Magnetorheological brush – a soft structure with highly tuneable stiffness Xiao Huang, ab Akshi Mohla, c Wei Hong, * bc Ashraf F. Bastawros c and Xi-Qiao Feng a By combining the field-stiffening effect of magnetorheological (MR) elastomers and the Euler buckling mechanism, we developed a brush-like magneto-active structure with highly tuneable stiffness. When the applied mechanical load is within a certain range, the effective stiffness of the structure can be tuned by several orders of magnitude with the applied magnetic field. The performance of the structure and its dependence on various synthesis parameters, such as the curing field and filler concentration, were investigated experimentally. It is found that the increase in the critical load for buckling is more than the contribution from the stiffening of the MR elastomer. To unravel the relationship between the stiffness increase and the applied field, a theoretical model with coupled mechanical deformation and magnetic field is established. The prediction of the model agrees well with experimental results. The theory may also be used to model the behaviour of other similar materials, such as MR gels. The MR brush developed in this research holds promise for potential applications in smart structures or devices that require mechanical stiffness to be tuneable in a relatively large range. As the amplification mechanism is independent of the base material, it could be used in conjunction with emerging MR materials for further enhanced performance. 1. Introduction Magnetorheological (MR) materials, including MR uids, MR elastomers and MR gels, are an important group of smart materials whose rheological properties can be varied rapidly and in most cases reversibly by applying a magnetic eld. 1–5 MR materials are typically comprised of micron-sized magnetic particles suspended in a non-magnetic matrix. For example, MR uids are liquid suspensions of magnetic particles. With their yield stresses and viscosities tuneable by two to three orders of magnitude under an applied magnetic eld, 6 MR uids have found applications in brakes, 7 dampers, 8 isolators, 9 etc. One major shortcoming of most MR uids is the settling of parti- cles. 5,10 With the uid component replaced by a so polymer, MR elastomers are solid analogues of MR uids. Depending on synthesis processes, the magnetic ller particles may disperse randomly or form aligned chains, resulting in isotropic or anisotropic MR elastomers, respectively. 2,11,12 The microstruc- tural parameters, such as the size, alignment, and composition of the ller, have signicant inuences on the MR effect. 13–17 Using solids as substrates, the sedimentation problem is cir- cumvented in MR elastomers at a cost of the tuneable range of the rheological properties. The reported relative change in the effective elastic modulus of MR elastomers is less than 100%, 11 much lower than that of MR uids. One way of increasing the relative change of stiffness is to use a soer material as the matrix. Signicant progress has been made recently by using highly swollen polymeric gels as matrices. 18,19 Just like in MR uids, the introduction of liquid solvent and other very so materials may also limit the stability and durability of the products. This paper reports an MR structure that enhances the performance of solid MR elastomers by combining the MR effect with the buckling instability. The principal working mechanism of the structure is illustrated in Fig. 1. Consider Fig. 1 MR-elastomer column under axial compression. When the load is between the critical value with a magnetic field and that without a magnetic field, the responses of the column differ drastically in these two scenarios. a AML and CNMM, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China b Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA c Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA. E-mail: whong@iastate.edu; Fax: +1-515-294-3262; Tel: +1-515-204-8850 Cite this: Soft Matter, 2014, 10, 1537 Received 9th August 2013 Accepted 10th December 2013 DOI: 10.1039/c3sm52159a www.rsc.org/softmatter This journal is © The Royal Society of Chemistry 2014 Soft Matter, 2014, 10, 1537–1543 | 1537 Soft Matter PAPER