Abstract
The transient response of magnetorheological (MR) materials, in general, is very important for design consideration in MR-based devices. Better response to magnetic fields is beneficial for a better response rate to the electrical current applied in the electromagnetic coil. As a result, MR-based devices would have a high response to external stimuli. In this work, the principal characteristics of magnetorheological greases (MRGs) which have two different particle shapes are experimentally investigated. One type of particle distributed in the grease medium is conventional spherical-shaped carbonyl iron (CI) particles, while the other is plate-like CI particles made using a high-energy rotary ball mill from spherical CI particles. A set of bidisperse MRG samples are firstly prepared by adjusting the weight percentage of the plate-like CI particles and mixing with the spherical CI particles. Subsequently, three important properties of MRGs in terms of their practical application are measured and compared between the two different particle shapes. The field-dependent apparent viscoelastic properties of the prepared MRG samples are measured, followed by the field-dependent storage and loss moduli using an oscillatory shear rheometer. In addition, the transient response time, which indicates the speed in the actuating period of MRGs, is measured by changing the strain amplitude. Then, a comparative assessment on the three properties are undertaken between two different particle shapes by presenting the corresponding results in the same plot. It is shown that the bidisperse MRG with plate-like CI particles exhibits an increase in the initial apparent viscosity as well as stiffness property compared to the MRG with spherical particles only.
Highlights
In recent years, many works have been undertaken on magnetorheological (MR) fluids
It is identified that the apparent viscosity of magnetorheological greases (MRGs), which is mostly dependent on the carbonyl iron (CI) particles, can be altered by changing the size, shape, and fractions of the particles suspended in the medium
It is remarked here that unlike MR fluid, MRG is a non-Newtonian suspension that has yield stress in the absence of a magnetic field owing to the thixotropy properties of the grease which can retard the free movement of the CI particles
Summary
Many works have been undertaken on magnetorheological (MR) fluids. MR fluids have attracted the interests of many researchers owing to their prompt ability to form chain structures reversibly from liquid-like to solid-like within milliseconds by applying an external magnetic field [1,2,3,4,5].The study on the rheological properties, such as shear stress, apparent viscosity, storage modulus, and MR effect, have been extensively investigated [6,7,8,9,10,11,12]. Several works on MRG have validated that the settling problem of the iron particles can be eliminated by utilizing grease which has thixotropic properties as a medium [24,25,26]. These properties restrict any reversible flow in the application devices without the presence of external force [27,28]. Most research works on MRGs so far have focussed on the field-dependent rheological properties at various magnetic field intensities [24].
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