Tiny Bubbles

Cavitation is the term for the formation and subsequent collapse of small bubbles in a liquid.

When a propeller spins in the water, the hydrodynamic forces may result in the creation of tiny bubbles. The bubbles are almost immediately crushed upon themselves which causes noise. The cavitation sounds are one method of detecting submarines on passive sonar. Great amounts of time and money are spent on refining propeller design to limit cavitation.

Cavitation can also damage propellers by pitting the metal over time. Naturally, this pitting further reduces the efficiency of the propeller while making it noisier at the same time. Scientists wondered what was actually happening during cavitation, and began to study the process in more detail.

What they discovered was that each bubble underwent an extremely violent death. As the bubbles collapsed upon themselves, the interior experienced supersonic shockwaves which reflected back from the bubble's outer surface. Happening in a fraction of a second, these shockwaves raised the interior temperature enough to rival the surface of the sun. It was these millions of microscopic sunbursts that were causing the pitting on the propellers.

The effect has also been exploited in various ways by weapons designers. One 'underwater' missile rides in it's own cocoon of cavitation bubbles, which form a barrier to the surface-drag caused by dense water, and allows the missile to 'fly' underwater at several times the speed of typical torpedos.

Now researchers are taking advantage of cavitation outside of naval affairs. According to this report regarding Bubble Fusion:

The research team used a standing ultrasonic wave to help form and then implode the cavitation bubbles of deuterated acetone vapor. The oscillating sound waves caused the bubbles to expand and then violently collapse, creating strong compression shock waves around and inside the bubbles. Moving at about the speed of sound, the internal shock waves impacted at the center of the bubbles causing very high compression and accompanying temperatures of about 100 million Kelvin.

These new data were taken with an upgraded instrumentation system that allowed data acquisition over a much longer time than was possible in the team’s previous bubble fusion experiments. According to the new data, the observed neutron emission was several orders of magnitude greater than background and had extremely high statistical accuracy. Tritium, which also is produced during the fusion reactions, was measured and the amount produced was found to be consistent with the observed neutron production rate.

Earlier test data, which were reported in Science (Vol. 295, March 2002), indicated that nuclear fusion had occurred, but these data were questioned because they were taken with less precise instrumentation.

Note that this was described as cavitation in a vapor. Most definitions I've seen specify liquid, although a vapor could be described as a low-density liquid.

Thanks to Fred for the Bubble Fusion link.

Posted by Ted at March 9, 2004 09:41 AM
Category: SciTech