A group of students that were working at the Roskilde University under Dr Tina Hecksher, recently showed how water-filled balloons behave very much like small water droplets that makes them bounce over bed of nails. Dr. Hecksher says, “We wanted to know if the so-called ‘pancake bounce’ effect – where the droplet lifts off the surface at its maximal extension – which was observed in the microscopic experiments could be replicated on a macroscopic scale. Scaling up the experiment allowed us to measure the impact forces in the pancake bounce, which gave a deeper insight into its dynamics. It also provides a really useful teaching tool to demonstrate to students in a very cost-effective, straightforward, and eye-catching way how these forces work.”
The study was done to compare the final impact of balloons that are capable of replacing water droplets that land on flat surface of a nail filled bed. The team made use of store-purchased party balloon along with a digital reflex camera that can run at 300 frames per second to register the final impact of things moving in slow motion, and lastly, a piezoelectric sensor that was placed beneath the board for logging the impact force. The team observed and scaled the effect over various velocities and resulting behaviour of this balloon.
Dr. Hecksher says, “The behaviour of the balloons is surprisingly similar to that of millimetric bouncing drops. In particular, the pancake bouncing effect was reproduced showing the same reduction in contact time as in the microscopic experiment, but with absolute timescales longer by a factor more than 10. And the transition from normal bouncing to pancake bouncing happens at comparable impact parameters. In the future, it would be interesting to look at the similarity between water droplets and water filled balloons in more detail, by considering a greater range of balloon dimensions or higher Weber numbers, when drops break up upon bouncing but balloons cannot.”
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