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Anisotropic structures are found in many natural materials, such as teeth, mollusk shells, and plants. Researchers are committed to developing new materials that contain aligned particles. These materials can exhibit enhanced magnetic, mechanical, optical, and diffusive properties. The anisotropic properties of ellipsoids allow them to be aligned by an external driving torque. In this study, a rotating magnetic field is used to align magnetic oblate spheroids, creating a planar anisotropy where all particles are aligned into the plane of the field. We developed an analytic solution describing the alignment dynamics that covers the entire possible frequency range of a rotating magnetic field. We also developed asymptotic solutions at both the high-frequency limit and the low-frequency limit of the rotating field, which can be applied in industrial implementations. The analytics are confirmed via finite difference numerics. Detailed single-particle experiments are shown to be in agreement with the model.
Our ongoing work investigates the dynamics of many-particle systems. If the magnetic field persists after the particles align, chaining of particles is observed. Since the induced magnetic particles can generate magnetic fields of their own, particles can either attract or repulse others, depending on their relative positions. The induced dipoles can be assumed to be identical, and the magnetic interacting force can be assumed to be pairwise. The lubrication force that prevents particles from colliding or overlapping can be included in the hydrodynamics for near-field interaction. With the knowledge of the dynamics, we strive to create novel metamaterial composites with anisotropic structures. Our current focus uses a three-axis electromagnetic coil system to align particles to the desired orientation, before the particles are frozen inside a solid matrix (e.g., using a UV-curable polymer solution).