As colloidal suspensions of ferromagnetic nanoparticles in a carrier liquid, ferrofluids combine the magnetic properties of solids with the flow properties of liquids. One outstanding feature of ferrofluids is the dramatic increase in viscosity that is induced by an externally applied magnetic field. However, in conventional ferrofluids this magnetoviscosity largely vanishes when the fluid is subjected to shear forces, because the latter disrupt the dynamic formation of the chain-like nanoparticle aggregates that are thought to be responsible for the viscosity enhancement. If these loose nanoparticle chains could somehow be replaced with stiff ferromagnetic nanowires or nanotubes, then we should expect the magnetoviscosity to manifest a significantly improved stability against shear thinning, thus making ferrofluids ideal working substances for adaptive damping systems or any other applications relying on switchable force transfer via a liquid.

We report the synthesis of nanotube ferrofluids by the metallization of a nanotube-shaped biotemplate—the Tobacco mosaic virus (TMV). The magnetoviscous properties of liquid-phase suspensions of the resulting nanotubes were studied using a squeeze-flow viscometer at magnetic fields up to 150 mT and frequencies up to 400 Hz. The measured viscosity behavior is compared to that of conventional ferrofluids and to suspensions of Fe nanorods prepared by aerosol condensation.

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