Application of monodisperse Fe₃O₄ submicrospheres in magnetorheological fluids

Highlights

• Soft ferrimagnetic Fe₃O₄ spheres were synthesized using solvothermal method.

• Flow behaviour of magnetorheological fluids depend on particle concentration (ϕ) and applied magnetic field (B).

• In low B regime, the particles interact and align along B exhibiting a sharp increase in yield strength (τ_Y).

• In intermediate B regime, the attraction between the particles increases showing a moderate increase in τ_Y.

• In high B regime, the particles attract strongly and rearrange to align along the field direction resulting in high τ_Y.

Abstract

Steady shear response of a magnetorheological fluid (MRF) system containing porous mono-disperse magnetite (Fe₃O₄) spheres synthesized by solvothermal method is demonstrated. In applied magnetic field the interaction between the spherical particles leads to form strong columnar structures enhancing the yield strength and viscosity of the MRFs. The yield strengths of the MRFs also scale up with the concentration of magnetic particles in the fluid. Considering magnetic dipolar interaction between the particles the magneto-mechanical response of the MRFs is explained. Unlike metallic iron particles, the low-density corrosion resistant soft-ferrimagnetic Fe₃O₄ spherical particles make our studied MRF system efficient and reliable for shock-mitigation/vibration-isolation applications.

Introduction

Magnetorheological fluids (MRFs) belong to a class of smart materials, which consist of meso-scale magnetic particles suspended in a non-magnetic, low viscous carrier medium. For an MRF, the viscosity rises from nearly free-flowing state in the absence of externally applied magnetic field to dramatically high values on the application of magnetic field. This is due to the magnetic coupling among the magnetic particles along the direction of applied magnetic field, imparting solid nature to the MRF. This resists the deformation of the MRF by any external source (mechanical force) [1]. Ideally, this change of physical state is very quick (milliseconds) response, reversible and repeatable over cycles of application/removal of magnetic fields.

MRFs find potential application in diverse fields such as automobile shock absorbers [2], [3], earthquake dampers [4], vibration isolators [5], rotor dampers in prosthetic knee [6], aircraft landing gears [7], [8], [9], actuators [10], optical polishing [11] and high torque transmission clutches [12], [13], [14] etc. besides other applications such as smart-membranes and biosuspensions [15], [16]. The shock-absorption, impact-mitigation or vibration-isolation in MRF-based devices is achieved via actuating an electromagnet synchronously with the onset of any external mechanical forces [17]. To make such mechatronic devices highly efficient, development of MRFs with high yield stress (strength) are necessary. High performance MRFs (with high yield strength) can be prepared by choosing magnetic particles with high magnetization [18]. Hence, conventional MRFs use micron-sized (1–10 μm) carbonyl iron (CI) particles, which provide high yield strength (τ_Y ∼ 10–100 kPa) [19], [20], [21]. However, Fe-based MRFs suffer from many disadvantages such as ready sedimentation and irreversible flocculation of Fe-particles in the carrier medium, poor oxidative and chemical stabilities. The attempt to overcome these limitations by coating the CI by ceramic oxides or by polymers inevitably deteriorate the magnetic properties and/or lubricity [22], [23], [24], [25], [26]. There are a few reports on CNT coated CI particles-based-MRFs which retain the yield strength as that of uncoated Fe particles-based MRFs coupled with high sedimentation stabilities [27], [28]. Another alternative to CNT coating is amorphous carbon covered Fe particles [29]. However, commercialization of such an MRF can be limited due to their high cost. All these factors suggest the need of alternate magnetic particles. Interestingly, ferrimagnetic oxides-based-MRFs are one of such alternatives, which have benefits of high thermo-oxidative, chemical and sedimentation stabilities [30], [31], [32], [33], [34], [35], [36], [37], [38], [39] as compared to metallic magnetic particles-based MRFs.

Magnetic oxide particles have low density (∼5.1 g/cc), high resistance to oxidation and corrosion, which allows much wider option for the choice of carrier media. The choice of carrier fluid is important as the speed of response, the chemical, physical and thermal stabilities and the lubricity of the MRFs are determined by the carrier fluid. Also, the MRFs based on uniform spheres are interesting for the fundamental understanding of the flow behaviour under magnetically activated conditions [40]. Considering these factors, we have prepared MRF by dispersing porous near-monodisperse sub-micron sized spherical Fe₃O₄ particles in low viscosity (∼140 cSt) silicone oil. These particles were synthesized by solvothermal method. The particles have high saturation magnetization and are soft magnetic in nature. As the shape, size and size distribution of the Fe₃O₄ particles can be controlled, the solvothermal method is the ultimate choice [41]. The choice of silicone oil was due to its versatility in obtaining the needed viscosity and operating temperature.