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Magnetic nanowires rotation in suspension under the influence of magnetic field
Nanomotors —such as drug delivery vehicles or self-assembled structures requires efficient manipulation of nanoparticle motion. Magnetic field manipulation provides a contactless and sensitive solution to the issue. Here, magnetic nanowires (MNWs) of various lengths and radii were fabricated via electrochemical deposition into nanoporous templates. As a working electrode one side of templates was sputtered with a Cu film. Suspensions were created by removing the MNWs from the template firstly by etching away the Cu film, and then dissolving the template in NaOH. The MNWs were then rinsed several times with deionized water, then redispersed using ultrasonic agitation and placed into the liquid to be studied. A rotating external magnetic field can apply a magnetic torque to balance the drag torque, caused by liquid, and rotate the MNWs in sync with the field. Magnetic torque arises when the magnetic field is not parallel to the nanowires. In addition, nanowire size has large effect on their rotation speed - a longer MNW will experience slower acceleration and larger drag, while a MNW with larger diameter rotates faster. At low frequency rotations of the magnetic field, the nanowires rotate synchronously with the applied field. When the frictional torque exceeds the magnetic torque, the MNW no longer synchronizes with the rotating external field. When the driving field frequency is very high the nanowire will keeps oscillating around its position. By exploring MNW diameters from 20- 200nm and lengths from 1-5μm and rotation frequencies from 60-200rpm in liquids from aqueous to viscous (including cryopreservation agents for the new field of nanowarming organs and tissues), a phase diagram of MNW control has been created.