My dog eats and drinks out of metal bowls. I’ve noticed that when I turn around after filling up the water bowl in the sink, any spit bubbles or backwash on the surface of the water stay oriented the same way (relative to everything else, not me). Why is this? This behavior doesn’t change regardless of how fast or slow, careful or not I am. I’m not sure what kind of metal the bowl is, but it’s about the size and shape of half a basketball. Where I live, the tap water is pretty damn clean. If there are any extraneous factors I haven’t thought to mention, let me know.
As mentioned, it’s basically inertia, friction, laminar flow. You turn the bowl, but the contents are liquid, so the bowl moves around them. Only the water touching the bowl itself is moved through friction (and the water touching that water, and so on, diminishing the further you get from the bowl).
If you turn the bowl quickly, you overcome most of that friction, and less water moves. Turn the bowl slowly, and friction keeps the water together.
For a fun visual, in a still liquid, you can see that these rotations are reversible, allowing you to “unmix” a liquid! https://youtu.be/p08_KlTKP50