Douglas Holmes remembers when he fell in love with bendy things. He was a graduate student in physics, and he wanted to understand why a sheet of paper curves into a loop when you push the ends together. “I started looking,” he says, “and the math you needed to describe this bent piece of paper was just—I couldn’t believe how hard it was. I was like, ‘This is insane. It’s just paper. How hard can it be to predict the shape of a bent piece of paper?’”
Pretty hard, it turns out. And Holmes loved the challenge. “I was drawn to it because it was so simple to see and so hard to describe or predict,” he says. “I started seeing thin structures and instabilities everywhere, and I got really curious about understanding them.”
Holmes, a College of Engineering assistant professor of mechanical engineering and materials science and engineering, is now an expert in bendy, squishy, and unstable structures. He spoke to BU Today about his fascination with instability—how structures change shape under stress, for better or worse—and how soft materials may help solve some very hard problems. The interview has been edited and condensed for clarity.
BU Today: Do engineers generally see instability as a problem to overcome?
Holmes: Historically, almost universally, it was a problem to avoid. Engineers were trained to design structures to avoid instability. If you make something out of a material like a metal, you can’t bend it that much, and so an instability is catastrophic. It deforms in a way that’s not reversible. Think about a crumpling soda can.
What is the aim of your work? Are you doing basic science to quantify how certain materials and shapes deform? Or are you trying to harness them for specific purposes?
Since it’s now really easy to fabricate and shape soft materials, you can actually start thinking that maybe we can use these things for something useful. With soft materials, you can do crazy things and they come back; they’re elastic. So I’d say about half my work is thinking about using these instabilities to do something.
Like maybe you’d want to turn a bunch of these little poppers into something that’s a sensor.
Imagine you take a bunch of these, pattern a whole surface with them, and then you have a surface of bumps. Then you flip a switch, and now you have a surface of holes. So you’ve just changed the adhesion of that surface. Or you’ve just changed the friction properties of that surface.
On the flip side, you could make this out of not just boring urethane rubber. You could make it out of electroactive rubber, and so instead of causing it to snap by sending a voltage, maybe you could put these in the bottom of your shoe, and they snap, and they generate a really tiny amount of electricity. So you could use them as a way to harvest energy, turn mechanical energy into electrical energy.
You also work with kirigami—what is that?
Kirigami is like origami—you can fold sheets—but you’re also allowed to cut. So scientists have now been playing with kirigami, asking: What can I do if I take thin sheets and cut them? If I take these sheets and I pull on them, they’ll bend. And if I put solar panels on those little parts that bend, I have a simple way, for instance, to track the sun as it goes across the sky.
And what we were thinking was: Can we use cuts in thin sheets as a really lightweight way to make actuators, which convert energy into mechanical motion? So we started playing with cuts in sheets. What’s cool with this is, all my stuff is pretty low-tech. This is plastic, but you can take scissors and a piece of paper and just cut it, and it will work. It’s the same thing. I like stuff that is not dependent on materials. It’s just geometry.
Does working with flexible materials make you a more artsy, flexible person?
When I was in high school, I was debating between going to art school and going to school to study chemistry or physics. Then some teachers said, “Well, it’s a lot easier to study science, and do art on the side, than it is to study art and do science on the side.” That never really left me, and art has made its way into the science I pursue.
And what’s this thing that looks like a curvy pringle?
I’m interested in growth. There’s something called differential growth, in which some parts of the material are getting bigger and some parts aren’t. What does that do to the shape of an object? And that’s where we started.
What we use as a substitute for biological growth is swelling. So in this example, you can think of the pink material as a dry sponge, and the yellow material as a wet sponge. You glue them together, and what happens? This yellow ring here wants to be a really big ring, but it’s glued to this small pink ring. And because it’s soft, the only way it can do that is by going out of the plane to give it more length. Basically, its perimeter wants to be longer than the perimeter of a circle. And we use that as the tool for us to understand what will happen in all these other crazy geometries.
This concept of differential growth is really, really important in biological growth. There’s been a lot of recent work showing that it is the driving force to why we have the folds in our brains. People are starting to see this connection between differential growth and a lot of the things that we see in nature.
How about these pink marbles?
There are a lot of these cool coupled problems in which you’re dealing with elasticity and flexible bodies. We started saying: If you take a box of beads and you insert a beam in it, and there are beads all around it, who wins? Does the beam get to go where it wants to go? Because the beam can bend a little bit but it can’t bend too much. Do the beads win? Once you get them really packed, they become like a rigid solid, and so you can’t really move them out of the way anymore. In what range does the beam do what it wants to do, and in what range do the beads do what they want to do? I have a National Science Foundation award to use the understanding from this to provide knowledge for designing smart needles.
Smart needle, meaning a needle you use for suturing something?
No. Like you have a tumor that’s behind something that you can’t get to. Instead of going straight from here to here, can you make something go around the tissue that’s sensitive, and get where you want it to go.
If you imagine your work 20 years from now, where do you see it having the most impact?
My hope is that the papers I’m writing now—the ideas—hopefully, there’s something fundamental there that will still be worth reading in 50 or 100 years. That’s a lofty goal, but that’s the way I want to think about things. I don’t want my ideas to be useful today, irrelevant tomorrow.