*3.3. Sweep Strain*

Figure 6 displays the behavior of all MRE samples at di fferent magnetic fields with the increase in the strain sweep. The MRE with A2 exhibited the highest storage modulus at all the magnetic fields applied in the o ff- and on-state conditions. Meanwhile, the control MRE exhibits a low storage modulus, where the decrease was up to 50% in the o ff-state condition and in the range of 47–50% in the on-state condition. On the other hand, the MRE with A1 demonstrated the lowest storage modulus, with 0.42 MPa in the o ff-state condition and a maximum storage modulus of 0.60 MPa in the on-state condition. However, the MREs with A1 and A2 follow the filler rule, in which the filler could contribute to a higher modulus of elasticity and strength than the matrix itself. Thus, in this study, the properties of the storage modulus were found to be changed by changing the weight percent of filler. A higher concentration of Mg in the nanosized Ni-Mg cobalt-ferrites increased the rheological performance of the storage modulus. However, this trend is contradicted for the MRE with A1 as the storage modulus was low as compared to the control MRE. The decrease in the storage modulus in the MRE with A1 might be due to the Ni concentration used in this MRE. As a matter of fact, the magnetic saturation of di fferent ferromagnetic materials, namely Fe, Co and Ni has decreased, which has been reported by previous researchers [38]. It is believed that although the Ni-Mg cobalt-ferrite is able to act as reinforcing filler owing to its large surface area and high aspect ratio, the size of the nanoparticles in the MRE might have an e ffect on the crosslink density, thus decreasing the storage modulus. Moreover, the decreasing trend of the storage modulus of MRE + A1 might also be due to the broken filler network, which lowers the storage modulus as compared to the control sample. This finding is similar to that observed by previous researchers [39,40]. Meanwhile, for the MRE + A2, it is believed that with the addition of these nanosized Ni-Mg cobalt-ferrites in the MRE, especially in the presence of magnetic field, the hetero-aggregation process occurred between nanosized Ni-Mg cobalt-ferrites and CIPs, thus leading to an enhancement of inter-particle interactions in the MRE. In addition, the void between the CIPs may be filled by the higher surface area and smaller size o ffered by the nanosized Ni-Mg cobalt-ferrites, therefore, enhancing the interaction between CIPs and nanosized Ni-Mg cobalt-ferrites. A possible mechanism of the above-mentioned phenomenon is shown in Figure 7.

**Figure 6.** *Cont.*

**Figure 6.** Storage modulus versus strain of all MRE samples (**a**) MRE, (**b**) MRE + A1, and (**c**) MRE + A2.

The mechanism of particle movement is illustrated in Figure 7. For the MRE + A1 and MRE + A2 samples, in the absence of a magnetic field, the CIPs and the nanosized Ni-Mg cobalt-ferrites tend to have a homogenous dispersion. However, when the magnetic field was applied, there are two possible situations that may have occurred. In the first—due to high magnetic saturation of the MREs—the CIPs tend to align according to the magnetic field direction, which is followed by the movement of the nanosized Ni-Mg cobalt-ferrites that will fill the void of the CIPs. In the second possible mechanism—due to the low magnetic moment of the nanosized Ni-Mg cobalt-ferrites and low Mg concentration—they will vibrate and experience slow microscopic movement which shown in Figure 7a. In this situation, the magnetic response as a result of the magnetic field will decrease and

produces an obstacle in the matrix, thus reducing the storage modulus capability. Meanwhile, in the MRE + A2 sample, in the presence of a magnetic field, the nanosized Ni-Mg cobalt-ferrites will fill the void between CIPs and strengthen the interaction within the MRE. Moreover, due to high concentration of Mg, this A2 experienced faster microscopic movement which resulted in small void occurred. In this situation, due to the high Mg concentration in the nanosized Ni-Mg cobalt-ferrites, the movement of the nanosized Ni-Mg cobalt-ferrites is increased and the reaction towards the magnetic field is also increased as depicted in Figure 7b. The higher Mg content is assumed to form better bonding due to the increment of compact structure in MRE thus resulted in higher storage modulus. This kind of similar finding has been reported previously by Agarwal et al. [41] in which higher concentration resulted in better mechanical performance.

**Figure 7.** Mechanism of particle movement in the MRE samples (**a**) MRE + A1 and (**b**) MRE + A2.

On the other hand, the loss factor of MRE samples is shown in Figure 8. The samples were subjected to the di fferent magnetic fields, ranging from 0 to 5 A. As shown in Figure 8a–c, it has been identified that the loss factor of all MRE samples is high at a strain of over 1%. In the o ff-state condition, MRE samples exhibit the lowest loss factor with the increase in the strain as compared to the on-state condition. On the contrary, in the on-state condition, MRE samples depicted a higher loss factor parallel to the increase of the magnetic field. However, the maximum loss factor decreased with the existence of nanosized Ni-Mg cobalt-ferrites in the MRE. This can be attributed to the existence of a magnetic field in which the CIPs and nanosized Ni-Mg cobalt-ferrites tended to vibrate and form a chain-like structure inside the matrix. The higher degree of macroscopic movement of the CIPs resulted in greater particle interaction and internal friction during the vibration. Thus, the energy generated from the vibrating surface of the particles was converted into heat and led to a decrease in the loss factor. Moreover, with the implementation of the nanosized Ni-Mg cobalt-ferrites in the MRE, it is believed that at a higher strain amplitude, the CIPs and nanoparticles are no longer stable due to the breakdown of the filler network. In other words, nanosized Ni-Mg cobalt-ferrites contributed to the increase in the loss factor with the increase in the strain. Therefore, it is important to determine the strain range when applying the MRE samples to the sensors, such as the strain sensor. Moreover, the response time of the sensor needs to be carefully considered if the signal from the MRE is used as a feedback signal in the control system.

(**b**) 

**Figure 8.** *Cont.*

 **Figure 8.** Loss factor versus strain at various magnetic fields for (**a**)MRE, (**b**)MRE+A1, and (**c**) MRE + A2.
