The realm of magnetorheological elastomers: experiments, theory and instabilities

In this talk, I will present a broad overview of our work on magnetorheological elastomers (MREs) including experiments, theory and numerical implementation. MREs are composite materials comprising metallic soft (e.g. iron) or hard (e.g. NdFeB) magnetic micron-sized particles. The latter have been called h-MREs. Out of these materials one can devise a number of interesting meso- and macrostructures, slender or not that can lead to a number of functionalities such as surface patterning, negative or positive swelling, network activation, negative Poisson ratio, evolving anisotropic magnetic properties, hierarchical buckling and others. The main idea lies in the ability to drive the systems by using two external loads, i.e. mechanical and magnetic ones and in particular to harness the resulting instabilities that are natural in magnetic structures. This allows, in turn, to reach a critically stable state by applying one of the fields (e.g. mechanical) and trigger patterning and instabilities at low energetic cost with the other (e.g. magnetic). As an example, we will discuss in more detail a recently obtained pattern, called crinkling. This pattern appears in slender magnetic structures such as MRE films bonded to soft (non-)magnetic substrates that are subjected to a combination of mechanical pre-compression and transverse to the film magnetic loads. In particular, the initial wrinkles evolve into crinkles, a bifurcation mode that is defined by the accompanied curvature localization and significant shearing of the side faces of the wrinkled geometry. Interestingly, the presence of the magnetic field prohibits the formation of creases and folds. The presentation will close with recent results on h-MREs and their potential ability to lead to unconventional responses mechanically and magnetically.