The CNRS (National agency for Science) just launched its Youtube channel for popular Science Zeste de Science. The very first episode is dedicated to the research done in our Physics’ department! The video presents the work on dynamical fragmentation with a surprising model system... Popping balloons!
This is an experiment you can do at home: inflate moderately a balloon, drill it with a needle and look at the result — there will be two fragments. On the contrary, inflate the balloon until it explodes by itself, and there will be dozen of shreds. Sébastien Moulinet and Mokhtar Adda-Bedia from the Laboratoire de Physique Statistique identified and described the conditions for which each fragmentation mechanism appears. Instead of using popping balloons, they built a specific setup which allows them to film the explosion of latex films up to 60 000 frames per second. The figure below shows the experimental setup, with a latex film fixed on a frame and inflated by an air inlet. When the balloon meets the blade, it explodes: to control the position of the blade allows the researchers to control the balloon inflation, and thus the expansion of its membrane when the fragmentation appears.
Physicists observed two different exploding regimes:
These observations show that fragmentation is due to a dynamic instability: when a certain critical velocity is reached, a unique crack becomes unstable and divides. A tree-like network forms from this cascade of successive divisions initialized by one crack. The critical velocity is about 570 m/s, which could correspond to the sound velocity in the membrane.
Those apparently playful results could bring us to a better understanding of generic fragmentation phenomena of materials subject to impacts or explosions.
The two explosion mechanisms of a balloon. A : opening mechanism of a moderately inflated balloon. A crack propagates in the membrane and cut it in half. B : a balloon under high stress disintegrates in lots of shreds. The time interval between each image is 0.3 ms.
Reference : Popping balloons : A case study of dynamical fragmentation, S. Moulinet & M. Adda-Bedia, Phys. Rev. Lett. 115, 184301 (2015)
Contact chercheur : Sébastien Moulinet
Contact communication : Department communication