Kilohertz Rotation of Nanorods Propelled by Ultrasound, Traced by Microvortex Advection of Nanoparticles

We measure the microvortical flows around gold nanorods propelled by ultrasound in water using polystyrene nanoparticles as optical tracers. We infer the rotational frequencies of such nanomotors assuming a hydrodynamic model of this interaction. In this way, we find that nanomotors rotate around their longitudinal axes at frequencies of up to approximate to 25 kHz, or approximate to 150 000 rpm, in the planar pressure node of a half-wavelength layered acoustic resonator driven at approximate to 3 MHz with an acoustic energy density of < 10 J.m(-3). The corresponding tangential speeds of up to approximate to 25 mm.s(-1) at a nanomotor radius of approximate to 160 rim are 2 orders of magnitude faster than the translational speeds of up to 20 mu m.s(-1). We also find that rotation and translation are independent modes of motion within experimental uncertainty. Our study is an important step toward understanding the behavior and fulfilling the potential of this dynamic nanotechnology for hydrodynamically interacting with biological media, as well as other applications involving nanoscale transport, mixing, drilling, assembly, and theology. Our results also establish the fastest reported rotation of a nanomotor in aqueous solution.