0
$\begingroup$

The drinking-water-tap at my workplace has a low pressure continuous stream of water. When I pour water into a glass which has at least about (depth) 5cm of water in it, surprisingly large air bubbles (2 or 3 mm diameter) get trapped near the bottom of the glass and jiggle around without surfacing. Why does this happen?

Here is a diagram:

diagram of water pouring into cup

Here is my conjecture for the underlying physics: The water flowing into the cup creates a current in the water which is analogous to the current in air created by blowing air. The air bubbles then play the roll of the ping pong balls in the well known Coandă effect demonstration (in which ping pong balls "levitate" when air is blown towards them). (See here.)

However, I am not convinced by my conjecture, because the air bubbles live near the bottom of the glass. Here the current should be flowing parallel to the boundary, actively pulling air bubbles away from the vertical stream of water. (With taller cups I could test such hypotheses, but I'm at work and all I have is a mug.)

Note that my question is not a duplicate of Why does water sometimes form bubbles when I pour it into a glass?, which concerns bubbles on the surface of the water.

$\endgroup$
0
$\begingroup$

The cloudiness might be caused by the water in the pipes being under a bit more pressure than the water in the glass, but is more likely due to tiny air bubbles in the water. Like any bubble, the air rises to the top of the water and goes into the air above, clearing up the water

$\endgroup$
  • $\begingroup$ No no these are concrete observations - I see a small number of large discrete bubbles that stay "trapped" at the bottom of the glass. There's no "cloudiness" because the bubbles are so large that it doesn't appear cloudy. The behavior is consistent in the sense that I've observed this multiple times in the past week. I can take a photo when I'm back at work tomorrow. $\endgroup$ – Harambe Jan 17 at 6:59
0
$\begingroup$

The air bubbles form when the water comng out of a tap (pressure or 20 psi while the ambient pressure is 14.7) and pour into the tap break the surface of the water and mix some of the air over the water into the water, and with the flow of water downwards the bubbles flow dawnwards and stick to the lower surface of the glass. The wiggling is due to the turbulent eddies at the bottom. As soon as the buoyancy becomes greater than the flow force exerted on them, they flow upwards.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.