Interview with: Prof Michael Dickey from North Carolina State University about liquid metal wires.
Prof Michael Dickey speaks to Materials Today about his recent paper published in the journal Extreme Mechanics Letters. Follow the link below, to listen to the interview, or right click to download. Click here to read the article, Drawing liquid metal wires at room temperature, which describes an extremely facile method to fabricate metallic wires at room temperature.
Stewart Bland: I'd like to get started by asking if you'd please introduce yourself and your group and tell us about your background.
Michael Dickey: Sure, I'd be happy to. My name is Michael Dickey and I'm a Professor of Chemical and Biomolecular Engineering at NC State University in Raleigh, North Carolina and although my training is in polymeric materials and nanofabrication my group has been studying liquid metals for about a decade now. These materials are interesting because they are both liquids and metals. Most people think of mercury when they hear the term liquid metal, which is a bit unfortunate because mercury is known to be toxic. Instead, our group and others have been studying liquid metal alloys of gallium, which are considered to have low toxicity relative to mercury. Importantly these metals form surface oxides that act like a thin shell and this oxide shell allows the metal to be patterned and manipulated into shapes that would not be possible with conventional liquids like water. We have been studying these materials and taking advantage of their properties to make soft and stretchable electronics in our group.
Stewart Bland: So in your recent study published in Extreme Mechanics Letters you reported on a new method to create flexible wires using polymers surrounded by liquid metal. Before we get into the technical details of how, can you tell us about the need for flexible electronics?
Michael Dickey: I'd be happy to. There is a lot of interest right now in making electronics that are flexible and in some extreme cases, stretchable or even soft. There are at least two reasons I can think of for this interest. The first is just simply to give electronics greater functionality. A simple example of that would be a phone that could be folded for example. The second is to put electronics in places where it is currently difficult. An example of that might be some electronics on or inside the body, maybe within or on clothing or in other things that we come across in our daily lives. If you sort of take a step back and think about it, our bodies and many of the things that we interact with in our daily lives are soft and deformable and, in contrast to that, most electronics are made from rigid materials.
Ultimately there is a mechanical mismatch between the electronics that we have and our bodies and the things that we experience day to day. So, our group and a number of other groups around the world have been looking at ways to make electronics with interesting mechanical properties and we've tossed around a couple of words here - flexible, stretchable, and also soft, and if you think about flexible, it's possible to make electronics flexible by simply making the components thin and there are plenty of examples of this in our day to day lives. For example, aluminium foil is flexible because it is thin even though bulk aluminium is a very rigid material. In fact, you can make bicycle frames out of aluminium. We've been really trying to go a step beyond flexible electronics to try to make conductors that are stretchable and also soft.
Stewart Bland: So you used a liquid metal core surrounded by a polymer and that's a liquid metal at room temperature. Can you tell us a bit more about these exotic metals?
Michael Dickey: We're studying alloys of gallium. Gallium is directly below aluminium on the Periodic Table, which is another way to say that they are related and have similar properties but there is one major difference. That is that gallium has a melting point of approximately 30 degrees Celsius, which means that if you were to hold it in your hand your body is sufficiently warm to melt the metal and turn it into a liquid. In our case, we ensure that it stays as a liquid at room temperature by adding other metals to it. In this case we add indium and adding those things together lowers the melting point below room temperature to ensure that it stays liquid throughout our experiments and our application.
Liquid metals, as you may know, have very large surface tension and that causes them to want to beat-up to minimise their surface energy. If you've ever had the misfortune of breaking a mercury thermometer, you'll know that the mercury will beat-up into the shape of a sphere due to its large surface tension. So, this is really a problem if you want to pattern a liquid metal into a useful shape such as a wire, for example. There are other examples of this beyond mercury. Even water has a pretty large surface tension so if you were to turn on your faucet, you would see that a cylinder of water comes out of the faucet but it eventually breaks up into droplets due to surface tension. So, if you want to make a wire or something like that, that is stable, that's a challenge with liquid. Fortunately, gallium has a property that allows it to be patterned into stable shapes and that is that gallium reacts rapidly with air to form a thin oxide skin on its surface. The skin is only a few nanometres thick so it is quite thin, but it allows the metal to be moulded and manipulated into stable shapes that are none-spherical such as wires and, not quite a perfect analogy, but I like to say that the oxide skin is similar to how a water bed contains mostly water but yet it is held into the shape of a bed by a thin plastic bag that surrounds its surface and so this is sort of similar, but on a much smaller length scale.
Stewart Bland: So, how do you go about creating these wires?
Michael Dickey: Simply stated, we just place a droplet of liquid metal on a piece of putty and stretch it. This process is very much like stretching a piece of bubble gum and in our case, because the liquid is a metal, it stretches along with the putty. So, you stretch the putty, you also stretch the metal and the two things move simultaneously. When you do the stretching, the oxide skin breaks and reforms as you elongate it and again, to reiterate, without the skin the metal would just beat-up into a sphere or drop but with it the metal can form stable wire shapes. This whole process was inspired by the processes that are used to make fibre optic cables. In that process, a cylinder of glass is heated and simultaneously pulled into the shape of a fibre. In our case, we did not need to heat the materials because they were already soft at room temperature. The whole process is literally then at room temperature by hand. The resulting wires that we form consist of liquid metal encapsulated in polymer and in our work we explored several different putty-like materials including those that could be cross-linked after stretching to lock the structures into place. In other words, once you stretched it, you don't want it to be a putty anymore.
You want it to have found desired mechanical properties. Depending on the chemistries we employed, the wires could either be stiff or they could be elastic or rubber-like. We also showed it was possible to stretch the wires into a variety of shapes so they don't have to just simply be a straight line. It could be something you stretch out into the shape of a plus or a star or some other shape.
We also showed that you can control the diameter of the wires based on how far you stretch them. I think the smallest we got was about 10 microns diameter, which is about an order of magnitude smaller than the diameter of a human hair. So, the wires could be large but they could also be very small at the extreme.
Stewart Bland: So what kind of applications could these wires be suited to and are there any specific pros and cons?
Michael Dickey: This approach allows for wires to be formed on demand, which might be useful for repairs or for the military, for example to create antennas in the field of operation. The materials can be stored in a compact shape. They can be held in your pocket or in a bag and then be elongated into whatever shape is needed on demand. Now one of the limitations here or a drawback is that liquid metals are more expensive than typical wire materials like copper. Personally, I don't envision this concept to replace existing wires and it really only makes sense to use this approach if the added features justify the added cost.
Stewart Bland: So what's next for this project?
Michael Dickey: The wires that we formed were all done by hand, which limited the length of the resulting wires and also limited the reproducibility. Ideally, it would be preferable to also use machinery to do the elongation and that is something we are currently looking at. One of the important things that I would like to point out is that this work was all done by really excellent graduate students and also in collaboration with some of my colleagues in my department and I'm really thankful for all their efforts.
Stewart Bland: Fantastic. Well, to finish then, I'd like to ask, as always, in your opinion what are the hot topics in materials science right now?
Michael Dickey: Well, I'm going to show off some that I'm a little bit partial here because it’s an area that I'm personally interested in but I am partial to soft materials and I think there is genuinely a lot of interest in this topic right now for a number of reasons. Where I live in the research triangle in North Carolina there is a lot of interest at the universities that are in this region. I'll just give you an example. In our group, we are interested in soft conductors and actuators including the liquid metal we just talked about. The human body provides once source of inspiration for this work since the body has, for example, nerve networks, memory sensors and many other complex mechanisms that are built entirely from soft materials and there really are very few man-made analogues that can mimic what our body can do using soft materials yet it would be interesting to make systems like these to create new devices that have the functionality that we find in the body built entirely from synthetic materials and to make interesting devices.
Stewart Bland: Fantastic. Well, thank you very much for joining us today. It's been a pleasure talking to you.
Michael Dickey: It's been great, thanks a lot for having me.