In 1991’s Terminator 2: Judgment Day, Robert Patrick’s T-1000, a shape-shifting robot assassin made from a “mimetic polyalloy,” introduced the masses to the idea of liquid metal. The polyalloy returns in the latest Terminator movie, Genisys, with even greater malleability and maliciousness. But the reality of liquid metal is quite tame—it’s often merely a shiny, noncorrosive substance that is used to make many everyday objects such as cellphones. Recently, however, researchers at North Carolina State University and China's Tsinghua University have developed an alloy that resembles, at least on the surface, the T-1000’s amorphous robot, and can actually move around and take on different shapes.
Sloan Science and Film spoke about the possibility of weaponized chameleon-like metals and the laws of physics that define such objects with Michael Dickey, Professor of Chemical Engineering at North Carolina State University, whose Dickey Group is studying new ways to pattern, actuate and control soft materials, such as gels, polymers, and liquid metals.
Sloan Science and Film: The Terminator movies have popularized this idea that liquid metals can change shapes and form a seemingly endless array of objects. I imagine when the first movies came out this was totally impossible, but what about now?
Michael Dickey: I usually show the clip from Terminator 2 to students, because it always gets people thinking about liquid metals. But it’s obvious that you’re looking at science fiction, because there are laws of physics that are being violated: Gravity would prevent liquids from going from a puddle to forming the head of a human. And, in addition to that, surface tension would prevent liquids from forming anything other than spherical-like shapes.
SSF: Why do liquids tend to form spherical shapes?
MD: Surface tension basically acts to minimize surface energy, and it does that by making liquids assume a shape that is generally round.
SSF: So what are the kinds of liquid metals you are using and manipulating? And how close are they to what is depicted in the films?
MD: I had the pleasure of talking to Gene Warren, the man who won the Academy Award for the effects on Terminator 2, and he confirmed what I thought: They used mercury as a starting point to use the computer rendering. In our case, we’ve avoided mercury, because it’s toxic. The metal we’re using is based on gallium, which is right beneath aluminum on the Periodic Table. It’s like aluminum, but its melting point is much lower. Gallium’s melting point is 30 degrees C, so if you were to leave it out, it would solidify, but the warmth of your body will make it melt. We mix it with Indium, another element nearby on the Periodic Table, so the melting point goes down, and it becomes a liquid. Superficially, it looks like mercury, without the toxicity.
The key thing for our purposes is that these liquids react very quickly with air. In the case of gallium, it forms an oxide, similar to rust, and this oxide is very much like a skin. The oxide is only a few nanometers thick; it’s so thin that you can’t even see it with your eyes. It’s a little bit like a waterbed, with liquid on the inside and a shell on the outside. And because of the shell, it has mechanical properties that allow you to shape the metal. We’ve been using this property to pattern the metal in all types of shapes: You can make wires, antennas, and at the end of the day, you still have a liquid. You can make electronic circuits that are extremely soft and stretchable, and in some cases, inject them into stretchable tubing, stretchable headphone wires, because the mechanism properties are in the encasing material rather than the metal.
If you want to get philosophical, the function of any material is usually dictated by its shape. A pile of sand doesn’t do you any good unless it’s in the shape of a window. We might be able to make an antenna that could change shape and change its properties. But this oxide layer is a blessing and a curse, it’s a blessing for us because it allows us to change the shape of the metal, but once you start moving the metal around, it behaves like wet paint, and sticks to a lot of surfaces. What we are excited about is that you can then manipulate the oxide layer with small voltages. There’s a little bit of nuance there: If you apply a small negative voltage, the oxide layer will go away, and the metal will go into a level of high surface tension; with a positive voltage, the surface tension gets really low, and the metal will flow like liquid.
SSF: What does oxidation exactly have to do with this process?
MD: Basically, we can deposit or remove the oxide layer that forms on the metal. When the metal is in direct contact with water, those two liquids don’t like each other, but if you can deposit the oxide, it creates a separation between the metal and the water, and makes them like each other a little better. It’s similar to how soap works when you have oil on your skin, and you try to wash it off: the soap is the substance that makes the oil and water more compatible.
SSF: And how does the voltage actually work?
MD: The voltage drives electrochemical reactions. So the metal wants to oxide spontaneously, which means it’s giving up electrons. You can drive that reaction by pulling the electrons out from the metal, or pushing the electrons into the metal.
The oxide is strong and can hold the shape to the metal, but once you put it in water or we put it in base, the base removes the oxide layer. An analogy I use is that it’s a little bit like trying to build a brick wall, where the brick wall is the oxide, and someone is constantly removing the bricks. Even though it’s mechanically strong, if you’re building it and breaking it down, it makes it a liquid. It took us about two years to figure out what was happening.
SSF: So what needs to happen to go from where we’re at now to creating something out of liquid metal as seen in the Terminator movies?
MD: There are really two dominant forces that affect the way the metal behaves: surface tension and gravity. The surface tension forces tend to get bigger when the droplets get smaller and the gravitational forces get bigger as the metal gets larger, so you have this sweet spot where the forces are in balance, at about few millimeters in size. When you’re bigger than that, gravitational forces get greater, so that ultimately limits what you can do. So if you have something the size of a beachball or a basketball full of liquid metal, the gravitational force would be so enormous it would just flow and form a pancake, with gravity causing it to flatten. So nobody really needs to worry about the Terminatordominating, because gravity dominates.
SSF: So what are the practical applications of your alloy?
MD: At the small scale, 1 mm or below, that’s about the size of metals used in electronics, wires, antennas, switches, so we’ve been trying to use this phenomenon for that purpose. Antennas are particularly interesting, because it’s just a piece of metal, and and the function of the antenna, such as frequency, is just the shape of the antenna, so we can move the metal around and it could be for tuning the antenna, or changing the length of the antenna. For another example, there’s a group at MIT that are using liquid metals for battery applications, or for removing heat from nuclear reactions, because metal has really good heat transfer properties.
SSF: So can’t you really weaponize liquid metal, creating small but lethal shape-changing weapons?
MD: You could change the shape of the metal from a square to a star, from a cylinder to a sphere, but that’s about the level of sophistication. At the end of the day, you still have a liquid, and to do anything, you would need it to, in principle, go from a liquid to a solid, but the length scales are so small. If you start to think about everyday objects or weapons, it would be very difficult to assume their shape. Most of the stuff we’ve been doing is on a much smaller scale.