Stretching and spinning droplets using sound

Mirror made from tiny polystyrene spheres. (Courtesy: Grzegorczyk et al., Phys. Rev. Lett. 112 023902)

By Hamish Johnston

There are two fantastic papers in Physical Review Letters this week that made me smile. Both of them are about controlling macroscopic objects using waves. While there are practical applications for both techniques, I can’t help thinking that the authors did the work for the sheer joy of it.

One paper describes how sound can be used to make droplets and small particles hover in the air, as well as spin and follow orbital paths. The work was done by Daniele Foresti and Dimos Poulikakos of the Swiss Federal Institute of Technology in Zurich (ETHZ) and involves a concept called acoustophoretics, which means “motion caused by sound”.

The ETHZ pair used standing waves of sound to create a trap that can hold a droplet or particle. By modulating the sound waves in both space and time, the researchers were able to set a particle spinning on its axis or even make it fly around in a tiny orbit.

When water droplets were used, their normally spherical shapes could be deformed into ellipsoidal shapes. These ellipsoids could then be set spinning. While the technique could be used for contactless materials handling and the study of substrate-independent chemical reactions in droplets, I think that making tiny particles hover, spin and whizz around sounds like a lot of fun!

The second paper is by Tomasz Grzegorczyk of BAE Systems, together with Johann Rohner and Jean-Marc Fournier of the Swiss Federal Institute of Technology in Lausanne. The trio has come up with a way of using laser light to assemble and bind about 150 spherical polystyrene spheres (diameter 3 μm) to create a mirror. The beads are suspended in water and trapped between two glass plates, which effectively restricts their motion to two dimensions. When illuminated with a 40 μm-diameter spot of green laser light, the spheres form a closed-packed structure, the surface of which acts like a mirror.

The idea is that eventually much larger mirrors could be made using this or a similar optical technique. These would be very useful for astronomers because in principle the shape of the mirror could be defined and controlled to great precision using just laser light, while the mirror itself would be extremely lightweight.

Of course, much more work needs to be done before a telescope-sized mirror could be built. So Grzegorczyk, Rohner and Fournier can look forward to more fun with beads and lasers.

The acoustophoretics paper is called “Acoustophoretic contactless elevation, orbital transport and spinning of matter in air” and the mirror paper is entitled “Optical mirror from laser-trapped mesoscopic particles“.