## 2 Effect of Magnetic Field on Metamorphosis of MCF Rubber
The size of the solidified MCF rubber in Figure 2c corresponded \(d\) = 8 mm (diameter), \(e\) = 13 mm (diameter), \(f\) = 1.5 mm, \(g\) = 13 mm in Figure 3. Before application of the electric field, MCF rubber liquid was attracted to the electrodes attached to Magnet 2 on the outside of the container, as shown in Figure 4. If both a magnet with the magnetic field strength of 130 mT used in previous studies [22] with a single surface magnet of 84.7 mT (Magnet 1) were used, the same solidified MCF rubber would be created. However, if magnetic field strengths of 312 mT and 177.7 mT (Magnet 3), or 490 mT were used, [22] together with a surface magnet of 191.3 mT (Magnet 4), solidified MCF rubber would not form. The magnetic field strength of Magnets 1-4 are listed in Table 1. We found that MCF rubber would not solidify when too large of a magnetic field strength was applied. When magnetic field strength was less than about 50 mT, MCF rubber liquid solidified. However, this was in the case of 21.7 wt % particles. As a result, the solidification of MCF rubber liquid was considered to depend on magnetic field strength. Figure 4: Photograph of MCF rubber liquid attracted to the electrodes before the application of electric field. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location** & **Magnet 1** & **Magnet 2** & **Magnet 3** & **Magnet 4** \\ \hline at b or c in Figure 1 (mT) & 34.5 & 48.3 & 80.1 & 107 \\ at a in Figure 1 (mT) & 22.0 & 28.1 & 43.4 & 55.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Magnetic field strength of Magnets 1-4. Figure 3: Schematic diagram of MCF rubber liquid solidified using the NM. The position for measuring magnetic field distributions shown in Figure 5 is also delineated. In the case of Magnet 2, the magnetic field distribution at position <a> and <b> in Figure 3 was measured; the results are shown in Figure 5. The yellow bands in Figure 5 correspond to the positions at which the solidified MCF rubber increased as in Figure 2c. These results suggest that the MCF rubber grows at the position between the maximum magnetic field strength and the maximum magnetic field gradient. It is considered that this is due to the aggregation of the Ni and Fe\({}_{3}\)O\({}_{4}\) particles by the force of the magnetic field strength and field gradient. The permanent magnet used had a circular maximum magnetic field and field gradient. Therefore, the solidified MCF rubber was cylindrical in shape (Figure 3). On the other hand, the enhancement of the solidified MCF rubber can be perceived at the lower side of the solidified MCF rubber shown in Figure 2c, as well as in the case without application of magnetic field shown in Figure 2b, due to the sedimentation of the particles in the MCF rubber. Therefore, large mass concentration is another factor that enhances the solidification of MCF rubber. Figure 6a shows MCF rubber solidified by the NM; this rubber was attracted on the anode of Magnet 2 in an MCF rubber liquid with 50 wt % larger mass concentrations of particles, consisting of 12-g Ni powder, 3-g MF (M-300) and 12-g NR-late. In contrast, particles in the MCF rubber liquid in Figure 2 were 21.7 wt %. The solidified MCF rubber in Figure 6 was larger than that in Figure 2c, reaching from anode to cathode with \(g=19\) mm. Thus, the higher the mass concentration, the larger the solidified MCF rubber. Figure 5: Magnetic field distribution of Magnet 2 (**a,b**) corresponding to the location labeled <a> and <b> in Figure 3.  
The solidified MCF rubber in Figure 6a,b, which shows the rubber between the electrodes, has the same wire-like shape as that shown in Figure 2c. Where the magnetic field is applied, needle-shaped magnetic clusters are created. Meanwhile, in areas outside of the magnetic field, such as the lower side of the solidified MCF rubber shown in Figure 1b, no needle-shaped magnetic clusters form, and the particles aggregate into a dense state.