The GRASP team (Group for Research and Applications in Statistical Physics), run by Professor Nicolas Vandewalle has published an astonishing article in the New Journal of Physics. In it the Liège physicists give an account of the ways they have managed to both make droplets dance and to direct and steer them. A far-fetched idea? Not really, if we consider the demands made of micro-analysis today.

The article’s title might make one think of a hoax in a journal looking to create a sensation: ’resonant and rolling droplet’. It is nonetheless the very serious and acclaimed New Journal of Physics which has published this article, which analyses and dissects the curious behaviour of droplets submitted to different excitation frequencies.

The equipment of the system used is as simple as it is ingenious: an I-Pod which sends a signal to an amplifier which starts to vibrate a glass container attached to a high-speaker. In the container is found an oil which is very viscous, to make sure that its surface never becomes deformed. The game consists of placing on the surface of the oil several other droplets of oil, this time as liquid as water, and of varying the frequencies applied. The experiment can be carried out with no matter what liquid but the liquid’s parameters will influence the results obtained. With the system used by GRASP it is observed that at a constant frequency of 50 Hz, the droplets are animated by a regular up and down movement. They are not very deformed at all, they do not burst and they never become mixed with the liquid contained in the glass: they bounce on its surface. According to the experimenters, this can continue for several days, until an external element (such as dust or a current of air) interrupts the process.

What phenomenon are we seeing at play here? The droplet crushes the layer of air which is formed between itself and the bath. The layer of air thus begins to empty but it does not have the time to disappear entirely before the droplet touches the surface. The vibrations of the tray make it move upwards and the layer of air can remake itself, and so on and so on: the droplet never touches the surface, as there is always a very thin layer of air which imposes itself between them. The same phenomenon also occurs between the droplets, which means that they only fuse together in certain conditions of excitation.

(1) Resonant and rolling droplet, S. Dorbolo, D. Terwagne, N. Vandewalle, and T. Gilet, New Journal of physics, 2008