You may be familiar with the types of pouring spout found in many professional cocktail bars, similar to the following:
The hole on the top of the spout is an air intake, which connects to the narrow pipe that protrudes into the bottle. Liquid can therefore flow out through the spout whilst air flows in through the intake—resulting in a smooth pour, uninterrupted by bubbles forming at the bottleneck.
But why does the liquid always flow out of the spout whilst air flows in through the intake? Why doesn't liquid ever flow out of the air intake whilst air flows in through the spout (or, for that matter, why doesn't air flow in through the spout at the same time as liquid flows out from it—resulting in bubbling as happens with an unspouted bottleneck)?
Is it simply because surface tension prevents the liquid from "squeezing" through the narrow aperture of the air intake pipe (and being "squeezed" past a bubble of air at the aperture of the spout)? If so, what is the maximum size for an aperture, like the air intake pipe, that will deny water from passing (and for one that, like the spout, will deny both water and air from passing together)?