Phys. Rev. Lett. 122, 043902 (2019)

The fact that light can control the motion of small particles has been known for some time. Going further, Albert Bae and collaborators have implemented a way to ‘juggle’ with two glycerol droplets using just a single laser.

When two droplets are caught and levitated by the laser beam, the first may be a little in front of the other so that it obscures the light from the second. The first droplet is accelerated by the beam as usual, but the partially eclipsed one is not so it drops and falls towards the centre line of the laser. At some point, the two particles cross and their roles are reversed, resulting in a rising-and-falling motion akin to balls being juggled. This effect can last for up to 30 min and the trajectory of the juggling can be controlled via the polarization of the laser, indicating that the technique can be used to probe two-particle interactions in a new way.