On the controllability of nanorod alignment in magnetic fluids

The controllability of orienting colloidal nanorods suspended in ferrofluid is investigated as a function of the external magnetic field intensity and the concentration of conventional ferrofluid. The expected value for the nanorod orientation was derived from the competition between the potential energy variation of an ellipsoid in a uniform magnetic field and randomizing rotational thermal energy. The agreement between experiment and theory is found to be quite accurate when the measured nanorod volume and vendor-supplied values for the ferrofluid’s magnetic susceptibility are used as inputs in the model. Our investigations confirm that local magnetic susceptibility surrounding the nanorod is equivalent to the bulk fluid susceptibility, indicating that ferrofluid composed of 12nm iron oxide particles can be treated as a continuum on the 100–1000nm length scale.