Meta thoughts about Metamaterials
There is something going on in the world of engineering. A theme, an idea, an approach. I speak of metamaterials. I've spent the better part of a month trying to wrap my head around this emerging 'field' or 'technology.' I placed both field and technology in quotations because the metamaterial approach is being applied to widely different fields of science and is being accomplished with as widely reaching technologies. That is to say, metamaterials are not confined to any particular field or set of technology.
So what the heck is a metamaterial?
Thanks for that, Google. Hmm. Negative refractive index? That sounds like the field of optics. That's funny, because this example that I recently watched was all about mechanics and really nothing about optics:
Then this type of metamaterial is all about generating surface features that can change depending on how you squash it:
And yet another metamaterial pertains to aiming radio as well as light (photonics:
Oh, and to confound my fragile mind yet further, here is a metamaterial that deals solely with how heat moves through a solid. They're calling it a thermal cloaking device... as in it makes things invisible... to heat?!?!
Here's some research into making frameworks that are either rigid or 'bendy' depending on how the individual subunits are oriented:
Then I saw a recent article about metamaterials designed to create specifically shaped sound fields that can actually levitate small objects:
This is ultimately what made me say STOP! I get it - metamaterials are awesome... But how do I get a grip on the basic concept?
I want to understand the overall abstract idea behind it, see how it applies to all these examples, and hopefully then be able to take it further. (How 'meta' a notion!)
Ok, first the word 'meta'
It gets thrown around like so many hipster tech terms. The original Greek preposition or prefix simply means "beyond" or "after." In our language, it also includes the concept of self-referential progress, especially when talking about art or storytelling. Art and stories that are self aware can be aptly described as 'meta'. A heady definition might be that 'meta refers to the act of identifying a common layer of abstraction that unites a set of concepts, and then using that abstraction to postulate new concepts that improve upon or complete the originals.'
Now for the 'metamaterials' word
Metamaterials have to do with using natural materials to construct new materials. Natural materials each have their own characteristic bulk properties. Ordinary materials are used to create tiny subunits which, when measured en masse, may have startlingly different bulk properties. In the case that these sub units are small works of engineering - such as small coils, tiny hinged junctions, or minute resonating chambers - and the bulk properties are measurably different from ordinary materials (negative refractive index, switchable rigidity, able to focus sound waves to a point, etc) then we can aptly call this stuff a 'metamaterial'.
Classifying natural materials
To build new and exciting metamaterials, it is first vital to understand our building blocks, the natural materials. Understanding of the material world is the central focus of nearly all disciplines of science and engineering, and a robust set of measures has been developed along the way. It could be said that science is the business of assigning measurements to things in nature.
For a given material, there are specific properties that can be best described by the fields of physics, engineering, chemistry, or bioscience. The properties go on to branch into different subcategories until we have a base property, such as heat capacity.
A designer of metamaterials will basically think of all of these characteristics as places to innovate! This can be seen in some of the examples above. For instance, that 'thermal cloaking device' is a metamaterial that innovates around the thermal conductance properties of a bulk material. Metamaterials that bend radio waves fit squarely into innovations around the wave properties of materials, such as diffraction, spectral absorption etc.
A really clear example of a metamaterial based around one specific material property is a thermal metamaterial designed by MIT.
The designers looked at the property of thermal expansion. Ordinary natural materials tend to expand when heated, and shrink when cooled. Some materials, however, expand and shrink more than others. Engineers created small cubical subunits using two such differing materials, arranged as a system of internal levers so that the expansion of one would counteract the expansion of another in a way that the total dimensions of the subunit remain the same. A block of material made by linking these subunits would not exhibit any change in size with a change in temperature. The subunits could even be redesigned so that the material would actually shrink when heated, and expand when cooled!
What are metamaterials even for?
A very good and open-ended question. What are materials for? Building things. Metamaterials expands the pallet of paint colors, so to speak. It broadens the variety of basic materials from which every day human objects can be manufactured. Thermal metamaterials could lead to more efficient thermoelectric devices for refrigeration and home heating. Optical metamaterials could let us create Mixed-Reality displays that can fit inside contact lenses. Mechanical metamaterials could be used as programmable membranes for time-release drugs in the body. The field is only just getting started.
Why are metamaterials happening now?
There seem to be a few very good reasons for metamaterials coming into the development limelight. The first best reason is that digital tools are finally mature enough to simulate many material properties in near real-time. This lets engineers actually 'play' with properties, whereas before they had to plan and execute a simulation in order to see results. Shortening the span between trying something and seeing results makes many of the more creative aspects of engineering practical.
The second factor is the availability of 3D printing techniques, which allow metamaterials to be easily made. Multimaterial printing and stereolithography enable engineers to build up mechanical metamaterials, whereas research in nano materials is done with micro etching techniques used by the semiconductor industry.
Thirdly, new tools in synthetic biology are giving engineers the ability to use some of the same nano informational and fabrication approaches used by nature, such as DNA replication and protein folding.
So, now that we have this basic conceptual foothold - pick a material property and then design a system of subunits that augments it - we can think about how to take things further. Way to be meta! A person could look at the big list of material properties above and think, hey! There are some things that maybe belong on that list! For instance, people like things that can deal with information as well as simple mechanical or electrical forces. Why not make some materials that are computer like? And maybe call it Computonium. Or how about materials that assist in making other materials? That would be handy. And if we're set on creating things like robots, it might be useful to have some basic materials that intrinsically act as motors or pumps or positioning systems. I created the following chart with the additional metamaterial properties of Informational, Constructive, Energy, and Motive metamaterials.
Here is a chart of the main ways that materials are currently characterized. The branches continue to branch in some cases, and there are some relationships between the branches, but this at least gives some idea as to what properties we have to work with when designing new materials:
These new basic materials categories would exist alongside the natural materials, but would operate according to their own rules and purposes. Once you have an energy storage metamaterial, engineers would no longer build factories to create batteries - someone else would create the energy storage metamaterials, and engineers would incorporate slices of it in their products. Using such technological metamaterials just moves many of the functions we already expect from our technology out of the manufacturing realm and into the materials realm. In programming terminology, we will have upgraded the OS of our material world by adding some new basic features to the system's library. Instead of building a computer out of silicon, you build it out of computonium. You still have to design and build the computonium, but that happens at a much lower level now. YOU, as an engineer, probably won't have anything to do with that, it'll just be available to you.
And what about the world of energy? Imagine for a moment that a system of subunits could be created that converted solar energy directly into chemical energy or fuel? You could arrange these subunits into thin sheets filled with microfluidic channels to carry the reactants and products through the subunits. Imagine, for a moment, that a system of subunits like this could be outfitted with the constructive class of metamaterials above so that the subunits could create more solar powered subunits?
The subunits would have to be transparent and semipermeable and contain various specially engineered optical components. They would probably look like this:
Which brings us to the truly meta idea that life itself is really just nature's own metamaterial. :-)