A thin stream of liquid moves horizontally through the air, held inside a tube that does not exist: only ultrasonic waves shape and support it. The result comes from the University of Helsinki, which built a prototype able to transport liquids, powders and even electrical discharges without anything touching the material.
The device is an 8-centimetre tube segment ringed by 256 small speakers. They emit ultrasonic waves at 40 kilohertz, a frequency people cannot hear. The liquid enters at the centre of one end, and the surrounding waves hold it together and in place. By changing how the speakers are driven, researchers can alter the path and shape of the "tube", even making it curve.
The physics relies on a Bessel-like acoustic field that stabilises the flow without material walls. In tests the system could meter liquid drop by drop and guide an electrical discharge along a chosen route. The segment can also be extended with additional modules to cover greater distances.
There are several reasons to avoid conventional pipes. Viscous liquids can clog them, corrosive liquids can damage them over time, and contact with walls can contaminate a product. For sterile substances, such as those used in medicine and pharmaceuticals, the problem is especially acute, which is why the technology is aimed mainly at medical and pharmaceutical uses.
The system is still at laboratory scale and is not a complete system for moving materials over long distances. The results were published in Proceedings of the National Academy of Sciences, in a study on contactless guidance of matter in its main states. The work was carried out with the Public University of Navarre and the University of Michigan.
The idea has a precedent in ultrasound levitation research. The EPFL lab showed with its 150-microgram microfliers how sound waves can support objects in the air without motors. Here the leap is different: what is held up is not a solid body but a flow of matter.
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