Ultrasound makes drops float

Acoustic levitation allows non-contact manipulation and mixing of objects

This green drop was created by free-floating mixing of two non-luminous starting drops © Dimos Poulikakos
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Ultrasound is not just for locating or medicine, it also lets objects float. This method of acoustic levitation has now been refined by Swiss researchers to the extent that they can freely move, manipulate and even mix drops and solids. This technique could be used, for example, for particularly sensitive materials, for example in biomedicine, as reported in the journal "Proceedings of the National Aacademy of Sciences".

Sound can produce a noticeable power - this can be observed on almost every major loudspeaker: If you place it on your back and, above all, turn up the basses completely, you begin to dance styrofoam pieces or other light objects placed on it: the vibrations generated by the sound keep picking up the pieces again and again. But this works even better when, instead of music, short-wave ultrasound is used, as Daniele Foresti from ETH Zurich and his colleagues demonstrate.

Node in the ultrasonic field

To this end, they arranged individually controllable, 15-millimeter-wide piezo loudspeakers with the sound opening upwards next to each other. At a certain distance from these they fastened a reflector plate. "This creates a standing wave between the speakers and the reflector, " the researchers explain.

These waves together create an acoustic field whose power is sufficient to overcome the gravity of smaller objects. These then float at certain points of the field, called nodes. That such acoustic levitation works was already known earlier. The novelty of the Swiss device, however, is that they can move and manipulate the nodes in the ultrasound field - and thus the particles floating in them.

This is how the acoustic levitation works: By changing the sound intensity, the ultrasonic field and thus also the positions of the drops change. © Foresti et al. / PNAS

In one experiment, for example, they injected two drops of water about one millimeter at opposite ends of the sound field. By making the smallest changes in the sound intensity of individual loudspeakers, she shifted the knots in the field so that both drops migrated toward each other and then finally merged. "We know of no other method or technology that allows the transport and manipulation of several drops in the air, " the researchers note. display

Floating instant coffee and DNA transfer

The whole thing works well with mixtures of solid and liquid objects, as they explain: Bring a small coffee instant coffee and a drop of water together in this field, the coffee in the water - and probably the first floating coffee maker is perfect. According to the researchers, this is precisely the advantage over other levitation methods, for example due to magnetic fields or electrical charges: the acoustic variant works independently of the material. It does not matter if the objects are liquid or solid, whether they conduct electricity or not, and whether they are magnetic or not.

Even a toothpick is levitated and moved by the ultrasonic field Foresti et al. / PNAS

The scientists demonstrated in another experiment that the method can also be used for biomedical research. They put a drop of a DNA solution and a drop of living cancer cells in the ultrasound field and made them both merge. Subsequent observations showed that some of the free DNA molecules had been introduced into the cells, a process that is fundamentally important for targeted gene manipulation. In another experiment, they made a fluorescent protein glow by fusing it with a special caustic.

This proves that even highly sensitive biological materials can be manipulated without contact and under controlled conditions without damaging or destroying them. "This concept paves the way for completely new process classes, from substrate-free biological and chemical reactions to new containerless mixing methods, " says Foresti and his colleagues. (Proceedings of the National Academy of Sciences (PNAS), 2013; doi: 10.1073 / pnas.1301860110)

(PNAS, 16.07.2013 - NPO)