Getting Head and Fingering: A Question about Carbonated Beverages.

I’m a soda drinker (or pop, depending on regional dialect). I like to consume it in mass quantaties in one sitting, so I use very large cups, and like them filled as much as practical. (One time I went to the grocery store, and bought 33 two-liter bottles of Mello Yello. Cashier asked if I was having a party, and gave me a funny look when I told her I wasn’t.)

When pouring soda, or any other carbonated beverage for that matter, one tends to get a nice head of foam as one pours. Now sometimes this is a good thing. I appreciate a good head of foam on a tall pilsner glass of a fine imported beer, for instance. However for my purposes of drinking inhuman amounts of soda, the foam is a bad thing. It lengthens my trip to the refrigerator, because I have to wait for the foam to disapate twice while pouring, in order to top off one of my 44 oz. cups.

There are pouring techniques that will minimize the foam, but I’ve found something else that works pretty good too: if I poke my finger in the foam it quickly starts to disapate. (Now just so I don’t gross anyone out, let me just state that I only do this with my own drinks, and I always make sure my hands are clean before I do this.)

My question is, why does sticking your finger into the head of foam accelerate its dispersion? I notice that the area that I stick my finger in turns darker than the surrounding foam, less white and more of the color of the beverage. This darker patch quickly spreads outward from where I poked my finger, until all the foam turns dark.

What exactly is happening here? My best guess is that I pop a bunch of the bubbles that make up the foam, and the soda that made up the walls of those bubbles spills down to the bubbles below them, making their walls thicker, adding more weight onto the bubbles below them, causing them to pop as well. This causes a chain reaction, the lighter bubbles at the top of the head falling in an avalanche that spreads outward from the point of finger insertion as the bubbles below them start colapsing. Does this theory make sense?

Color me disappointed. I was expecting something more juicy and all I get is a fizzle.

Funny, my last girlfriend said that to me too.

I think it has more to do with the oil on your skin being a solvent to the carbonation bubbles.

The explanation I’ve heard is that the bubbles in the foam break because of the natural oil on your skin. It’s an old bartender’s trick. On his 50s TV program, Jackie Gleason did a regular sketch playing “Joe the bartender”. He would talk to the camera as if it were a patron (a Mr. Dunnehy, IIRC), and he would always draw a beer and stick his finger in it. This usually got a laugh.

Ok, let’s see, Tree Boy. Adding oil to an aqueous solution tends to lower surface tension, and with lower surface tension the gas pressure in side the bubble would push the walls outward, thining them, and thinner walls are more prone to breakage than thicker walls.

So by poking my finger into the foam, I pop some bubbles, and push others aside. In the process, the soda from the walls of the bubbles I popped and the soda in the walls of the bubbles I push aside pick up a bit of oil from my finger. The extra soda from the popped bubbles is picked up by the surronding bubbles, and the combination of the extra solution and the weaker surface tension makes them swell.

These swolen bubbles have less surface area to volume, and because the structure of a bubble is concentrated on it’s surface, these bubbles are therefore weaker, and unable to support the weight of the bubbles above. They pop, spreading the adding more finger oil laced soda to the walls of the bubbles next to them, continuing the chain reaction.

Makes sense.

Oil oil oil oil oil the cleanest human hands have a smal amount f oil on them (dirty human hands are downright stinky) and if you put a little oil onto a mass of water-solution bubbles, the ones on top lose their structural integrity (remember in biology the explanation for the outer lipid shell of cells? Lipids (oils, fats) have a water-hating and a water-not-minding end). The bubbles are big enough so that they can only survive by attaching to eachother - tiny bubbles of soda su=olution can stand on their own for a little while, but the big mass of them gets a little oil in and suddenly the bubbles can’t stand to touch eachother! So they move aside a little and lose their support and break into some free air and some water solution.

cdhostage if lipids in water make the bubbles in foam hate each other, how come I can get a nice foam from adding dishwashing liquid (lots of lipids in that) to the water in my sink as I fill it? I agree that it’s probably the oil causing the colapse, but I don’t think the mechanism is the hydrophobic end of the lipid molecules. I think it’s surface tension.