Levitation may look like magic, but there are a number of scientific tricks behind it. Magnetic systems are usually behind gimmicky consumer products like floating lightbulbs and speakers, optical levitation turns up in more academic pursuits like quantum computing, and acoustics could help suspend tiny particles to make better drugs. These techniques only work with certain objects, but researchers at the University of Chicago have developed a method to levitate basically anything, using differences in temperature.
"Magnetic levitation only works on magnetic particles, and optical levitation only works on objects that can be polarized by light, but with our first-of-its-kind method, we demonstrate a method to levitate generic objects," says Cheng Chin, one of the researchers on the team.
Balls of ceramic, plastic and glass, ice particles, seeds and pieces of lint have been used to demonstrate the technique, and the team found that the levitated particles could be held aloft for over an hour rather than a matter of minutes, and wouldn't wobble around sideways.
The researchers achieved this versatile levitation through the process of thermophoresis, which manipulates particles by placing them between sources of different temperatures. In this case, the objects were placed in a vacuum between two plates – the bottom one, made of copper, was left at room temperature, while the top plate contained liquid nitrogen, cooling a stainless steel container to -300º F (-184º C). The relative heat would flow from the bottom plate toward the top one, lifting the particles along with it.
"The large temperature gradient leads to a force that balances gravity and results in stable levitation," says Frankie Fung, lead author of the study. "We managed to quantify the thermophoretic force and found reasonable agreement with what is predicted by theory. This will allow us to explore the possibilities of levitating different types of objects."
As useful as the team's system is, precise measurements are required to ensure it works to its full capacity. The relative size of the plates needs to be considered, and they need to be placed at just the right distance apart, to give the warm air room to circulate. And the bottom plate has to be perfectly horizontal, otherwise the thermal gradient – the difference in temperature – could point off to one side, sending the levitating objects off-center.
"Only within a narrow range of pressure, temperature gradient and plate geometric factors can we reach stable and long levitation," says Chin. "Different particles also require fine adjustment of the parameters."
The device could allow scientists to experiment with the effects of microgravity on objects, chemicals and organisms, without needing to take them into space. It could also lead to new ways to move objects without touching them, which could be useful in cases where contamination is an issue.
"It offers new avenues for mass assembly of tiny parts for micro-electro-mechanical systems, for example, and to measure small forces within such systems," says Thomas Witten, a UChicago professor who wasn't involved in the study. "Also, it forces us to re-examine how 'driven gases,' such as gases driven by heat flow, can differ from ordinary gases. Driven gases hold promise to create new forms of interaction between suspended particles."
The next step for the researchers is to try to use the technique to levitate objects over 0.4 in (1 cm) in size, and study how they may react to each other in that environment.
"Our increased understanding of the thermophoretic force will help us investigate the interactions and binding affinities between the particles we observed," says Mykhaylo Usatyuk, a co-author of the study. "We are excited about the future research directions we can follow with our system."
The research was published in the journal Applied Physics Letters.
Source: University of Chicago
