(477f) Assembly of Magnetic Microspheres Under Combined Electric and Magnetic Fields

With control over both space and time, colloidal structures can remarkably enrich their prospect of being used as either model systems for scientific study or functional materials for practical application. Previous work mainly focused on the assembly of colloidal structures under specific types of external fields. Here, we take advantage of both electric and magnetic fields where almost all types of particles can be responsive to the electric field due to a mismatch in dielectric and conductive properties between particles and solvent, while the magnetic field can be conveniently manipulated in three dimensions. We can tune the dipolar attraction and repulsion between particles independently when we apply these two fields orthogonally on paramagnetic microspheres. As a result, we obtain high density of well-aligned but individually separated long chains at low and intermediate particle concentrations. In comparison, one only gets either non-close packed hexagonal arrays or bundles of chains under individual fields. The inter- and intra-chain spacings can be tuned by adjusting the particle concentration, relative field strengths of both fields, and the frequency of the electric field. At high particle concentrations, the microspheres form small clusters under electric fields due to anisotropic dipolar interactions. Subsequently, those small colloidal clusters, as unique building blocks, can be further assembled into hierarchical structures under magnetic fields. To better understand how particles respond to the orthogonally applied electric and magnetic fields, we also perform Monte Carlo simulations and model the dipolar interactions using Stockmayer-like potentials. Both phase diagrams and simulated colloidal structures match well with experimental observations. Our results demonstrate the potential in using combined fields to make diversified types of highly aligned structures for applications in high strength composites or optical materials.