Wednesday, March 07, 2007

The shower-curtain and Coanda effects, and velcro and randomness

This post and the next one are a great demonstration of what happens to my brain when I leave it on for too long, and why I never get anything done.

The shower curtain effect is the name for that annoying tendency for said curtain to bow inwards towards you when you're running the water. I usually try to wet and stick it to the bottom and sides of the tub, but sometimes surface tension can't hold it. Apparently people have actually investigated this in Scientific American (okay, and the IgNobel). I'm going to have to experiment tomorrow morning when I go in for my shower - I haven't been paying attention to the shower curtain, but I'm going to try to see if there's some kind of experiment we can set up to figure out what's going on. Sweet!

Somewhat related (one possible explanation for the shower curtain effect) is the Coanda effect, which is described as "boundary layer attachment." Although I know what the three words mean separately in a fluids sense, I have no idea what the phrase means when it's put together. Apparently you can demonstrate both Coanda and Venturi effects by putting the back of a spoon in running water. Here's how it works, to the best of my ability to understand it:

  1. Water running straight down in a nice stream, thanks to gravity.
  2. You bring the back of the spoon close to the water. As the spoon nears the water stream, the channel of air between the spoon and the water stream gets narrower.
  3. Air is moving through this channel (because the water is flowing beside it, dragging air molecules alongside). As you narrow the channel, the airspeed through this channel increases (it's the equivalent of blowing into the open mouth of a funnel; the same amount of air has to go through a smaller area).
  4. Bernoulli's principle says that faster-moving fluid (air, in this case) has lower pressure. The Venturi effect says that this creates a vacuum - in this case, a vacuum pulling things towards the spoon.
  5. The water stream is pulled towards the spoon because of the Venturi effect. It gets closer and closer until - whoop, it touches the spoon!
  6. Now the Coanda effect kicks in, meaning that the water follows the curve of the spoon (it will drip off the tip of the spoon) instead of just touching it at a tangent and continuing to drop down...

Okay, so I still don't understand the Coanda effect. Is the water sticking to the spoon in some sort of moving version of surface tension, is that it? There's a thin layer of water that directly touches the spoon and is pretty stationary, and the moving stream of water actually flows on top of that, and the water-spoon bond sticks because of... surface tension, and the still-water-moving-water bond sticks because of... surface tension? Ack. I don't know the first thing about fluids. I wish I did. I get the feeling that my knowledge of partial differential equations would get so much better if I learned about fluids.

And someday I have to learn about how airplanes work. (This being related to the Coanda effect.) Sometime later, though, maybe when I go for my pilot's license (and I will someday; I'm going to learn how to fly at some point). I wish I had time to follow all these mental threads down.

And surface tension is... molecules being attracted to each other, because - well, they just are, somehow? Is it the charges on the molecules being attracted together? Ach, I know I sound like an idiot right now; I don't know anything about this stuff. I need to learn chemistry; I want to be able to understand this stuff, and understand food chemistry when Debbie talks about it in Foodlab. Maybe I can bribe Karen, Tim, or Jessie with chocolate cake sometime and beg for some explanations, or stop by Chris or Debbie's office with cake slices and lots of questions. I should set aside time to read a chem textbook first, though. Oh, man. I just want to learn stuff, just wander around and teach people stuff, ask people to teach me stuff.

Last night Alex Davis blew my mind by talking about how magnetism is really relativity acting on electrons. Ask A Scientist and UIUC explain it better than I can. Alex said he learned this from Steve Holt. I'm not surprised; last year I wandered by Steve's office and he waved me in all excited, and five minutes later my mind was reeling because he'd just derived the speed of light from Maxwell's equations on a sheet of paper in front of me for no apparent reason. I was just walking by and he thought it would be fun. I freakin' love our professors.

Why is it that I always learn more outside of class? Always, always, always?

Okay. I'm going to work now. Actually, I'm going to go to class. I'm two hours late.


Nikki said...

I believe that the water sticks to the spoon basically because of intermolecular forces [IMFs]. Since air is by and large composed of diatomic molecules [N2, O2, stuff like that] it has very weak IMFs, while a spoon has much stronger IMFs, being metallic and more prone to holding some sort of charge. Water likes charge, because of its structure, so it's more likely to glomp onto the spoon, and other waters are more attracted to water than to air [the wonders of H-bonding], which is where all of water's magic surface tension and cohesion comes from. They will follow the other water molecules as long as the forces between the molecules are stronger than gravity pulling them downward.

More or less.

Mel said...

And after a long dinner conversation with Nikki & Co. (during which Chris Morse joins in and talks about electronegativity and ends up with the Schroedinger equation after I kept on asking "but why?" with regards to how 8 becomes a magic number in terms of valence electrons), chemistry is slightly less vague; I have pictures of London Dispersion Forces and hydrogen bonding swimming in my notebook, and as an added bonus more people understand low-pass filters now.

The world is a bathtub!

David Klempner said...

Ah, now that I can actually post, it looks like you've cleared that up.

Effects a lot like london dispersion/etc abound on that scale; subtle asymmetries tend to have noticeable effects.

Doping of semiconductors is another example.

I'm also a little surprised you weren't aware of the tie between relativity and magnetism.

Of course, the fact that we'd tend to look at magnetism as the relativistic effects of electricity says more about our view of the world; special relativity is kinda more fundamental than E&M.

Mel said...

Update, because I only remembered to notice yesterday: my shower curtain is far too heavy-gauge to conclusively demonstrate the shower-curtain effect. Must wait until I use a bathroom with cheaper plastic...