# Study of Magnetorheological Grease's Thermomagnetic Coupling Rheology
## 3 Rheological Experiment of MnFe\({}_{2}\)O\({}_{4}\) Magnetorheological Gresae  
The rheological test of magnetorheological grease was conducted using an Anton Paar MCR301 rotational rheometer, manufactured by Anton Paar GmbH, Ostfildern, Germany. The experimental setup, as shown in Figure 1, employed a flat plate testing configuration. The temperature control module featured a water bath heating system, capable of maintaining temperatures within the range of 5 \({}^{\circ}\)C to 85 \({}^{\circ}\)C. The magnetic field control module consisted of an energized coil, capable of delivering a maximum output current of 5 A, corresponding to a magnetic field intensity of 1.1 T. In the figure, the upper platform represents the lower plate of a PP20/MRD test head with a test diameter of 10 mm. During the experimental procedure, an appropriate quantity of magnetorheological grease was placed between the lower plate of the PP20/MRD test head and the lower platform. The desired temperature and magnetic field intensity were achieved by adjusting the water bath temperature and current magnitude, respectively. Real-time data captured by the PP20/MRD measuring system during the test were fed back to the main unit, allowing for the acquisition of experimental results. \begin{table}
\begin{tabular}{c c} \hline
**Item** & **Parameter** \\ \hline Base oil & Mineral oil \\ Type of thickening agent & Lithium 12-hydroxystearate \\ Thickener content/\% & 4.8 (mass fraction) \\ Viscosity (20 \({}^{\circ}\)C, 10 \(\mathrm{s}^{-1}\))/mm\({}^{2}\cdot\mathrm{s}^{-1}\) & 13 \\ Dropping point/\({}^{\circ}\)C & 173 \\ Consistency class & 00 \\ Working cone penetration/dmm & 417 \\ \hline \end{tabular}
\end{table} 
Table 1: Main composition and technical parameters of grease. During the experiments, we considered uncertainties such as variations in grease volume, ambient temperature differences, and vibrations. To address these, we carefully controlled the operating process, recorded ambient temperature, and monitored the experimental platform for disturbances. We made efforts to minimize their impact on the data. To investigate the rheological behavior of magnetorheological grease under the influence of thermomagnetic coupling, a rotational rheometer was utilized to conduct magnetic field scanning experiments at different temperatures. Additionally, rate scanning experiments were performed on the magnetorheological grease under various temperature and magnetic field intensity conditions. The magnetic field saturation intensity of the magnetorheological grease was found to be 440 mT. Consequently, five magnetic field intensities (0 mT, 55 mT, 110 mT, 220 mT, and 440 mT) were selected for the rheological tests, with the corresponding current values for the magnetic field control module presented in Table 2. The temperature control module of the rheometer employed a water bath heating system, providing a temperature range of 5 \({}^{\circ}\)C to 85 \({}^{\circ}\)C. Hence, four temperature conditions (20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C) were chosen for the experiments. The experimental procedure for the magnetic field scanning of the magnetorheological grease was in accordance with the schematic diagram shown in Figure 2. At a shear rate of 10 s\({}^{-1}\), the experiments were conducted at four different temperature conditions: 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C. The current was linearly increased from 0 to 5 A, resulting in a linear increase in magnetic field intensity from 0 to 1.1 T. To ensure the accuracy of the experimental data, a pre-shearing process was performed for 72 s at 0 current, followed by a 480 s ramp-up period of the current. Data points were collected every 12 s to examine the changes in shear stress and apparent viscosity with variations in magnetic field intensity at each temperature.