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In a landmark discovery, researchers from the University of Wollongong (UOW) have realized non-contact handling of liquid metal. Metals can be controlled to move in any direction and manipulated into unique levitating shapes such as loops and squares using small voltage and a magnet.
The liquid metal used is galinstan, an alloy of gallium indium and tin, which favors the formation of droplets due to its high surface tension. Under the application of a small trigger voltage, this liquid metal becomes a wire because the voltage causes electrochemical oxidation, which lowers the surface tension of the metal.
The research team was led by eminent Professor Xiaolin Wang, Node Leader and Theme Leader at the ARC Center of Excellence for Future Low Power Electronic Technologies (FLEET), and Director of the Institute of superconducting and electronic materials from the UOW within the Australian Institute of Innovative Materials. He noted that by combining electromagnetic induction and fluid dynamics, the team was able to manipulate liquid metal in controllable ways and move like soft robotics.
Liquid metal research has been inspired by biological systems as well as science fiction, including the shape-shifting “T-1000” liquid metal robot in the James Cameron-directed film. Terminator 2. “This research is more than science fiction, we designed and realized this non-contact method for liquids, providing a new way to manipulate and shape fluids,” said Professor Wang.
Because these reactions require an electric current to flow through the wire, it becomes possible to apply a force to the wire via the application of a magnetic field (i.e. electromagnetic induction; the same mechanism that drives the movement in an electric motor). Thus, wires can be manipulated to move in a controllable path, and can even be suspended (against gravity) around the circumference of the applied magnetic field, taking on controlled designed shapes.
UOW PhD student Yahua He was lead author of the study, published in the January issue of Proceedings of the National Academy of Sciences of the United States of America (PNAS), one of the world’s leading journals for the multidisciplinary research. He noted that the contactless manipulation of liquid metal allows researchers to harness and visualize electromagnetism in new ways.
The ability to control liquid metal flows without contact also enables new strategies for shaping electronically conductive fluids for advanced manufacturing and dynamic electronic structures. Non-contact manufacturing and manipulation methods can minimize unwanted disturbances to objects being studied or manipulated. Previously developed non-contact technologies include manipulation of objects by acoustic manipulation or optical tweezers.
However, to date, free-flowing liquid streams have been particularly difficult to manipulate without contact. Achieving highly controlled changes in the directionality or complex shaping of liquids, especially without disturbing the cross-sectional shape of the flow, was the challenge for the UOW team.
Once the team started working on this, they realized there was a lot more behind it. Liquid metal wires are formed by applying a small voltage (about 1 volt). However, the team discovered that considerable electrical current (up to 70 mA) could be measured in the resulting wires.
“There was a creative leap at this point, as the team realized that electromagnetic induction could be used to control wires of liquid metal without contact. This was the key to ultimately successfully solving the challenge, developing thus a new strategy for shaping contactless fluids,” he added.
This contactless manipulation is made possible by the material’s unique fluid and metallic dynamic properties. As flexible conductors and current carriers, wires exhibit minimal resistance to manipulation via the Lorentz force under a controlling magnetic field. Thus, the researchers were able to manipulate the wires in a designed way.
Co-author Professor Michael Dickey of North Carolina State University said this very low resistance to movement allowed for exceptionally fine control of the resulting shapes. He said that usually liquid streams break up into droplets. For example, streams of water from a faucet or a hose start out as a cylinder, but quickly break down into droplets. However, the liquid metal thread has a rope-like property, similar to ribbons waving in the air. This property has allowed researchers to manipulate the flow of liquid metal into continuous loops and other shapes.