Assist. Prof. Dr. Mojca Vilfan (IJS, FMF): Dynamics in ferromagnetic ferrofluids

About a decade ago, it was shown that stable suspensions of magnetic nanoplatelets in anisotropic fluids, such as nematic liquid crystals, exhibit ferromagnetic behaviour. Similar ferromagnetic properties can also be achieved in suspensions of magnetic nanoplatelets in alcohol, provided the nanoparticle concentration is sufficiently high. We investigated the dynamics of such ferromagnetic ferrofluids in the presence of an alternating external magnetic field. When observed with polarisation microscopy, regular and periodic stripes are visible and identified as elongated magnetic domains. Even more intriguingly, flow along the stripes is detected and visualised as the transport of microparticles. The flow was studied by performing space-time correlation spectroscopy of thermally excited fluctuations. The measured flow velocity reached the values of several μm/s. More complex behaviour is observed when the ferromagnetic fluids are exposed to a rotating magnetic field, where a wide variety of different dynamic patterns, including lines, dots, vortices, and static winding lines, is observed.
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