Scientists at the National Institute of Standards and Technology (NIST) have discovered that a gold nanorod submerged in water and exposed to high-frequency ultrasound waves can spin at an incredible speed of 150,000 RPM, about ten times faster than the previous record. The advance could lead to powerful nanomotors with important applications in medicine, high-speed machining, and the mixing of materials.
Take a rod only a few nanometers in size and find a way to make it spin as fast as possible, for as long as possible, and controlling it as precisely as possible. What you get is a nanomotor, a device that could one day be used to power hordes of tiny robots to build complex nanostructured materials or deliver drugs directly from inside a living cell.
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Nanomotors have made giant strides in recent years: they've gotten much smaller and more reliable, and we can now also power them in many different ways. Available options include electricity, magnetic fields, blasting them with photons and, more recently, using ultrasound to rotate rods while they're submerged in water, which could prove very useful in a biological environment.
Previous studies have shown that applying a combination of ultrasound and magnetic fields can control both the spin and the forward motion of the nanorods, but nobody could tell just how fast they were spinning. Now, researchers at NIST have found that, despite being submerged in water, the rods are spinning at an impressive 150,000 RMP, which is 10 times faster than any nanoscale object submerged in liquid ever reported.
To clock the motor's speed, the researchers used gold rods which were 2 micrometers long and 300 nanometer wide. The rods were submerged in water and mixed with polystyrene nanoparticles, and positioned just above a speaker-type shaker.
Vibrating the shaker at a frequency of 3 MHz, the nanoparticles started spinning, creating vortexes in the surrounding water that swept up the polystyrene nanoparticles. The scientists were able to infer how fast the rods were rotating by measuring the speed of the polystyrene particles, as well as their distance from the nanorods.
The sheer speed at which the nanomotor is operating opens up the possibility of using the device not just for medical applications, which was the original intent, but also for high-speed machining and the mixing of materials.
But in order to get a consistent performance out of them, the size of the rods must be controlled very precisely. According to NIST, in fact, even a small change in dimensions can result is very large deviations in the motor's speed.
The researchers will now focus on understanding exactly why the motors rotate (which is not yet well understood) and how the vortexes around the rods affects their interactions with each other.
A paper published in the journal ACS Nano describes the advance.