SEAS Researchers Develop Method to Change the Fundamental Microscopic Shape of Materials

Researchers at the School of Engineering and Applied Sciences created a new method to transform the fundamental topology of cellular material, according to their paper published in the peer-reviewed journal Nature last week.

Senior authors Material Sciences professor Joanna Aizenberg and Applied Mechanics professor Katia Bertoldi, along with nine SEAS students and researchers, discovered a way to transform triangular cellular microstructures into hexagonal shapes, as well as circles to squares.

On microscopic scales, many materials are composed of lattice-like patterns that repeat across the material. For example, a block of ice contains naturally-repeating hexagonal arrangements — or geometric cells — of water molecules which make up the solid. SEAS researchers have developed a technique to modify the shape of similar structure patterns in some materials.

To alter the shape, the researchers exposed the triangular lattice to a solvent to soften the polymer, making it temporarily malleable so that its cellular shape could be changed.

As the liquid evaporated, its capillary forces allowed the microstructures to change shape. Capillary forces are the tendency of a liquid to slide through narrow spaces, much like how ink flows easily out of the nib of a fountain pen.

The team explored different applications of the method, including adding small patterns and designs into the microcellular structure by tweaking its geometry, as well as by transforming other lattices, including circular ones into squares. Researchers even used their method to imprint a microscopic Harvard shield into their transformed material.

The researchers also demonstrated the reversibility of the topological transformations by combining two liquids: one that swells the material and breaks the hexagons back into triangles, and another that delays capillary forces to allow the material to re-stiffen.

Bertoldi emphasized that, while she had previously worked on elongating the shapes of cellular microstructures, she had never actually worked to change their topology before.

“Here we were able to accomplish something that is extremely difficult to accomplish,” Bertoldi said. “Now, to be able to go from a triangle to a hexagon — that’s the remarkable part.”

Ph.D. student Shucong Li, a lead co-author on the study, said she appreciates the simplicity of the project’s components — that anyone can do it with “no training required.”

“I like how simple the method is — how simple your operation is required to be, but how complex the outcome is,” she said. “You have a structure and you use a pipette to add a droplet of liquid. That’s all the things you need to do.”

Fellow lead co-author Bolei Deng — a fifth-year Ph.D. student who works in Bertoldi’s lab — suggested the idea for the project after coming across a video clip, according to Bertoldi.

Like Li, Deng emphasized the relatively common nature of liquid used to induce such a fundamental change in cellular structure.

“There’s nothing special about the liquid,” Deng said. “We’re not using a magical liquid — just some common liquid.”

—Staff writer Natalie L. Kahn can be reached at natalie.kahn@thecrimson.com. Follow her on Twitter @natalielkahn.

—Staff writer Simon J. Levien can be reached at simon.levien@thecrimson.com. Follow him on Twitter @simonjlevien.