title: Internal deformation in Hybrid-materials
Creators: name: Schmidt, Henrik
Publisher: name: UNSPECIFIED
Year: 2021

description: Magnetic-hybrid materials generally consist of non-magnetic carrier materials interspersed with a magnetic filler material. If the carrier material is an elastomer they are called more specifically magnetorheologic elastomer (MREs). Under an applied magnetic field, those materials can respond in two ways: The magnetostriction and the magnetorheological (MR) effect. Magnetostriction is the ability to deform when subjected to a magnetic field. The MR effect is the alternation of mechanical properties like tensile strain under an applied magnetic field. To obtain strong magnetostrictive effects, the particles inside the MRE have to rearrange in position and orientation under the influence of a magnetic field. To be applicable e.g. as an actuator, it is of utter importance to understand the link between particle movement and macroscopic deformation. This work extends the previous studies of Huang and Puljiz et al. [69, 133, 135]. Similar to [133, 135] the sample were prepared as layer system of polymer layers [polydimethylsiloxan (PDMS)] and particles. This facilitated particle positioning. At first, MRE samples comprising two paramagnetic nickel particles with initial inter-particle distance of roughly one particle diameter were prepared in order to keep the amount of unknown parameters small. Subsequently, the sample complexity was increased by introducing more particles into the system. As the magnetic forces not only depends on the distance between the particles but also on their position relative to the external field, the system was exposed to a slowly, stepwise rotating magnetic field of 180 mT. Resolving the particle attachment and detachment and measuring the angles between particle axis and external field for all corresponding magnetic field orientations, I identified a strong magneto-active configuration. This strong magneto-active configuration is characterized by a large inter-particle distance change generated by a small alternation of the external magnetic field orientation. It is sensitive to the initial inter-particle distance, Young’s modulus of the matrix, and magnetic field strength and thus, was only stable in a defined parameter range, i.e. for a defined ratio between elastic and magnetic forces. To determine the influence of additional particles, the particle number in this linear arrangement was successively increased up to fourteen particles. Particles did not form a continuous particle chain but particle groups of different particle numbers. Thereby, the distance to neighbor particles determined whether particles were able to get into contact or not. Also matrix mediated interaction was observed. In addition, in five by five particle lattices, particle groups of more than five particles formed. For magnetic fields orientated along the lateral lattice direction the influence of lattice irregularities was shown. Furthermore, a correlation between the mean particle number per group and the area the rim particles surround, existed. It was an inverse correlation, i.e. the larger the mean particle number per group the smaller the area. Concluding, magnetically hollow spheres and magneto-active membranes comprising a dense packed monolayer of particles, were prepared. The hollow spheres showed large and direction changable deformations. The membrane was used as drop size selector and showed a significantly larger stiffness as the pure PDMS matrix.