Making an invisible, silent, and heat-proof cylinder—for science

Chris Lee
A 3D printed metamaterial.
It has been a long time since I've been grabbed by a metamaterial. For those of you who don't know or remember what metamaterials are, they are the things that make invisibility cloaks work. Unfortunately, I got tired of writing about incrementally better invisibility cloaks and stopped paying attention.
So it was with a glorious sense of anticipation that I attended the opening talk at the Physics@FOM 2017 conference. Martin Wegener, who delivered it, hails from Karlsruhe Institute of Technology in Germany. Wegener has quite a reputation in the field of optics, but for this talk, he took a kind of scattered approach to show how the ideas behind metamaterials apply everywhere and can be used to design entirely new functions into simple materials—materials that may find their way into almost every aspect of your life one day.

What are metamaterials?

A metamaterial is both a very simple and a very complex object to describe. The simple level is very simple: mix two materials to obtain a new material with distinct properties. But, to borrow Wegener's analogy, imagine that we take two materials, one very dense, like lead, and the other less dense, like air. I can mix the two by drilling holes in a bar of lead. At the end of the process, the density of my new material lies somewhere between the density of air and lead. But, with a metamaterial, this combination can result in strange properties—the equivalent of a density that is larger than that of lead or less than that of air.
This is where the complexity comes in. No simple combination of materials will do anything like that. Instead, you have to consider the scale of the phenomena you are interested in. In our density example, we might care about density for the sake of the propagation of sound waves. To create a specialized material for sound, we need to combine our lead and air in a controlled fashion on length scales that are much smaller than the wavelength of sound that we care about.
That much has been known for a long time, but in recent years, the field of metamaterials has really exploded, thanks to two ideas.

Bend your space

The first idea comes from general relativity. In general relativity, we consider space to be curved, and its curvature is increased by massive objects. As I fly through space, I fly in straight lines, but thanks to the curvature of space, I trace out some complicated path as I go past a more massive object. To put it differently: I always take the shortest path between two locations. On a plane, that's a straight line; in curved space, it's a curved trajectory.
How does this help? Well imagine that I want to create some invisible area in space. To prevent light from interacting with the object, I have to make sure that all paths curve around that volume so that it looks like no hidden volume exists. General relativity can tell us what the shape of such a surface looks like. And (remarkably) a link between the equations of electromagnetism and general relativity tell us what the material properties need to be at each point in space to create the equivalent of curvature.
So, in principle, all sorts of previously unimaginable optics can be created, among them invisibility cloaks.

Tomography doesn’t work (but don’t tell your doctor)

The second idea comes from tomography. Any time you get a 3D image from ultrasound, the image is reconstructed from sound waves that are detected outside of the volume being imaged. According to Wegener, a long-standing question in tomography was "is the reconstruction unique?"
Now, since it works, you, I, and Wegener all believed that yes, it probably is. But a couple of years ago, mathematicians proved that this was absolutely not the case. The heart of the proof was the idea that a particular form of equation maintained a type of invariance, no matter how space was curved or stretched. That means that tomographic reconstructions can be fooled by hidden volumes.
The critical equation occurs everywhere: it applies to sound waves, heat propagation, light, and quantum mechanics. You name the physics, and Wegener can probably show you how that type of equation appears. Suddenly, it became very simple to take the idea of stretching space and apply it to the creation of desirable properties in almost any domain—light, sound, or whatever. You could then use this equivalence to determine how the properties of the material needed to vary in order to obtain the intended behavior.

A tour of hidden objects

Since that revelation, Wegener has taken to hiding things in all manner of ways, with a huge amount of success. Not just success in the "hey, that's pretty cool" way. But in the "can you scale that up and put it on my device?" way.
There were many striking examples, and I don't want to turn this into a listicle, so let me discuss just three. Wegener showed how to prevent heat from moving into a volume. Now you might think this is just insulation, but it's not. At the border of an insulated volume, you have a temperature gradient. These gradients then distort the temperature field around the object in a way that, if you were to measure the temperature, you would know there had to be an object in the center.
The metamaterial, which consists of rings of copper (high thermal conductivity) and plastic (low thermal conductivity), guides the energy flow around the hidden volume in such a way that on all sides, the temperature gradient looks exactly as if there was no object between a heat source and the cold sink.
You might think that this doesn't have much use, but there are many cases in which some critical component that can't be allowed to get too hot has to be located somewhere near a heat source. For the lifetime of the heat source, good thermal conductivity is required to clear the heat away. This metamaterial goes some way to achieving that goal. It allows heat to flow around it undisturbed and doesn't allow the heat into the center volume.
Another example does the same thing with light. In this case, the shells consist of highly scattering and highly transparent materials. Think of layers of paint sandwiched between layers of glass, but on a scale of just a few nanometers. This structure will guide light around the volume encased in the shell. This technology is so robust that it has been scaled up to objects that you can hold in your hand, and Wegener showed a picture of this working in non-laboratory settings. However, that demonstration involved placing the object in a translucent medium (think a partially opaque bathroom window) as well. So, it worked there, but I don't think it would work in a clear window.
(Mind you, how would you know... and come to think of it, once they put their device on the lab bench, how do they find it again?)
On the other hand, this already works well enough that it would be useful for covering conducting electrodes that go on the front of solar panels, to give one example. And, in complex optical environments, like the front face of a large OLED screen, it would probably work as well, provided the electrodes weren't that big.
Sound waves are a bit more challenging. This is because solid materials support three types of sound waves: longitudinal waves, where material moves forward and backward (these are the sort that travel through air), and two types of shear wave where the material moves from side-to-side. These can only travel through solids. The issue is that you can create a structure that works for one type of wave, but it won't work for the other two. Critically, if a longitudinal wave approaches at a glancing angle, it looks like both a shear wave and a longitudinal wave from the perspective of anyone watching. So any material has to work for all these waves.
To get something to work for sound waves, Wegener had to create a solid material that didn't support shear waves. It turns out that you can do this by joining needles together at the points to create a lattice. You can't just put the needles together any old way—they have to have a specific structure (diamond-like). The result is surprisingly strong and can transmit longitudinal waves. The points where the needles connect, however, don't resist sideways motion, so shear waves die out very quickly. So, the shear wave components that are created at boundaries and at imperfections are absorbed by the structure.
This material also has to be structured to guide the remaining longitudinal waves. In the end, you need a metamaterial of a metamaterial. The result is that a volume is shielded from both longitudinal and shear waves, where the longitudinal waves pass around the object as if it weren't there.

Thinking big

At the end of his talk, Wegener outlined some work that I've written about before: using giant metamaterials to guide surface waves from earthquakes. Apparently, that work is still in progress and is looking hopeful. At the time, I was pretty skeptical. But, given the advances presented here and the fact that these are now devices that are not at the limits of fabrication, I'm starting to become a believer.
This talk was presented at Physics@FOM, Veldhoven, 2017
Making an invisible, silent, and heat-proof cylinder—for science Reviewed by Bizpodia on 21:03 Rating: 5

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