# Why does air get trapped below water pouring into a glass?

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:

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.

• 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 Commented Oct 9, 2021 at 8:01
• 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. Commented Feb 7, 2023 at 18:21
• You're seeing a vortex. It's a clump of bubbles in the water flow, just Commented Jul 22 at 4:09