We all know the experience of buying a nice bottle of Coke, pouring it into a cup and that MF just foams up like crazy and out of the cup.
Other times, when we pour it slowly with an angle, NO FOAM! crazy stuff.
Why is that? What makes it foam or not foam?
In addition, you can poke the foam and not only does it stop foaming more, but the foam also dissipates much faster! HOW?! WHY?!
Man if this happened during the middle ages someone would be burned for witchcraft.
In: 1
When you think of something dissolving in a liquid, you normally only think of this being possible for solids, like sugar or salt. But it’s actually possible for gases to dissolve in liquids, too.
At the surface of a liquid where it meets a gas (such as air), the liquid is being bombarded by gas molecules as they zip and zoom around in the gas above. Some of these gas molecules may be moving at just the right speed and angle to punch into the liquid. Once punched in, they get knocked about by the surrounding liquid molecules like a pinball in a pinball table. Some gas molecules that end up in this situation may stay trapped like this for a long time. Only when the random pinballing takes them close to the surface do they get the opportunity to leave again, which could take ages.
I should take care to note that gas punching into the liquid from the surface isn’t the way sodas and beers get their fizziness. It’s either injected by a machine or is created as a natural byproduct of the production process. I only brought it up as a way to intuit how liquids can dissolve gases. It is a thing that happens at all liquid/gas boundaries to some degree, but not enough to make the liquids fizzy.
As the liquid sits and the gas inside it pinballs around, some of those gas molecules make it to the surface and escape. This is why fizzy drinks “go flat” when they sit for some time. If they’re in a closed container, though, the gas won’t be able to leave, it will simply collect in a bubble and build up pressure. This increased pressure causes the gas molecules to bombard the liquid harder and more frequently, causing more of them to get re-trapped in the liquid. At some point the rising pressure causes the escaping gas to balance out the re-entering gas, reaching an equilibrium. This is part of why every soda you ever open is pressurized and makes a hiss when you open it, the pressure keeps it from going flat while it sits around in its container.
Now, pouring. If you pour the liquid slowly down the side of a glass, it won’t be very jostled. It just gently slides down the wall of the glass and pools up at the bottom with little fuss. But if you pour the liquid sloppily in a tall stream, the surface tension of the liquid causes the stream to break up and pull together into droplets as the liquid falls. These droplets then crash into the pool of liquid at the bottom of the glass, which causes tiny pockets of air to get stirred into the liquid.
Now remember what I said before about how if a dissolved gas molecule finds its way to the surface, it will escape and zip away. This was fine when our liquid only had one exposed surface at the top of the container, where maybe only the top millimeter of water interacted with that surface. But now, thanks to our sloppy pouring, we have a bunch of microscopic air bubbles stirred in all throughout the liquid. Each one of these tiny air bubbles acts just like the surface up top, so dissolved gas molecules throughout the entire liquid suddenly have a route to escape. Gas diffuses into the bubbles. This makes them grow in size, which increases their surface area, which makes them grow more, in an exponential feedback loop cycle. At some point the bubbles grow so large that they must float up due to buoyancy. Hundreds of thousands of bubbles are doing this all at once, growing and rising to the surface, which ultimately creates the foam.
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