# How come milk pours smoothly from a bag?

I have seen the commercial about the new beer can where it is vented by puncturing a whole on the can (air behind water). If you just pour normally with normal cans it will get bottle necked and the pour is not smooth.

This morning I opened a new bag of milk by cutting a small opening at the top corner of the bag. I noticed the milk always pours smoothly through that one opening.

Why is the pour smooth coming out of a bag but not from a can?

I am uncertain what I should be tagging this with, if someone can add some relevant tags it would be appreciated. Also remove this paragraph once done, thanks

• Are you talking about something like this ?
– TZDZ
Commented Nov 27, 2014 at 15:37
• @TZDZ yes a bag of milk like that. I cut a small opening on the top left corner for example and I put it in a jug and pour into a glass or bowl. Milk comes in bags like that in North America, not sure about other countries Commented Nov 27, 2014 at 15:39

The question is therefore : why doesn't a fluid flow out a bottle smoothly ?

The "bottleneck" phenomenon is caused by the lack of pressure in the can/bottle. As the liquid flows out, the pressure inside decreases because the volume of the container is fixed. When the pressure inside the bottle reach a given threshold, the outside air tends to flow in the container (ie. a bubble enters). And so on.

Since your bag is not solid, its volume decreases when the milk comes out and the pressure inside remains close enough to the outside pressure so that there is no suction effect.

A second opening at the opposite side of the bottle will allow the air to enter smoothly and thus prevents this discontinuous pattern (the inside pressure will stay in quasi-equilibrium with outside pressure thanks to the air flow from the second opening).

I hope I understood well the question.

• Oh yes I get it now, your understanding of the question is correct. Commented Nov 27, 2014 at 17:09

Because the bag deflates as milk leaves it, the volume of the bag decreases and the pressure remains constant so the milk pours smoothly. When pouring from a can, which does not deform like the bag, the pressure inside the can decreases as liquid leaves the can. The pressure differential creates a potential that pulls air into the can, interrupting the flow of the liquid and restoring atmospheric pressure inside the can. Then the process repeats, which gives the "glug glug glug" sound as you pour from the can.