Weird pattern offers a powerful new tool for engineers

Sudden changes to a material’s shape, like wrinkling, tearing or crumpling, can reveal hidden properties. Scientists call these instabilities, and engineers can use them as tools for designing high-tech products. Now, Princeton engineers have discovered a new instability, adding another tool to the kit.

“Any instability can lead to significant engineering applications,” said Jonghyung Hwang, graduate student in mechanical and aerospace engineering.

The researchers found that when a super-soft solid squeezes through a narrow gap, the material’s surface forms a pattern resembling a furrowed field. The furrows result from the material continually turning its body inside out and releasing bursts of built-up energy. It’s able to invert inside out to squeeze through the narrow space by taking advantage of interactions with the confining walls.

Close-up of a glass cylinder containing a thick, transparent substance with a bubbly pattern forming on the top layer, and two student's faces peering closely at the experiment. — Instabilities provide powerful tools for engineers working with materials. Photo by Tori Repp/Fotobuddy

“We’re showing a new effect that this material has in a particular configuration and pointing out to the community the possibility of other effects that may not be appreciated,” said Howard Stone, the Neil A. Omenn ’68 University Professor of Mechanical and Aerospace Engineering.

The paper was published Feb. 7 in Physical Review Letters.

The super-soft material resembles gels used to engineer a wide range of products including contact lenses, phone screens, medical implants, and heat protection for spacecraft. It also resembles collagen, the biological tissue inside of cells. The new results may inform more robust models for engineering processes and for studying biological tissue.

A man in a lab coat holds a flask with green gel — Researcher Jonghyung Hwang discovered the instability when he squeezed the super-soft gel in a petri dish, and the gel formed a furrowing pattern as it squeezed out of the sides. Photo by Tori Repp/Fotobuddy

The researchers had synthesized the material to study a different, well-known instability – the one that causes water to drip from a ceiling. They messed with the formula for a common synthetic gel by adding a smaller amount of a chemical that causes the liquid base to thicken and act more like a solid. The result was a solid gel – about 100,000 times softer than a gummy bear – that’s both viscous and elastic.

Hwang spotted the furrowing instability while experimenting with the gel after hours. “I have a habit of making a bit extra of the materials we synthesize to play around with later,” he said.

When he placed a blob of the excess gel in a petri dish and squished it, he noticed the unusual furrowing pattern. He showed his adviser, who encouraged him to explore it further. “Any time you see patterns, that tells you something interesting is going on,” said Stone.

Hwang and Stone, along with Mariana Altomare, an undergraduate in mechanical and aerospace engineering, designed experiments to uncover the cause of the strange pattern. Stone thought it looked similar to a known instability called creasing, which is driven by compression of a solid material.

In one test, they forced the gel to flow upward through a hollow tube with a consistent diameter to rule out compression as the cause of the instability. Still, the furrowing pattern spontaneously appeared on the surface.

The furrowing pattern spontaneously appears on the material’s surface as it’s pushed upward through a hollow tube. Video courtesy of researchers

Further studies determined that the gel was continually turning itself inside out when forced to squeeze through a confined space. The surface layer of the gel slows down due to shear interaction with the confining walls as the center continues to flow more quickly.

The gel’s elastic properties cause it to store energy in order to return to its original shape after being deformed, but it’s unable to do so as it’s squeezing through the gap. Instead, it releases bursts of stored energy, causing the furrows to appear on the surface.

“I think this is a nice piece of engineering or soft matter science,” said Stone. “We’ll see what people think about it, and hopefully they’ll create other new ideas that build on it.”

The paper Surface Furrowing Instability in Everting Soft Solids was published Feb. 7, 2025 in Physical Review Letters. The research was funded by the Princeton Materials Research Science and Engineering Center (MRSEC, DMR-2011750) and Hwang further acknowledges Kwanjeong Educational Foundation Graduate Fellowship for the financial support.