Fluid Dynamics Is Weird: Bathroom Sink Edition

One of the things about being a physicist that makes it tough to have any sensible work-life balance is that I’m constantly seeing little things and thinking “Oooh! Physics!” then getting distracted from what I’m actually supposed to be doing. Take, for example, our bathroom sink.

I have noticed, from time to time, a weird effect where the stream of water coming out of the faucet, which normally is fairly straight, will spread out when the water level in the container below gets close to the faucet. This turns out to be damnably difficult to replicate, though, and I’ve spent more time than I care to admit screwing around with the sink trying to get it right. I finally got it yesterday, though, as you can see in this video (which also has a clip from a later failed attempt):

And here’s a couple of screencaps (also visible as the “featured image” above if you’re not reading via RSS) showing what I’m talking about:

Two screencaps from the video showing the way the stream from the faucet spreads when water level of the container gets close to the nozzle.

On the left, the cup of water is lower down, and not completely full, and the stream from the faucet is basically straight, narrowing slightly. On the right, the cup’s completely full, and close to the faucet, and you can see that the stream fans out just before it enters the water.

The one semi-consistent thing I’ve noticed is that this seems to only happen with hot water; I’ve never gotten it to work with cold (the second clip is cold water, done deliberately to show that it’s not a consistent thing). The flow rates in the two clips probably aren’t identical, but that’s a hard thing to match with the handles on our sink.

Anyway, I have absolutely no idea why this happens; I’m mostly just glad to have evidence that I didn’t imagine it. So I’m going to throw it out there and see if any of my wise and worldly readers can explain what the hell is going on.

And now, actual work.