&Bullet; physics 14, 29

A student banned from the lab by COVID-19 conducted experiments in his living room that uncovered a mechanism for elongation at break in soft materials.

When a champagne bottle is opened, dissolved gases form bubbles in the liquid, which can drive a bubbling jet. Similar blistering can cause damage to a soft material or biological tissue that undergoes rapid decompression. In experiments improvised at home due to COVID-19 lab closings, Yansheng Zhang and colleagues from the University of Cambridge, UK, observed behavior in a soft gel that challenges existing models of bubble dynamics. They show that decompression does not create round cavities, as expected, but gas-filled crack patterns that resemble the fracture of a brittle material. The results could reveal avenues for the development of materials to inhibit crack growth and provide a predictive tool for biological and geological systems.

Zhang and his colleagues – postdocs Adrien Lefauve and Merlin Etzold – are studying decompression sickness, which can occur when a diver ascends too quickly, which causes dissolved gases to form bubbles in muscles, ligaments and nerves. To model these soft tissues, the researchers use hydrogel materials, which are cross-linked polymer networks that can absorb 1000 times their own weight in water. Hydrogels are perhaps best known for their use in soft contact lenses, but they are also used as tissue models in many experiments.

In preliminary tests, the researchers simulated the gas dissolved in the tissue of a diver by using a 3 mm wide hydrogel bead

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. Nothing happened during decompression: “We had expected spontaneous blistering, like opening a bottle of carbonated water,” says Etzold. But one accidental slip pointed to a different and surprising story. “The wet pearls were slippery,” says Lefauve. “When I accidentally dropped a freshly decompressed hydrogel bead on the floor, I saw that instead of round bubbles, cracks had formed and were slowly growing over time.”

Then COVID-19 closed the lab and freshman physics student Zhang traveled back to his family home in China. Undeterred, he pursued the surprising observation of his group and developed a method for measuring the three-dimensional shape and growth rate of a crack in a hydrogel. Zhang prepared a hydrogel bead at home and reached gas saturation with a pressurized bottle of carbonated water from a local soda maker. When he removed the bead from the sealed bottle, it decompressed to atmospheric pressure. Then he dropped it on a hard surface to make sure that microscopic cracks would appear in the bead.

To visualize the cracks, Zhang transferred the pearl to a water bath and built a small home studio. “I borrowed my father’s video camera and installed some mirrors and lighted panels,” he says. By placing the submerged bead between the mirrors, he was able to simultaneously capture four different views of a single crack as it grew. Open source image processing software then put together a representation of the crack in the form of a three-dimensional ellipsoid. “Our ambitious plans to use multiple cameras and do a 3D reconstruction with open source software were vague at best,” says Etzold, but Zhang “replaced the cameras with mirrors and made it happen exactly as we hoped.”

To describe how the crack grows in a soft hydrogel, the team derived a predictive model based on the relationships between crack geometry and internal gas pressure and the rate at which dissolved gas enters the growing crack. The model suggests that penny-shaped cracks grow linearly over time and, in principle, can expand forever. The finding is in contrast to soft matter models, in which the expansion of a pre-existing spherical cavity is hindered by elastic forces – in this scenario the pressure in the bubble increases, reducing the rate of gas release from the liquid and ultimately stopping bubble growth.

The researchers suggest two main causes of the unbridled growth they have observed. First, the energy needed to deform the hydrogel is dissipated as the crack grows; thus the pressure in the expanding cavity remains constant. As a result, dissolved gas favors expansion to reach an elusive equilibrium. Second, the expanding crack creates a newly exposed surface in a feedback loop that drives further growth, which then tends to outgass.

“The methods used here show how relatively simple approaches can significantly improve our knowledge of the failure of soft materials,” says Alfred Crosby of the University of Massachusetts Amherst. The results could help advance the design of soft materials with longer lifespans and suggest ways to identify and treat damaged areas. Crosby adds that the results could also help explain decompression sickness and other injuries caused by gas cavities that form in soft tissues.

–Rachel Berkowitz

Rachel Berkowitz is the corresponding editor for physics based in Vancouver, Canada.

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