FOCUS

Video—Juggling Droplets

Physics 12, 10
A pair of microscopic liquid droplets suspended by a laser beam can execute a surprisingly stable “juggling” pattern for up to 30 minutes.
A. Bae et al., Phys. Rev. Lett. (2019)
Lilliputian carnival routine. Tiny droplets follow stable, interleaving orbits as they interact with each other and a laser beam (see video below).

A pair of tiny droplets of glycerol in an optical trap can interact with each other and with the trap’s laser beam to produce a sort of “juggling” pattern, as observed by a trio of physicists. The 30-micrometer-diameter droplets follow side-by-side, kidney-shaped orbits that lie within a single plane. Kelken Chang of the University of Gothenburg, Sweden, and his colleagues discovered the phenomenon by accident and combined experiments with simulations to explain it.

A. Bae et al., Phys. Rev. Lett. (2019)
A pair of 30-micrometer-wide glycerol droplets can be “juggled” in a laser beam for up to half an hour.

The laser beam points upward and is most intense along the vertical center line. The beam acts in two ways, with a force that pushes up on the droplets against gravity and a refraction-based force that pulls them toward regions of higher intensity. When one droplet is above the other, the upper droplet becomes partially eclipsed by its lower partner and falls from lack of light to hold it up. But as the droplet falls, it also gets drawn away from the center line by the more intense light at the edge of the lower droplet’s shadow. Meanwhile, the lower droplet rises and is pulled toward the center line by the unblocked laser beam. The two droplets eventually switch roles. The juggling oscillation occurs at a frequency of about 34 Hz and can last for up to a half hour.

Chang says that the long-time stability of the juggling is in part a result of the team’s precise control of the droplet sizes, thanks to a precision nozzle that creates them, which allows the droplet pair to be nearly identical. As for the usefulness of the experiment, Chang is blunt: “There is no importance whatsoever to light juggling,” he says. “It was a joy making this serendipitous discovery, being puzzled by it, and finally finding a simple solution to it.”

This research is published in Physical Review Letters.

–David Ehrenstein

David Ehrenstein is a Senior Editor for Physics.


Subject Areas

Optics

Related Articles

Hearing the Quantum Difference
Quantum Physics

Hearing the Quantum Difference

At very low volume, a quantum optical microphone performs better than a classical device, and humans can hear the difference. Read More »

Small Molecules Twirl Freely in a Helium Droplet
Optics

Small Molecules Twirl Freely in a Helium Droplet

A laser nudges deuterium molecules into friction-free motion within a superfluid. Read More »

Carving a Space-Time Crystal in a Plasma
Plasma Physics

Carving a Space-Time Crystal in a Plasma

A theorized scheme uses lasers to produce space-time quasicrystals in a plasma—density fluctuations that could be used as diffraction gratings for high-intensity laser pulses. Read More »

More Articles