Chemical reaction levitated by sound
Scientists in Switzerland have designed an acoustic levitator capable of mixing substances as they hover.
The device is built of two sound-emitting platforms that trap the substances between them.
Sound waves move upward until they are reflected by the platform above. The upward- and downward-moving waves overlap, reinforcing each other in places and cancelling out in others. This traps materials in place.
The development is described in PNAS.
To demonstrate its capabilities, the researchers used it to mix a single drop of water with a single coffee grain and to bring together a minuscule grain of sodium metal with a drop of water, causing a tiny but explosive reaction. They even managed the levitate a toothpick.
Their device though needs to use very powerful sound waves to levitate even these very small droplets and particles.
Lead researcher Daniele Foresti from the Swiss Federal Institute of Technology (ETH) in Zurich, explained that to "cancel out gravity" in this way required 160 decibels of sound (you would receive about 120 decibels in the front row of a rock concert or from a jet engine).
"That's why we use ultrasound," Dr Foresti said.
"[Using frequencies beyond those that human ears can detect] means we won't hear it, so we can work with without any ear protection."
Acoustic levitators were invented in the 1980s by Nasa scientists looking for ways to simulate weightlessness. But this diminutive device is the first to be able to move and control the material it levitates.
"We have total control of the acoustic field inside," Dr Foresti said.
By making the acoustic platform of many tiny squares - each 15mm x 15mm square being its own sound-emitting device - the researchers are able to "pass" a ball of material from one of these platforms to another.
The main benefit of this remotely controlled chemical reaction is that it is contactless, meaning chemical tests could be carried out without any possibility of contamination.
"One of the tests we would like to perform is DNA transfection," said Dr Foresti.
This is the process of introducing DNA into a cell, which is a medically useful process but one that is very sensitive to contamination.
"Some of the transfection agents [chemical substances needed to carry DNA into cells] can be rendered ineffective even by plastics," said Dr Foresti,
"So this could be a way to overcome that problem."