why does liquids “glug” when you pour them from a bottle?
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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.
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