why does liquids “glug” when you pour them from a bottle?
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Air has to go in when the liquid comes out or you get a vacuum in the bottle and it is a literal bottleneck as liquid goes one way and air goes the other.
Liquid out. Air in.
Air pressure out is more than air pressure in. So as air forces itself in, it makes a bubble. Glug is bubble popping.
its just the pressure equalising.
as you remove liquid form the bottle, the pressure in the air pocket at the top of the bottle drops as it expands to fill the void left by the poured liquid. At some point, the pressure difference is great enough it starts to pull the liquid “back up”, with enough strength that air is able to force its way though the opening, and a bubble of higher pressure air rises up through the liquid to join the air pocket, which increases the pressure enough to allow some more liquid to flow out before the whole process begins again.
if their is a clear passage for the air to get in, such as the opening not being completely full of liquid, or a second opening, you dont get the glug as the air can equalise smoothly and the pour is much smoother. try it with a plastic drinks bottle over a sink, first just as it is make, then a small hole in the bottom. you will find the one hole set up glugs, but the two holes bottle will not glug if both holes are open. Or if you pour from it slowly, you will get a smooth pour right up until the liquid fully seals the hole, then it will start glugging.
The Water is Escaping the bottle, but because the water is taking up all the space around the only opening in the bottle, nothing can enter the bottle to replace it (not even air). This creates an area of low pressure in the bottle which continues decreasing in pressure until it reaches a level of criticality.
At this point, air from outside the bottle is literally pulled in to the bottle at a greater force than the liquid trying to leave the bottle, it pushed the water out of the way to get in to the bottle. This is whaty creates the glugging sound.
Metaphorically speaking, the bottle is taking a huge breath, like you would need to after breathing out. You can avoid this by pouring slowly and leaving an area of air above the liquid level as is leaves the bottle.
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