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Nanoparticles set spinning record

31 Jul 2018
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(Courtesy: iStock/taseffski)

If a jet engine spins faster than about 1000 Hz, the forces on its outer edge may rip it apart. But two research teams – one in Switzerland, the other in the US and China – have independently made nanoparticles rotate at over a billion Hertz, making them the fastest rotations ever produced.

Such ultrafast nanorotors could be useful for testing material properties, as well as verifying theories of frictional damping on the nanoscale. The dumbbell-shaped nanoparticles of the US-Chinese group can also form ultrasensitive torsion balances – the force sensors used to measure gravity in the 18th century. They could therefore potentially detect quantum effects in gravitation and other tiny force effects.

Both teams trapped silica nanoparticles around 100 nm in size using optical tweezers, a well-established technique that exploits the force of focused laser beams to manipulate everything from atoms to cells. By rotating the polarization of the beams, the researchers turned the optical tweezers into “optical spanners”.

“This had been done about twenty years ago in a liquid, but what sets the ultimate spinning speed the particle can reach is how much it’s damped.” explains René Reimann of ETH Zurich. “If you’re sitting in a liquid all these molecules are braking it by friction, so it doesn’t spin super-fast.” Adding extra laser power to overcome this friction risks damaging the particle through overheating. Researchers have more recently tried spinning particles in vacuum, but their rotation rates have not exceeded 10 MHz.

Systems like this, where you have really good control over either torsion or rotation, would be really useful – both in terms of sensing and on the fundamental physics side

Andrew Geraci, Northwestern University

In the new research, both groups used extremely high vacuum, very pure silica nanoparticles, and trapping lasers with a wavelength of precisely 1550 nm – the transparency window for silica. “We were trying to minimize the absorption as much as possible,” explains Tongcang Li of Purdue University in Indiana, a senior member of the US–Chinese group. By minimizing heating in all these ways, both groups achieved rotation rates over 1 GHz.

Nanoparticles minimize spinning stress

One reason the particles can spin so fast without flying apart is their nanoscale size, which means that their edges move less quickly than a larger object at any given rotation rate – which means that they experience less stress. “If you do the tests with a bigger probe, you’re typically limited by defects like scratches and tiny cracks on the surface that fly apart long before you reach the ultimate limit set by the atomic bonds,” Reimann explains. “If you move to a much smaller particle, it’s much simpler to prepare a probe that is defect free.” Moreover, he says, theoretically unforeseen effects may cause matter to behave differently at the nanoscale. “It’s an interesting fundamental question,” he says.

Reimann and colleagues have now acquired a detector capable of measuring rotation rates of up to 20 GHz. In theory, this should allow them to spin particles fast enough to rip apart the atomic bonds. “It would be very interesting to see at which speed these things explode, and whether it’s always the same, or whether it varies from particles to particle,” he says.

In principle – at sufficiently high rotation rates and sufficiently low pressures – it may also be possible to measure so called vacuum friction. This is a theoretically-predicted braking force arising from virtual particle–antiparticle pairs appearing momentarily from fluctuations in the quantum vacuum. However, Reimann believes this is some way from being experimentally tested.

Torsion in the balance

While the ETH Zurich researchers used spherical nanoparticles, the US–Chinese group produced nano-sized dumbbells by joining together two spherical nanoparticles. In linearly polarized light, the long axis of these nano-dumbbells aligned with the light’s polarization axis and simply wobbled because of collisions with the remaining air molecules. The researchers calculate that their system forms a torsion balance similar to that used to Henry Cavendish to measure the strength of gravity in 1798, but nearly 20 orders of magnitude more sensitive. It is even, they calculate, around 100 times more sensitive than state-of-the-art torsion balances today.

This could have multiple uses. For example, theoreticians predicted over 40 years ago that anisotropy in the quantum vacuum between birefringent materials should lead to a tiny rotational force called the Casimir torque, but this has never been observed experimentally: “Our calculations show the sensitivity of our system should be enough to detect the Casimir torque,” explains Li. Others have even suggested similar devices could potentially detect quantum effects in gravitation.

Andrew Geraci of Northwestern University in Illinois, who was not involved in either paper , is cautiously impressed. “Some of the technical details are not reported at this point in a very clear way, but I wouldn’t consider that a major shortcoming – just a sign that this is a relatively new area and people haven’t had time to tease out all these details yet,” he says. “There are a lot of things one can imagine doing where systems like this, where you have really good control over either torsion or rotation, would be really useful – both in terms of sensing and on the fundamental physics side.”

The papers by the Swiss and the US–Chinese teams are published in Physical Review Letters.


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