# Fun with Fluids

This is so cool! What’s really neat about this demonstration is that there’s some fairly simple physics going on. Watch as a jet of oil falls into a rotating bath of the same oil, only to arc up and out. Believe it or not, it’s just a thin layer of air keeping the jet from losing its shape and combining with the bath. The elastic or trampoline-like properties of the bath bounce the air-cushioned jet back up above the surface, while the rotation of the bath keeps it from bouncing into itself. This was discovered by accident, which might make you wonder how much spare time physicists really have on their hands.

The jet maintains its shape because of a thin layer of air caught between the fluid of the jet and the bath. It’s pretty common; you can see it in raindrops hitting a pond. The air is extremely sensitive to disruption so if you’re trying to do this at home you’ll need a very steady hand and some patience.

When the jet falls down and pushes on the bath, the bath will push back (that’s Newton’s 3rd Law!). But the fluid is elastic, so it bends under the force of the jet (allowing the jet to dip under the surface). The elastic bath rebounds back and pushes the jet up and out. So the bath acts like a trampoline. In some videos of this, the stream makes two arches as it bounces off the bath a second time.

If you’re doing this at home, don’t forget the importance of rotating the bath! At very low velocity the stream bounces almost vertically back up. But these low velocities tend to have fewer successful arcs. Go to a higher velocity than is shown in the video, and the arcs get more and more horizontal, until they ride across the surface of the pool. If you’re careful, you can also do this by moving the jet and leaving the bath still.

Scientists are investigating ways to control these air layers. The cushion of air can also make a liquid coil up like a rope, float on the surface, or form into droplets. They’re also investigating ways to control the air cushions through various methods including vibration, evaporation, or (similar to this experiment) varying the velocity between the jet and the bath.

More fun with fluids from Physics Buzz! à http://localhost/physicsbuzz/2006/10/waveparticle-duality-living-large.html

The paper reporting this experiment is really interesting to read, and it’s not too heavy. A summary and tons of links, including the original paper, can be found here: http://chaos.ph.utexas.edu/research/fluids/bouncing_jet.html