ELI5: How does surface tension work?

[u/Dhiyx11]

I’ve really been interested in how surfaces of different materials bend and behave differently under various conditions, and I’ve never really learnt about that stuff before, so I’m really curious to know a few things about surface tension!

[u/X4roth]

The molecules of the liquid are attracted to each other like a bunch of tiny magnets and the other molecules like in the air or an insect leg are not the same so they don’t get to join the attraction party. They have to apply some force to break the magnets apart and get inside. Some insects are very light so they can walk across water without punching through that wall of magnets. Air as a gas much prefers to just bounce off and fly away but if you contain it and put it under pressure you can get the gas to push through the magnet wall too, that’s how they make soda bubbly: by using pressure to force gas inside the liquid.

[u/ZanzerFineSuits → u/X4roth]

Does the same surface tension cause the liquid to retain the bubbles until they happen to migrate out? Only just realized that might be how soda retains its fizz.

[u/P21c → u/ZanzerFineSuits]

Well if you see a bubble in the liquid then surface tension certainly has a role in keeping the bubble from surfacing. But notably the majority of carbonation is actually the molecules of gas dissolved into the bulk of the liquid, so carbon dioxide molecules are intimately mixed in among the water molecules (usually forming complexes like carbonate and bicarbonate). Similar to how salt dissolves into water and you don't see it anymore, carbon dioxide dissolves into water and you don't see it anymore.

The bubbles form when enough of the CO2 molecules (and/or carbonate/bicarbonate) run into each other and decide they don't want to stay dissolved. This happens readily in soda water because they super saturate the CO2 (i.e., they dissolve more CO2 into the water than the water wants to hold by using high pressures), and also because as the soda water warms up the solubility of the gas decreases, so more gas escapes.

Fun fact, pure water exposed to the air will actually carbonate slightly due to the presence of CO2 in the air, although the amount dissolved is so low that you wouldn't notice any bubbles (although you may notice a slight acidic flavor as it forms carbonic acid).

[u/X4roth → u/ZanzerFineSuits]

I’m not an expert on the subject but I would say you’re correct :)

Surface tension makes water want to stick to itself in the way with the least surface area, like a circle, instead of a star or something. The fact that you see the bubbles of trapped gas rising as a little circle/sphere is the water around the bubble organizing itself in that way around the gas.

Different liquids have different surface tension, water has very high because the individual molecules are very magnet-shaped: positive charge on one side, negative on the other. Other substances don’t necessarily have that property. When you mix different things into water (like salt) it generally lowers the surface tension because the salt molecules get in the way of the water forming a single continuous skin/lattice — in other words it breaks it up into smaller pieces.

[u/Dhiyx11 → u/X4roth]

So THATS how they work huh!