Gallium-based liquid metal manipulated without physical contact

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay.  In the search for better electrical devices, one thing to consider is how to prevent wires and connections from breaking. Soft electronics could be the future for mobile and wearable technology and one aspect of this research looks a lot like something from science fiction.  

MICHAEL DICKEY: There’s a lot of interest now in trying to make some of the functionality that you find in rigid devices into soft devices that can be stretched and bent and stuff, very much like human tissue. If you could get the metal to actually move and change shape then you can make reconfigurable structures.

LAURA LEAY: Professor Michael Dickey from North Carolina State University is interested in determining the fundamental properties of materials, and finding new ways to exploit these properties. In collaboration with researchers in Australia, at the University of Wollongong and the University of New South Wales, Michael has been working with gallium.

MICHAEL DICKEY: Gallium is probably the most interesting material in terms of the number of unique properties. One of the things for this particular work that’s important is that it’s got the largest surface tension of any liquid at room temperature.

LAURA LEAY: The high surface tension of this liquid metal means that, when submerged in an aqueous solution, the liquid gallium will form a sphere; in air it oxidizes which causes it to spread and coat a surface.

MICHAEL DICKEY: How do you take something like this and make it into the geometry of, like, a human hair? And then how do you manipulate it because it’s just a liquid; it’s very fragile? We discovered that if you apply a voltage to it relative to another wire that’s in the water, that will cause the surface to oxidize. We think that those oxide species actually lower the surface tension.

LAURA LEAY: This means that a substance that tends to form a sphere, when fed by gravity through a thin nozzle, which is surrounded by aqueous solution under the right conditions, will instead flow into the shape of a wire. 

MICHAEL DICKEY: What’s remarkable is that the surface tension is effectively zero. That’s really weird. Most liquids that we encounter in our day to day life have surface tension. There’s no energetic penalty for it to increase its surface area. In the absence of surface tension there’s no driving force for it to break up and turn into droplets.

LAURA LEAY: Passing an electrical current through the liquid metal wire also means that a magnetic field is created and this means that the wire can be shaped using external magnets, following the Lorentz force, or the left-hand rule.

MICHAEL DICKEY: If you have current going through one finger which in this case is the wire and then the other finger would be a magnet, and that generates a force in the other direction. So all is we do is we take a magnet and we put it behind that wire.

LAURA LEAY: The result is a liquid metal wire the width of a human hair that can be manipulated without physically touching it.

MICHAEL DICKEY: You’re taking a liquid that should just form droplets and you’re making it into a wire. That’s already amazing. But then you’re able to manipulate it, basically against its natural direction of flow, against gravity, and actually suspend it temporarily. There’s something sort of magical about this electrochemical reaction that is causing the surface tension to be very, very low. 

LAURA LEAY: Although the voltage applied to the liquid metal causes constant oxidation on the surface of the wire, which is what lowers the surface tension, the exact mechanism remains unknown.

MICHAEL DICKEY: We spent a lot of time trying to understand it and it’s really tricky because whatever we’re putting on to the surface is lowering the surface tension but as soon as we turn the voltage off whatever’s on the surface dissolves away. And because we’re doing this inside of a liquid it’s not like we can do spectroscopy, or something, to figure out what’s on the surface.

LAURA LEAY: Future work could see these wires extruded with a polymer layer around them so that they will retain their cylindrical shape when removed from the aqueous medium, and so could lead to soft electrical wire. But more importantly, it will lead to a better understanding of the properties of matter. This work was published in a recent issue of the Proceedings of the National Academy of Sciences. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.