We report the viscosity (η) and thermal conductivity (k) enhancement in a stable magnetic nanofluid containing particles size <10nm as a function of volume fraction (vol %), shear rate, magnetic field, and time. Without field, the η enhancement of nanoparticle is much larger than that of k. In a nanofluid with 0.078 vol %, the k/η ratio is tunable from 0.725 to 2.35 by controlling the applied field. Our results show that magnetic nanofluids with tunable k/η ratio can be used as multifunctional “smart materials” for miniature cooling cum damping applications.

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