Magnetic field tunable structures in dispersions containing superparamagnetic nanoparticles

A ferrofluid is a stable colloidal suspension containing superparamagnetic nanoparticles dispersed in a carrier fluid. Magnetic field induced structural transitions in ferrofluids have been a topic of interest from both the fundamental and practical point of view. The self-assembly of magnetic nanoparticles under an external magnetic field gives rise to interesting optical properties. Particle-size polydispersity is inevitable in any ferrofluid, and it plays an important role in field-induced structure formation. In summary, the influence of particle size polydispersity on field-induced structures and structural transitions in magnetic fluids (ferrofluids) is studied using phase contrast optical microscopy, light scattering and Brownian dynamics simulations. Three different ferrofluids containing superparamagnetic nanoparticles of different polydispersity indices (PDI) are used. In a ferrofluid with high PDI (~0.79), thin chains, thick chains, and sheets are formed on increasing the in-plane magnetic field, whereas isotropic bubbles, and hexagonal and lamellar/stripe structures are formed on increasing an out-of-plane magnetic field over the same range