The manipulation of small objects in suspension, such as nanowires of 10 µm in length, is in the realm of ultralow Reynolds numbers (defined as Re = Dvr/h, where, D, v, r, and h are the size of particle, relative velocity, density of medium and viscous coefficient, respectively), where the drag force overwhelms the motion of the entities. For 10 µm long nanowires, Re is only 10^-5 (for a human swimmer Re = 10^4). It is even more challenging to controllably rotate nanoscale entities, especially with high rotation speeds.
In this work, we demonstrate a versatile method for executing controllable high-speed rotation in nanowires using a rotating electric field provided by AC voltages applied to a quadruple electrode, at which four phase-shifted AC voltages were applied as shown schematically in Fig. 1(a). The rotating electric field causes controlled rotation of nanowires as shown by the overlapped images in Fig. 1(b).
Fig. 1: (a) Schematics of nanowires suspended in water rotated by quadrupole electrodes, at which four phase-shifted AC voltages are applied. (b) Overlapped images at 1/30 sec interval of free (right) and one end fixed (left) rotating Au nanowires at 2.5 V, 80 kHz.
The rotation of the nanowires can be instantly switched on or off with precisely controlled chirality, rotation speeds [to at least 1800 revolution per minute (rpm), limited by the CCD camera], and total angle of rotation. We further show that this method can rotate various elongated metallic entities, including nonmagnetic and magnetic nanowires as well as multiwall carbon nanotubes (MWCNT). We have also demonstrated a micromotor using a bent Au nanowire in a rotating electric field as shown in Fig. 2. This electrically controlled motor, perhaps the smallest to date, is relevant to small fluidic or mechanical devices, such as micro electromechanical systems (MEMS).
Fig. 2: (a) Schematic of a micromotor with a bent nanowire anchored at the center of the electrodes, (b) driving a dust particle.
PUBLICATIONS:
D. L. Fan, F. Q. Zhu, R. C. Çammarata and C. L. Chien, Controllable High-Speed Rotation of Nanowires, Phys. Rev. Lett., 94, 247208 (2005).
