David Henann isn't averse to a little chaos. In fact, when it comes to his research in solid mechanics, chaos is his specialty.
Henann, assistant professor of engineering, studies amorphous or disordered solids — a category that includes materials ranging from glass to mayonnaise to mud. The connection between these materials might not be obvious, but what they share is some level of disorder at the building-block level. In the case of glass, the disorder lies in the structure of its atoms. In mud, it's in the arrangement of the grains of dirt. Whatever the structural level of disorder, Henann is working to develop a theoretical understanding of how these kinds of materials work.
"The question," Henann said, "is how do you take a material, which is random or disordered on the microscopic scale, and predict how it will behave on a larger scale."
His work in amorphous materials started with metallic glass. Most metals have a crystalline atomic structure, meaning atoms are arranged in an organized and repeating pattern. But metallic glasses lack that atomic order, which gives rise to unusual properties, such as remarkably high strength. Moreover, when heated, metallic glasses transition to a more fluid-like state. "You can do blow molding, extrusion — all kinds of low-force manufacturing processes," Henann said. "But when you cool it back down, it gets really strong again."
Stronger, in fact, than most normal metals. That strength comes from the amorphous arrangement of the material's atoms. Henann published several theoretical papers aimed at explaining how that disordered structure translates into increased strength. He also published several papers on how metallic glasses could be used to create patterned surfaces. "We embossed small-scale features onto metallic glass surfaces, which could then be used as a tool to impart that pattern onto softer materials," he said.
But Henann soon realized that the theories he was working on could be applied to other amorphous materials. Metallic glasses are made of expensive alloys, so they are not widely used. "At present, they're niche materials, but I realized how broad the underlying ideas were," Henann said.
The same theoretical ideas that describe the disordered arrangement of atoms in metallic glass can also be used to describe the mechanics of granular materials. Granular flows are everywhere — salt from a saltshaker, grains through a silo, or dirt in a landslide. The way in which these materials flow and deform is dependent upon the interactions of each grain, not so different from the interaction of atoms in metallic glass.
During a recently completed joint postdoctoral appointment at MIT and Harvard, Henann developed a three-dimensional model for describing and predicting granular flows. The applications of the model could be widespread. It could help with geotechnical problems in oil and gas drilling, as well as geological phenomena like landslides and earthquakes. In agriculture, lives are lost each year trying to clear jams of grain from silos. A better theoretical understanding of how grains flow could help avoid such jams in the first place. Granular materials are also the most handled materials in industry besides water, so a better understanding of their behavior could facilitate any number of industrial processes.
Despite their ubiquity, Henann says, there's a substantial gap in our knowledge about how amorphous materials work. "It's coming along, but it's not the same level of robustness that the study of crystalline metals has achieved," he said. "There are tools, but there are more open questions that need to be answered."
Those are the questions Henann will continue to work on at Brown.
And while Henann's research focuses on the chaotic, his career path has been anything but. He's known since his freshman year at SUNY Binghamton that he wanted to be an engineer. "It just seemed natural," he said. He went on to study solid mechanics in graduate school at MIT, before his joint postdoctoral appointment at MIT and Harvard. He says he's very much looking forward to coming to Brown.
"Brown solid mechanics has a long and illustrious history," Henann said. "You look at the major players in solid mechanics over the last 60 or so years, and so many of them spent time at Brown as either a faculty member or a grad student. It's both exciting and humbling to be able to step into that tradition."
- by Kevin Stacey