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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.

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  • $\begingroup$ I have seen it too, isn't the bubbles formed at the sides of the container? I see it usually when there's a large distance between the level of the water and the level at which the water is falling from $\endgroup$
    – Adil Mohammed
    Commented Oct 9, 2021 at 8:01
  • $\begingroup$ My guess would be that similar to the way a styrofoam ball can leviate in an airstream. The from the water keeps the bubbles from rising due to the drag, and there is a restoring force keeping them in the middle of the current due to the velocity profile. $\endgroup$
    – Sebastian Riese
    Commented Feb 7, 2023 at 18:21
  • $\begingroup$ You're seeing a vortex. It's a clump of bubbles in the water flow, just $\endgroup$
    – Whit3rd
    Commented Jul 22 at 4:09

4 Answers 4

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Tap water contains dissolved oxygen. Since the oxygen is dissolved, there will be no bubbles seen at first (bubbles are an air-water mixture). The solubility of oxygen in water is temperature dependent and decreases with increasing temperature. If cold tap water is poured into a glass at room temperature, oxygen gas will be released from solution and create bubbles as the water warms. The interface between the glass and the water will be the warmest and bubbles will form as the water and glass reach equilibrium

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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

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    $\begingroup$ 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
    Commented Jan 17, 2019 at 6:59
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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.

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You're seeing a vortex. It's a clump of bubbles in the water flow, just as a smoke ring is a cluster of smoke particles in air flow. The circulation is a trap for its contents.

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