Magnetic field gradient and sample size effect on magneto-mechanical response for magnetic elastomers

At present, there exists several magnetically responsive composites whose physical properties can be controlled by a magnetic stimulus. Magnetic soft materials, mainly consisting of soft polymeric matrices and magnetic particles, tend to exhibit considerable change in elastic modulus when a magnetic field is applied. The field can only be applied either by using permanent magnets or by use of an electromagnet. The former tends to produce a weak magneto-mechanical response due to the inhomogeneity of its magnetic field. Consequently, the size correlation of magnetic elastomer and permanent magnet is quite important for the magneto-mechanical response. Previous studies have incisively emphasized on this. More so, magnetic elastomers with significant magneto-mechanical response tend to exhibit anomalous deformation under the magnetic field. Such works have directed that the magneto-mechanical response clearly levels off when the diameter of magnetic elastomers exceeds that of permanent magnets. This in turn insinuates that the distribution of magnetic fields at the fringe dominates the magneto-mechanical response.

Researchers led by professor Tetsu Mitsumata at Niigata University in Japan investigated the size effect of disk-shaped magnetic elastomers, placed on a permanent magnet, on their magneto-mechanical response for the permanent magnet. They hoped to further study the size-dependent magneto-mechanical response of magnetic elastomers. Their work is published in the research journal, Reactive and Functional Polymers.

The research team initiated their experimental procedure by synthesizing the magnetic elastomers. Unidirectional compression measurements were then undertaken using permanent magnets having different diameters, magnetic elastomers with various volume fractions of magnetic particle and permanent magnets with various geometrical shapes. Eventually, they carried out magnetic field analysis by measuring the magnetic field strength in both horizontal and vertical direction.

The authors observed that the size-dependent magneto-mechanical response was strongly associated with the characteristic distribution of magnetic fields generated by permanent magnets. Moreover, they noted that the magnetic field measurements and numerical simulation revealed that the magnetic field gradient appears at the fringe of permanent magnets not only in the horizontal direction but also in the vertical one. After undertaking image analyses, the research team observed that the bottom of the magnetic elastomers was restricted to deform by the compression. The restricted deformation was seen to be due to the steep gradient of magnetic fields at the fringe of permanent magnet, probably the field gradient in the vertical direction.

Professor Tetsu Mitsumata and colleagues successfully presented a report on the magneto-mechanical response of magnetic elastomers with various diameters by using permanent magnets of different diameters. Based on the unidirectional compression measurements, it has been seen that a considerable magneto-mechanical response is achieved when the diameter of the magnetic elastomer is equal to that of the permanent magnet. Therefore, we firmly believe that the present results provide a smart design for fabricating devices with variable elasticity consisting of magnetic elastomers and permanent magnets.