# Foam in a cup of soda

1. May 25, 2006

### daniel_i_l

Why is there less foam the second time you fill a cup with soda than the first time.
At first I thought that it had to do with the temperture of the cup changing(it cools down after filling it the first time) but I get the same resault with warm coke or with a cold cup?
Any ideas?
Thanks.

2. May 25, 2006

### chroot

Staff Emeritus
There could be many "systematic errors." If you're pouring from the same bottle for both the first and second cups, it's possible that the way you're holding the bottle the second time (it is lighter and needs to be held at a different angle) is affecting the results.

You'd really need to start over with a totally fresh bottle of soda with each "run" of the experiment, to isolate the cup as the only variable.

- Warren

3. May 26, 2006

### Meir Achuz

I think the foam is caused by dirt or dust on the surface of the cup.
I think there is less foam with a really clean glass.
When you refill the cup, there is a coating of soda on the surface, which probably keeps the new soda from interacting with the surface.

4. May 26, 2006

### Gokul43201

Staff Emeritus
If you're pouring from a bottle (as opposed to filling from a fountain), it's possibly just the soda in the bottle going flat (every time you open the bottle, the soda outgases some) with time of exposure and number of prior openings.

5. May 26, 2006

### dav2008

Why would dirt cause foam? That doesn't make any sense.

The obvious answer is that when you first open the bottle of soda it's at its maximum carbonation level. It's also possible that the change in pressure when you first open it allows more bubbles to be released so if you pour it right away you get more foam. Once you let it sit for a bit, that initial level level of carbonation isn't there anymore so you don't get as much foam. (Which is pretty much what Gokul said)

6. May 26, 2006

### brewnog

Pouring into a wet glass makes much less foam than pouring into a dry glass, regardless of the condition of the bottle. Try it by wetting the glass with water, and using a fresh bottle.

Not sure of why though. Something to do with nucleation of bubbles from the glass surface, purely as a guess.

7. May 26, 2006

### DaveC426913

It makes perfect sense if you understand how bubbles are formed in liquids. Google or Wiki "nucleation sites"

Personally, I think that a cup that has previously had liquid in it has been "wetted" - all its nucleation sites are covered or blocked with a thin layer of liquid. Less available nucleation sites, less foam. [Edit: ... as I now see brewnog points out before me]

Last edited: May 26, 2006
8. May 26, 2006

### dav2008

Yeah I made that comment before thinking. I guess what I wanted to say was that the amount of dust in a glass wouldn't be enough to make that big of a difference.