Magnetorheological (mr) Materials Structure

Magnetorheological (MR) materials are smart-composite materials that are usually Comprised of highly magnetizable micron sized particles (up to 50% volume) dispersed into a non-magnetizable fluid medium. This arrangement means the fluid exhibits a reversible and practically instant transition from a low viscosity liquid to a virtually solid state with exact controllability once placed under an external magnetic field. Generally speaking, MR fluids particulates must have large saturation magnetization and small remnant magnetization as well as be active over a wide temperature range, and be stable against settling, irreversible flocculation and chemical degradation. Based upon these criteria for the magnetic constituent of an MR fluid Carbonyl Iron particles are commonly used for MR fluids because of their large saturation magnetization (M =2.216[4]) (cobalt and Nickle are also commonly used). In conjunction with the magnetisable particulates the other 3 main constituents MR fluids are comprised of are: the carrying fluid (mineral or silicone oil), dispersants (to minimize particle coagulation) and gel forming additives.

Due to the nature of the magnetorheological effect an incredible level precision in the controllability of their viscosity as the strength of the magnetic field can be manipulated down to the most minute level. this controllability in conjunction with their low energy demand and wide temperature range makes them incredibly desirable for numerous engineering applications particularly those that require active control of vibrations and transfer of torque typical examples include: shock absorbers, breaks, clutches and control valves; despite their attractiveness in many applications there remains a challenge to their commercialization namely getting the greatest possible yield stress for minimum input energy. Due to current MR fluids innately, suboptimal yield stress and the value of stronger MR fluids any improvement is greatly sought after.

A well-documented attribute of MR fluids is that as the spherical particulates get larger so too does the yield stress of the MR fluit; However, the huge drawback of increasing the size of the dispersant is the increased instability of the fluid as the density differential between the dispersant and its suspension causes exponentially increasing sedimentation rates thus rendering the use of larger particulates impractical. One potential solution to this problem is the use of microwire structures. Usually around 200-300 nm diameter and ranging from 3-13micrometres in length, they show the same increase in yield stress but greatly decreased sedimentation rates compared to their spherical analogue or potentially the use of a mixture of sizes in a bidisperse or poly dispersant MR fluid.

Microwire structure

Bell et al 2008 conducted research into this topic. Their method was to use two distinct length distributions of pure iron microwires being 5.4 ± 5.2 μm and 7.6 ± 5.1 μm each having a diameter 260± 30 nm. Using Spherical iron particles with 1–3 μm diameter in a silicone oil suspension were used to replicate conventional MR fluids as a control. Their method of experiment was to use an Anton-Paar Physica MCR300 parallel plate rheometer equipped with an MRD180 for the rheological measurements with A gap of 1 mm was maintained between the plates. A Hall probe (FW Bell FH301) was placed within the gap to calibrate the input current of the electromagnet of the rheometer. A magnetic flux density of up to 0.7 T was used for the fluids. The temperature of all samples was maintained at 25 ◦C. A 0.3 ml fluid sample was placed between the rheometer plates. Rotational tests were carried out to determine the flow curves (shear stress versus shear rate) simulating a MR fluid in use in the shear mode (see figure 1 for examples of the modes). A viscous carrier fluid (silicone oil, 0.45 Pa s) was used to avoid expulsion of the fluid from between the plates at high shear rates as well as to avoid sedimentation of the particles before testing.

Results found from their investigation

As can be seen in figure 2, at a saturated magnetic flux density, the yield stress of the 5.4 μm microwires was found to be 0.65, 2.23, and 4.76 kPa for the 2, 4, and 6 vol% suspensions, respectively. For the 7.6 μm wires, the yield stress increases to 8.2 kPa for the 6 vol% suspension. Comparing, these values to those obtained for the spherical suspension control the obtained value for which was 4.