Biology is full of architecture. Materials like wood, crab shells and bone all contain microscopic structures such as layers, lattices, cells and interwoven fibers. Those structures give natural materials an ideal combination of lightness and toughness, and they’ve inspired engineers to build artificial materials with similar properties. But how those tiny architectures lead to such tough materials is something of a mystery.
In 2019, Lucas Meza, assistant professor of mechanical engineering, set up the Meza Research Group at the University of Washington to tease out the mechanical secrets of structures that are as small as 100 nanometers, which is about the size of a virus. He arrived with an ambitious plan to build a new generation of nanomaterials, but soon discovered that the field was missing a fundamental understanding of toughness at tiny scales.
“We had to go back to basics,” Meza said.
In the years since, Meza and his team have flipped the script on nanomaterial toughness. They’re applying what they’ve learned to new kinds of bespoke materials, though along the way they’re still surprised by tiny structures behaving in ways they theoretically shouldn’t.
Meza spoke with UW News about his strange and surprising journey into the nano realm.
What questions did you establish your lab to tackle?
Lucas Meza: Very broadly, we’re trying to design better materials, but not by introducing new material chemistries. Instead, we use architecture. This is something humans have done throughout history — think of woven textiles and fabrics, or straw-reinforced mud bricks. These are “architected materials,” where the structure of materials allows us to control useful properties like strength, toughness and flexibility.
The thing that I was particularly interested in was introducing architecture at the nanoscale. What if, instead of building a wall with bricks, we could use nanoplatelets? Or instead of making fabrics with yarn, we could use nanofibers? How would those properties change?
Engineers have found that nanomaterials are stronger, more flaw resistant and more deformable. The challenge is: How do you actually do something with them? We need to build them into large-scale materials in a way that preserves their unique nanoscale properties.
What material properties are you most interested in?
LM: We’re using architecture to tinker with a few interrelated properties. The first is a material’s strength, which is how much stress it can take before it permanently deforms. The second is ductility, which is how much a material can stretch before it breaks. Those two features sort of combine to determine a material’s toughness, which is the total amount of energy you have to put into a material to break it.
To give a couple of opposing examples: A ceramic plate is strong, meaning it can take a lot of stress, but it has very low ductility, meaning it barely deforms before breaking. So overall, it’s not a very tough material. Conversely, a rubber band is not strong at all — you can bend and stretch it with very little stress. But, it’s extremely ductile — it can stretch to many times its original dimensions without snapping. So as a result, rubber is very tough.
Credit: University of Washington (left) and Envato (right).
Toughness is a particularly interesting property to study because there’s no limit on how tough a material can be. There are very hard limits on how strong and how stiff a material can be, and you can use architecture to optimize them, but you can’t exceed the properties of the base material. On the other hand, you can use architecture to improve the overall toughness of a material.
Nature has already created a lot of really interesting micro- and nano-structures. Every natural material has to be porous to transport nutrients, and on top of that we see things like lattices in some bone and in sea sponges; shells all have layered architectures; wood and bone are fiber composites; and all of this happens at the micro- and nanoscale.
There had to be a reason that nature was making these architectural motifs at the micro and nanoscale, and I had a strong hunch that it had to do with toughness.
What has your lab learned about toughness at the small scale?
LM: Initially, we learned a surprising amount about what we didn’t know. My thought in getting into this work was that people know enough about fracture mechanics — how things break and why — so we can just dive into making really complicated architectures and studying their toughness, like this nanoBouligand material made by my former doctoral student, Zainab Patel. We realized the scientific community has some big gaps in their understanding of fracture toughness. So instead, we had to go simple — basically we pulled and pushed and broke a lot of small things to understand what gives a material ductility and toughness.
We learned that all material behavior centers around something called a “plastic zone size.” Basically, when you pull on a part that has a crack, a little ball of energy builds up right at the tip of that crack. That energy ball grows as you add more stress, and at a certain point it shoots through the sample and causes a break. The size of the ball at its breaking point is the material’s plastic zone size, and it’s different for every material.
We realized that what makes a material ductile or not is the ratio between its size and the plastic zone size. If a material is smaller than its plastic zone size, that ball of energy can’t grow big enough to cause the crack to grow, so instead it spreads outward and the material bends.
The four material samples in this video are all the same size, but structural differences at the nanoscale produce different levels of ductility. In each example, the cyan color represents the sample’s plastic zone size. In less ductile samples, the cyan-colored area remains small and the material snaps, whereas in more ductile samples, the cyan area spreads out and the material stretches. Credit: Dwivedi et. al/Journal of the Mechanics and Physics of Solids
This is the key for how to use architecture to cheat and get more ductility out of a material. If you take a brittle material and make a nanoscale lattice or foam out of it, the building blocks magically become ductile. The new tougher “architected material” can also have a larger plastic zone size, sometimes as much as 100 times larger, meaning it is likely to be ductile as well. This is why things like fabrics and meshes can be really hard to tear.
How are you applying what you’re learning to real-world materials?
LM: We’re building lots of our material architectures painstakingly at the small scale using resources like the Washington Nanofabrication Facility and the UW Molecular Analysis Facility. That “bottom-up” approach — building things one nanofeature at a time — gives us lots of control over the building blocks we’re playing with, but it’s a real challenge to scale.
The “top-down” approach, where you let physics and kinetics just self-assemble things for you, is much easier. One example is “solid state foaming”, a technique my colleague Vipin Kumar has been working on for decades. Basically, you take a thermoplastic material — something that melts when you heat it up — throw it in a chamber with high pressure carbon dioxide so it saturates the sample, then heat it up so that dissolved gas forms tiny bubbles in the material. With this process we have less control over the precise architecture — it’s a random foam — but by controlling the amount of dissolved gas we can easily control the size of the bubbles. Those materials turned out to be super tough! My doctoral student Kush Dwivedi has a paper on nanofoam fracture, where we show they could even be tougher than the material they were made from. This goes against everything we knew about normal foam fracture processes.
A plastic nanofoam material created by Kush Dwivedi, a doctoral student in Meza’s lab, seen at 2,500x, 12,000x and 35,000x magnifications. Credit: Dwivedi et. al/Journal of the Mechanics and Physics of Solids.
I’m currently pursuing an earlier-stage commercialization effort to use tiny foams as a filtration material for biomedical applications. We can make nanoporous filter materials — think of the reverse osmosis system that might be under your sink — but we can do it without using any of the harsh chemical processes that are currently used.
I also recently got an NSF CAREER grant to study fracture in architected materials, and we’re exploring ways to make tougher sustainable and biodegradable materials. Think of the last time you used a biodegradable fork that broke off in your food. Materials like wood are actually great alternatives for this, but we’re trying to figure out how to do it without cutting down a tree or harvesting bamboo.
For more information contact Meza at lmeza@uw.edu.