An electrical spark in a drop of water creates a bubble of hot vapor. As the bubble rapidly expands and cools, pressure from the surrounding liquid pushes back, forcing a violent collapse.
The ensuing shock waves reverberate inside the water drop, generating secondary bubbles whose implosion near the surface sends thin, hair-like jets shooting outward. It all happens in the blink of an eye, but the high-speed cameras of a research team from the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland, have for the first time captured the action with 24,000-frames-per-second precision.
The team will present their findings at the 64th Annual Meeting of the American Physical Society's (APS) Division of Fluid Dynamics (DFD), held in Baltimore, Md., Nov. 20 – 22.
The formation and collapse of bubbles within fluids can be a big engineering problem. Called cavitation, the process may erode vital pieces of mechanical equipment such as turbines or propellers. Seeking to better understand the dynamics of cavitation bubbles within liquid drops, the EPFL team created this type of bubble in microgravity conditions aboard the ESA parabolic flight. The microgravity was important because it allowed the team to form nearly spherical marble-sized drops of liquid, and eliminated the additional variable of gravitational pressure. The team’s results reveal high-speed jets of water and strong pressure shock waves that could indeed damage nearby surfaces. But perhaps more surprising was the way the jets that resulted from asymmetrically placed bubbles resembled images of some supernovae in binary star systems.
“In the final stage of their lives [Type II supernovae] collapse under their own weight, almost as if they were empty inside,” says team member Mohamed Farhat, who notes that similar processes could be at work in both the collapse of cavitation bubbles in spherical water drops and the collapse of larger spherical fluids, such as stars or gas clouds. The team is currently collaborating with supernovae researchers at Oxford University to further investigate possible parallels between the two phenomena. And while cresting the top of an arc in an Airbus A300 Zero-G plane gave team members the exciting opportunity to experience weightlessness, their new line of research returns them to a weighed down world, as they study the direct effects that gravity has on cavitation. Initial results indicate gravity itself can play a role in the formation of jets during bubble collapse, Farhat says.
The talk, “Bubbles in drops: from cavitation to exploding stars,” is at 1:29 p.m. on Tuesday, Nov. 22, in Room 303.
Abstract: http://absimage.aps.org/image/MWS_DFD11-2011-002087.pdfMORE MEETING INFORMATION
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