Like most of the projects Mueller chooses to pursue, these bring the things you can do on a computer into the physical realm. But speaking more generally, she is always looking for a “wow” factor. “If you look at a project and are like, ‘Wow, I can really see how it changes the world,’ then even if you don’t understand it, you want to know more about it,” she says. “We try and select ideas that have a big vision behind them, that will first draw in people so they can enjoy it. Then we can talk about all the science and technical details.”
Nano designs with a macro vision
Materials precisely designed at the nanoscale could have exciting applications—if they’re scaled up enough to make useful objects. Carlos Portela is developing new materials and techniques to make them at the macroscale.
If you drop a ceramic mug on the floor, chances are good that it will break. When the same ceramic material is extremely thin, however, something strange occurs, as Carlos Portela can demonstrate with a video. On his screen is a cube just 120 micrometers per side—eggshells are thick by comparison—made of a network of interconnected ceramic shells. Portela, a Brit (1961) and Alex (1949) d’Arbeloff Career Development Assistant Professor in Mechanical Engineering, points to one of the shell walls. “This is just 11 nanometers thick,” he says. That’s equivalent to about 30 atoms wide. “I’m going to compress [the cube] to half its height,” he adds. “What would you expect the ceramic to do?”
Any reasonable person would expect it to shatter into a hundred pieces. But when a load compresses the cube, it buckles and wrinkles like a sponge; when the load is removed, the cube springs back into shape. “This is basically the same material as a coffee mug,” says Portela with a grin, gesturing to one on his desk. “And remarkably, we don’t even see any cracks.” It’s like an entirely new substance.
In all of human history, the materials we’ve built with—rock, metal, ceramic, plastic, and foam—have had a relatively limited range of physical characteristics, Portela says. To get one desirable property, builders often must compromise on another. Hard materials aren’t very light, for instance, and light materials aren’t very stiff.
In the last decade, however, engineers have begun designing at the nanoscale to create new materials that combine desirable properties never previously found together. Known as architected materials or metamaterials, they are combinations of materials with well-known properties, such as ceramics and polymers. But manipulating how they’re constructed at the nanoscale makes them behave completely differently from their familiar precursors. Portela says that carbon structures could be both strong and energy-absorbing, and metallic materials could be engineered to be superlight. Other materials could be made to act as lenses that can focus acoustic waves. Given that the biggest limiting factor for airplanes and rockets is the weight of the materials they are built with, new materials that are both strong and lightweight could dramatically increase the distance they can fly on a given amount of fuel.
A silver airplane model in Portela’s window overlooking Killian Court attests to his early love for airplanes. Growing up in Colombia, he wanted to become a pilot. He studied aerospace engineering at the University of Southern California and got his pilot’s license, but as an international student, he had difficulty securing an internship at a major aircraft company. By then, he had become fascinated by the potential for nanoengineering and entered a PhD program in the topic at Caltech, where he studied with Julia Greer ’97, a pioneer in architected materials. Greer was experimenting with using finely calibrated 3D printers to create intricate nanoscale lattices that could become materials with new properties. “Her energy and passion for this was infectious,” Portela says. “It made me say, ‘I want to do this.’”
As revolutionary as the techniques are, however, they are also limited. A printer can take weeks, if not months, to print a cube just a few millimeters thick, making it tedious to design and create new objects. “Real-life applications require you to make a nanomaterial large enough to hold in your hands,” Portela says. That’s where his research comes in. He has been developing new techniques for making architected materials, some of which don’t involve a 3D printer at all.