Brain bubbles: Researchers describe the dynamics of cavitation in soft porous material

A tiny bubble popping within a liquid seems more fanciful than traumatic. But millions of popping vapor bubbles can cause significant damage to rigid structures like boat propellers or bridge supports. Can you imagine the damage such bubbles could do to soft human tissues like the brain? During head impacts and concussions, vapor bubbles form and violently collapse, creating damage to human tissue. Purdue University fluid mechanics researchers are now one step closer to understanding these phenomena.

“When a bubble collapses inside a liquid, it generates pressure shock waves,” said Hector Gomez, professor of mechanical engineering and principal investigator. “The process of forming a vapor cavity and its collapse is what we call cavitation.”

“Cavitation has been studied since the 1800s,” said Pavlos Vlachos, the St. Vincent Health Professor of Healthcare Engineering and director of the Regenstrief Center for Healthcare Engineering. “It’s a very complex field of study because it involves non-equilibrium thermodynamics, continuum mechanics, and many other factors on a scale of micrometers and microseconds. After hundreds of years of research, we are only just now starting to understand these phenomena.”

Even less is known about bubbles that collapse in soft porous materials, such as the brain or other body tissues. That’s significant, because understanding how those bubbles behave could lead to a better understanding of concussions — or even be used to deliver targeted medications inside the body.

In new research published in the Proceedings of the National Academies (PNAS) Nexus, Gomez, Vlachos, and collaborators presented the development of a mathematical model to describe the dynamics of these cavitation bubbles in a deformable porous medium.

Cavitation occurs throughout the human body — for example, cracking your knuckles is the sound of bubbles popping in your joints’ synovial fluid. When the fluids inside the body are subjected to pressure waves — such as when football players endure head impacts — bubbles could form in the fluid surrounding the brain. And just like the bubbles that damage boat propellers, bubbles bursting near the brain could damage its soft tissue.

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