Coarsening dynamics of ferromagnetic granular networks - experiment and simulation

• Applicant: Privatdozent Dr. Reinhard Richter
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Theoretical Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 453252565

WHAT: Similar to magnetic dipoles (De Gennes & Pincus, 1970), magnetised steel spheres self-assemble due to anisotropic magnetic interactions. If one shakes a mixture of glass and steel beads with a supercritical vibration amplitude, inter-particle collisions hinder the aggregation and keep the mixture in a “gas phase” (Blair & Kudrolli, 2003). Below a critical vibration amplitude, magnetic forces lead, first, to the formation of individual chains and rings that merge into a network as the system relaxes. If given enough time, the network eventually compacts into crystalline-like islands of magnetic beads. This scenario closely resembles the viscoelastic phase separation (VPS) introduced by Tanaka (2000) for molecular mixtures. There phase separation arises from the different shear viscosities of both components, labelled as dynamic asymmetry by Tanaka. Also in our mixture magnetic forces rise the shear viscosity of the ferrogranulate in contrast to the glass phase. In our project, we raise the fundamental question: “Can the coarsening dynamics of ferrogranulate be described as VPS in the macroscale?” With this aim, we study the time evolution of order parameters: network specific efficiency, node degrees, number of loops and phase separation characteristic wave numbers, diffusion coefficients, and cluster sizes. By applying magnetic fields (parallel and perpendicular to the vibration plane) or by changing the composition of the ferrogranulate, we want to control/induce/eliminate the VPS in these systems. HOW: We perform experiments and simulations. For the experiment a flat vessel is mechanically vibrated and observed by a camera. Data are obtained via image processing, a tailor made magnetometer, and magneto-optical detection. In molecular dynamics simulations the glass beads are modelled as repulsive spheres, whereas a new model of composite core-shell magnetically susceptible particles is developed to mimic steel beads. Both methods are complementary: in the experiments, the complex magnetic nature of the steel beads fully manifests itself, however, it is difficult to avoid finite size effects and change the properties of the beads; in the simulations one can easily sample the full range of system parameters, but the simplifications in the inter-particle interactions are unavoidable. WHY: Our project pioneers in connecting the four different research areas: granular matter, magnetic nanoparticles/magnetic fluids, phase transitions and complex networks. In this way, this project shall not only explain the VPS in a ferromagnetic granulate, but shall also reveal the underlying physical mechanism inherent to any system in which the magnetic forces, be that the ones acting between nanoparticles or large macroscopic objects, lead to the emergence of a dynamic asymmetry and to aggregation in the system. Examples span from sedimentation in magnetorheological suspensions to agglomeration of magnetic stardust in the early stage of planet formation

• DFG Programme: Research Grants
• International Connection: Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Cooperation Partner: Professorin Dr. Sofia Kantorovich