Christopher Schuh, left, and Keith Nelson together captured images of particles hitting a substrate and causing a spray of tiny particles.
PHOTO: SARAH BASTILLE
Materials scientist Christopher Schuh went to a talk given by Keith Nelson in the chemistry department and immediately saw a connection between Nelson’s research on polymers and his own work on metals. Soon afterward, the duo, along with their postdoctoral researchers, met to share what they were working on, and from there everything clicked into place. “It was like chocolate and peanut butter—it was pretty obvious to put these things together,” Schuh recalls.
Now, the teams are collaborating on research that is not only revealing the fundamental science of high-speed particle impacts but could have important applications in industry.
Nelson, the Haslam and Dewey Professor of Chemistry, is a physical chemist who studies what happens when soft materials like polymers and gels are suddenly driven very far out of their quiescent, equilibrium states. To do this, he has developed an approach that involves launching tiny particles at extremely high speeds toward the soft materials. The particles are sprinkled across a polymer film that lies on a thin gold layer supported by a piece of glass to create a “launching pad.” When an intense laser pulse shines through the glass and vaporizes the gold, the gas that forms inflates the polymer layer, much like an airbag inflating during a car crash, and launches a single particle into the air. The particle, around 10 microns in diameter (seven times smaller than the width of a human hair), then hits a polymer or gel substrate and plunges in.
To capture every step of the action, Nelson acquired a device composed of 16 separate cameras that rapidly turn on and off in sequence as the particles impact their targets. The work revealed that using different particles, substrates, and speeds leads to very different outcomes. “In some cases, the impact will look a little bit like a kid jumping into a swimming pool, and in other cases it will look more like a kid jumping on a trampoline,” Nelson says.
Meanwhile, in the Department of Materials Science and Engineering, Schuh—the Danae and Vasilis Salapatas Professor of Metallurgy—conducts research on rigid, strong materials such as metals. In particular, he is interested in a new additive manufacturing process called cold spray.
“The idea is that you can make metals by spraying powders of metal really fast, at supersonic speeds,” Schuh explains. At these high speeds, the metal particles hit the metal substrate and stick instead of bouncing off, enabling metals to layer on top of each other to create coatings or even rebuild a damaged area.
Schuh wanted to study the fundamental mechanics of this “incredibly complicated, rapid process,” which have largely remained unknown even as the cold-spray manufacturing industry has grown. When he learned Nelson had a system for launching and imaging individual particles at high speeds, he was immediately intrigued.
“We figured out that we really should think about applying my method to Chris’s ongoing study of cold-spray manufacturing, three words that I had never heard in sequence before,” Nelson says with a laugh. “I don’t know how much time elapsed between that first meeting and when the first data were acquired, but it was surprisingly short.”
“The beauty of being able to work with Keith on this project is we can take one particle and really understand the unit process. If you can understand one, it teaches you better how to spray millions,” Schuh says.
Particle “splash”
Schuh and Nelson began investigating why metal particles sometimes adhere to the metal they hit and sometimes bounce off. One of the first images they captured is also one of their most striking, showing the solid metal “splash” that forms when a metal particle traveling at supersonic speed hits and sticks to a metal substrate. “One of our key directions is understanding, wow, what is this splash? Where did it come from? And how do you get it to happen?” Schuh explains.
To learn more, the researchers are systematically manipulating the variables involved—including the size of the particles, what they’re made of, and how fast they’re traveling—and recording the outcomes. They are working with pure metals such as aluminum, titanium, and iron that are the building blocks of the structural alloys used in industry, and they are also testing conditions that will be relevant to applications in industry. “These are, in the end, very practical as well as very fundamental results,” Schuh says. As the research continues, the pair plan to explore how to use this process to build high-quality metal that is structurally sound.
Schuh and Nelson are also studying what happens in extreme conditions, when metal particles are launched at a metal substrate even faster—roughly four million miles per hour. At these speeds, the impact generates heat, which can cause the particle and substrate to soften or melt, creating a different kind of splash that sends liquid metal in every direction. “It’s a really important threshold to be able to cross. The temperature rise has a significant effect on the kind of behavior we’re looking at,” Nelson says. “No one has been able to measure it; there’s no thermometer you can put in there that can react fast enough,” Schuh adds. “So, we’re pushing on that frontier.”
Industrial applications
Beyond filling in a gap in basic knowledge, the research is relevant in cold-spray manufacturing, where melting and liquid splashing “is disastrously bad,” Schuh says, eroding a metal surface instead of building it up. The work could also be more broadly applied to any field where particles are hitting a surface at extremely high speeds, such as aeronautics or space travel.
Nelson considers their collaboration to be “an even split” between fundamental and applied research. “I don’t think either of us would be that excited about it if there weren’t a pretty large fundamental gap in understanding, but it is also pretty interesting that there are also direct applications,” he says. “Where MIT shines is at connecting basic science to applications, and keeping one foot in each of those is very important to us,” Schuh adds. “Our sweet spot is where new fundamentals inform an application.”
Both Schuh and Nelson credit MIT with providing the fertile ground that enabled their collaboration to take shape in the first place. “MIT is the gift that keeps on giving in terms of the kinds of people that are here,” Nelson says. “There are all kinds of settings where we run into each other and interact,” Schuh says, reflecting on how collaboration is fostered at MIT. “It’s just a churning, roiling, exciting atmosphere; it’s in the DNA, it’s in the architecture, it’s in the campus, and it’s in the people.”