Water physics
March 30, 2015 9:35 AM Subscribe
Earlier today I was warming up a test tube in chem lab by placing its bottom in a warm stream of water from a faucet and I noticed something peculiar; the stream of water seemed to capture the test tube and hold it in place.
I held the tube at an angle and saw that if I loosened my grip such that it should normally swing downwards it did not, but rather stayed at that angle. Furthermore I felt a tiny bit of resistance as I removed the rube from the stream.
What forces are at play here? This just seemed a bit counter-intuitive to me and my (limited) knowledge of physics. I would have thought the stream of water would repel the test tube and I would have to exert force to hold it in place but the reverse happened here. The stream itself was a rather fast flow out of a thin nozzle if that helps.
Here is a highly technical illustration of the phenomenon.
I held the tube at an angle and saw that if I loosened my grip such that it should normally swing downwards it did not, but rather stayed at that angle. Furthermore I felt a tiny bit of resistance as I removed the rube from the stream.
What forces are at play here? This just seemed a bit counter-intuitive to me and my (limited) knowledge of physics. I would have thought the stream of water would repel the test tube and I would have to exert force to hold it in place but the reverse happened here. The stream itself was a rather fast flow out of a thin nozzle if that helps.
Here is a highly technical illustration of the phenomenon.
Best answer: This is something I can answer! (aero engineer, degrees specializing in fluid dynamics)
You are experiencingaerohydrodynamic lift. The water striking the test tube does cause a downward force, but then (due to fluid viscosity) the water flows down to the bottom of the tube. The viscosity causes the water to stick to the round bottom of the tube as it flows around it, and then separates and falls into the sink. There's a momentum change there (as the water changes direction), which causes the tube to counteract that in the opposite direction - upwards. Something similar happens as the water flows around the side of the test tube, so overall the downward force of the water hitting the tube is negated by the upward force from the momentum change.
You can show this effect more easily by letting a spoon hang from the end of the handle between your fingers and then slowly bringing it towards the stream from a faucet (convex side facing the stream). Just as the spoon touches the stream, it will suck itself in.
posted by backseatpilot at 9:49 AM on March 30, 2015 [18 favorites]
You are experiencing
You can show this effect more easily by letting a spoon hang from the end of the handle between your fingers and then slowly bringing it towards the stream from a faucet (convex side facing the stream). Just as the spoon touches the stream, it will suck itself in.
posted by backseatpilot at 9:49 AM on March 30, 2015 [18 favorites]
Oddly enough, there's an analogous behavior in light. Tightly-focused light produces what is essentially a low-pressure spot - a wall-less vacuum, if you will. IIRC the idea is that the atoms at the tiny volumen where light is most focused heat up quickly, leaving the area (thermal energy = molecular velocity), while molecules a scootch away are barely heated at all. The result is a static, continually replenished low-pressure spot.
A former company I worked at discovered this as the source of frequent blockage of their laser systems - dirt particles were attracted to this spot, and would then block up the tiny hole the light was to go through.
It can be used as "laser tweezers" to pick up ultra-small particles.
posted by IAmBroom at 9:43 PM on March 30, 2015
A former company I worked at discovered this as the source of frequent blockage of their laser systems - dirt particles were attracted to this spot, and would then block up the tiny hole the light was to go through.
It can be used as "laser tweezers" to pick up ultra-small particles.
posted by IAmBroom at 9:43 PM on March 30, 2015
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https://www.youtube.com/watch?v=lZz06t9hq60
posted by justkevin at 9:39 AM on March 30, 2015 [1 favorite]