Sound could be the key in building tomorrow's nanostructures

Researchers from Penn State University have found a way to precisely manipulate tiny objects using sound rather than optical instruments with a quick, energy-effective and technologically-simple technique that could have important applications in the fields of nanotechnology and biological research.

As shown with lab-on-a-chip devices, sound is proving a great way to manipulate small objects, and for a very precise reason: while light manipulation is technologically-easy to achieve, its high frequency in the electromagnetic spectrum means high energy levels are involved, which translate into a significant waste of energy for most applications. Sound waves, on the other hand, have much lower frequencies and therefore bear less energy, meaning a technique that harnesses sound rather than light will have a significantly better energy efficiency.

"Current methods for moving individual cells or tiny beads include such devices as optical tweezers, which require a lot of energy and could damage or even kill live cells," assistant professor of engineering science and mechanics Tony Jun Huang, who was part of the research team, explained. "Acoustic tweezers are much smaller than optical tweezers and use 500,000 times less energy."

But what makes the technique all the more interesting, particularly for possible applications in nanotechnology, is that it can be easily tweaked to position particles into well-defined patterns, providing a cheap and simple way to build nanoscale structures quickly and reliably.

The method exploits wave interference and involves setting up two or more sound sources producing acoustic waves that are stable in time: the waves interfere with one another in a way that depends on the number and relative position of the sound sources, and stable peaks and troughs in sound pressure are formed.

By controlling the sound sources it is possible to change the position of the troughs, therefore obtaining a different positioning of the particles: for instance, by placing two sources of longitudinal waves at a 90° angle will produce troughs that are evenly spaced in rows and columns, as illustrated in the picture above. Because sound waves have pressure, they can quickly push tiny objects to the nearest trough, placing them in the designated area.

These 'acoustic tweezers' also have the advantage of a reduced size — 'smaller than a dime' — so that it could be embedded on a chip using standard manufacturing processes. Moreover, the low energies involved mean there is a reduced risk of damaging or altering the objects, which makes this technique useful for biological applications.

The team tested the accuracy of its technique and found it to be effective regardless of the particles' electrical, magnetic and optical properties, as well as their shape, and verified that most cells or particles reach their final position within only a few seconds.