensing, medical imaging, medicinal therapy, magnetic nanoemulsions, and magnetic impedance. Other applications include water treatment, energy harvesting and transfer, and vibration control. To progress from synthesis to applications, research is still ongoing to ensure control of the ferrofluids’ properties.

Investigation of the Rheological Properties of Zn-Ferrite/Perfluoropolyether Oil-Based Ferrofluids.

The rheological properties of ferrofluids are related to various applications, such as sealing and loudspeakers, and have therefore attracted widespread attention. However, the rheological properties and their influence on the mechanisms of perfluoropolyether oil (PFPE oil)-based ferrofluids are complicated and not clear. Here, a series of PFPE oil-based ferrofluids were synthesized via a chemical co-precipitation method, and their rheological properties were revealed, systematically. The results indicate that the prepared Zn-ferrite particles have an average size of 12.1 nm, within a range of 4–18 nm, and that the ferrofluids have excellent dispersion stability. The activity of the ferrofluids changes from Newtonian to non-Newtonian, then to solid-like with increasing w from 10 wt% to 45.5 wt%, owing to their variation in microstructures. The viscosity of the ferrofluids increases with increasing Mw (the molecular weight of base liquid PFPE oil polymer), attributed to the increase in entanglements between PFPE oil molecules. The magnetization temperature variation of Zn-ferrite nanoparticles and viscosity temperature variation of PFPE oil together contribute to the viscosity temperature change in ferrofluids. The viscosity of the ferrofluids basically remains unchanged when shear rate is above 50 s−1, with increasing magnetic field strength; however, it first increases and then levels off when the rate is under 10 s−1, revealing that the shear rate and magnetic field strength together affect viscosity. The viscosity and its alteration in Zn-ferrite/PFPE oil-based ferrofluids could be deduced through our work, which will be greatly significant in basic theoretical research and in various applications.

The effect of nanoparticle concentration on the rheological properties of paraffin-based Co3O4 ferrofluids

A series of ferrofluids were made using oleic acid-capped Co3O4 (cobalt oxide) nanoparticles in liquid paraffin by a high-energy milling/sonication method. The average physical size of the dry Co3O4 nanoparticles was determined to be 21 nm by measuring the peak broadening of the (311) crystal plane from X-ray diffraction data of the nanopowder. The results of measurements by an alternative gradient force magnetometer indicated that the dry nanoparticles were paramagnetic. The prepared colloids were characterized by dynamic light scattering and found to contain aggregated nanoparticles having between 51 and 835 individual particles per aggregate. The rheological properties of these ferrofluids were then studied using a standard rotating rheometer, both in the presence and absence of an externally applied magnetic field. Magnetoviscous and thixotropic effects on the ferrofluids viscosity were studied as both the nanoparticle concentration and the applied magnetic field was varied. Nanoparticle aggregation and its effect on the rheological properties of these ferrofluids is discussed (i.e. the interactions between nanoparticles and aggregates). At nanoparticle concentrations above 30 wt%, a combined thixotropic/magnetoviscous effect on the fluids viscosity was observed. The ferrofluids exhibited a thixotropic behaviour with little magnetoviscous effect at more dilute nanoparticle concentrations, 25 wt%.

Preparation and characterization of Fe 3O 4/polystyrene composite particles via inverse emulsion polymerization

Superparamagnetic magnetite/polystyrene (PS) composite particles were prepared by inverse emulsion polymerization with water-based ferrofluid as dispersing phase and organic solvent and styrene (St) as continuous phase. The resultant brownish Fe 3O 4/PS emulsion showed magnetic behavior in an applied magnetic field. The obtained magnetic Fe 3O 4/PS microspheres were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetry (TG). The magnetic Fe 3O 4/PS microspheres with a diameter of 200 nm were observed. The results showed that magnetite particles were well encapsulated in PS and the composite particles have high magnetite contents, which were more than 15%. In addition, the influences of magnetic field intensity on the rheological properties of ferrofluids were investigated using a rotating rheometer.

New Driving Unit for the Direct Measurement of Yield Stress with a Stress Controlled Rheometer

Abstract Investigations of rheological properties of ferrofluids have shown strong changes of the viscosity in magnetic fluids with an applied magnetic field. The change of the viscosity – the magnetoviscous effect – can theoretically be described with chain and structure formation under the influence of a magnetic field. Moreover, the formation of these structures leads to the appearance of viscoelastic effects or other non-Newtonian features like yield stress in ferrofluids with an applied magnetic field. With a shear rate controlled rheometer – as it as been used in former experiments – the yield stress could not be investigated directly. Therefore the results concerning a field dependent yield stress based on an extrapolation of shear controlled measurements. For the direct investigations of the yield stress, a dedicated stress controlled rheometer is required, allowing direct investigations of the magnitude and field dependence of this effect. In this work the design of the stress controlled rheometer with its main parameters has been described in detail. The rheological investigations with differently composed fluids show that the stress controlled rheometer enables direct measurements of even small yield stresses in ferrofluids as well as large effects like they are found in magnetorheological fluids (MRF).

Rheological properties of ferrofluids with microstructures

This paper presents results of a theoretical study of the effects of linear chain-like as well asbulk drop-like heterogeneous aggregates on the rheological properties and behaviour offerrofluids. The results demonstrate that the appearance of both these internal structuresleads to a strong (one–two orders of magnitude) increase of the ferrofluid effectiveviscosity under the action of the magnetic field applied parallel to the gradient ofthe ferrofluid flow. When the ferrofluid fills a thin channel (gap) placed into anormal magnetic field, the drop-like structures can overlap with the channel. In thecase of a rigid connection between the drop-like domains and the channel walls,the appearance of elastic and yield stress effects on the ferrofluid is expected.