# 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.