Alternatively, pour the first (cold) beer. Then pour a room temp beer. Then pour a cold beer. If beer number 3 still froths less, you can eliminate temperature as the determinate.
My guess is - as others mentioned - that the stainless steel is not as smooth as glass, so there are a lot more points for bubbles to form on. (Recall that the bubble chamber for observing atomic particles was invented when a physicist pondered how and why bubbles formed from certain points in his class of beer - so your curiosity has a good pedigree…) Once you’ve filled the mug and drained it, there is a coating remaining that prevents the second helping from forming as many bubbles.
The “cold” theory is odd since basically, gas dissolves more and better in cold than hot liquids, as opposed to solids where (except for oddities like salt) hot liquid dissolves more solids than cold. The polar regions are particularly friendly to whales because the plankton they feed on grows better in the cold ocean water with it’s higher content of dissolved oxygen.
As for diet cola, I recall reading in the early days of diet drinks - the syrupy effect of sugar-water helps form more stable bubbles and create a “head” for non-diet soft drinks, because the liquid is “thicker”. Without sugar, diet drinks tended not to fizz, so part of the formula is to add a chemical to help make bubbles longer-lasting and stable, so diet soda looks like “real” soda.
There’s another possibility: the oils from your lips and mouth left after you drank the first beer remain in the residue and are acting as a surfactant to cut the foam from the second beer poured. You can see how fast the bubbles start disappearing by rubbing your finger against your nose and touching it to the foam.
This video shows what I’m talking about. There may not be as much as on your nose, but there is some oil on any patch of your skin which could be cutting the fizz. The first beer poured didn’t have that surfactant because the mug was clean, so the foam lasted longer.
Stainless steel has some odd surface properties that causes it to cling on to certain things; that thing where you can deodorise your hands after peeling garlic, by rubbing them on a stainless steel spoon (or indeed a stainless steel ‘soap’ bar ) does actually work.
So… could it be that something in the first pour of beer is staying stuck to the inner surface of the vessel, forming a very thin layer that affects the way the second pour interacts?
Could it be that the colder initial mug dissipates heat slower (thermal inertia) creating a larger head that is temperature dependent before collapse, compared to the second mug with a higher temperature that causes head (bubble) decomposition at a faster rate?
From your OP, it appears you know the answer. Please share.
Like I said, test the temperature theory by pouring (and disposing of) a second glass of warm beer. So then the third, cold beer is being poured into a warm mug like the first one - but the surface has been wetted, so if the foaming is caused by dry stainless, then you still will not get as much foam. If the foaming is due to cold beer hitting warm surface, you will get the same foam.
A temperature test has been suggested; can you also do a “wetness test” by quickly drying the mug out with a paper towel?
I personally favor the wet mug theory; it was the first thing I thought of when I read the OP, before reading any further. I haven’t got the physics background to explain it (though I follow and agree with @md-2000’s explanation). It’s just instinct after spending years in the kitchen; that’s no substitute for science, of course.
It’s definitely the wetness. I never use cold glassware. If I pour an especially frothy beer (like Leffe) into a dry glass it foams like mad no matter how carefully poured. A quick rinse of the glass with tap water solves the problem.
I don’t know if it’s a matter of physics or just plain taste, but I dislike beer out of a can (metal). It tastes different to me. In fact, I won’t even buy beer in cans, only in bottles.
IIRC the reason is that there is a microscopic bit of detergent left on the mug from when you washed it, just enough to cause the foaming by nucleation. That first beer rinses off that layer of detergent and so less foam the second time.
While many of the ingredients are the same between diet coke and normal coke, there is one obvious difference:
a small amount of a complex organic compound as sweetener, vs a large amount of dissolved sucrose
This obviously changes some physical properties of the mixture - namely surface tension and viscosity.
It’s obvious if you have ever made a strong sugar syrup - you have water at a rolling boil, start adding sugar, and the nature of the rolling boil changes to something far smoother and less bumpy. The more sugar you add, the more the change.
For diet coke, this makes the foam more stable, which is why diet coke is favoured for Mentos explosions as opposed to normal coke. Plus it’s easier to clean up because it isn’t sticky in the same way.
Don’t drink from the can, pour it into a glass. Modern cans are lined on the inside but if you’re drinking from the can you’re putting your mouth on untreated metal.
Many of these answers are beer/ stainless steel oriented, and that’s fine because that’s the OP. However, the same phenomenon happens with sparkling wine in a glass. There must be a connection.
I’m in favor of nucleation sites on a dry surface, you know, if we’re voting.
Like I posted earlier - I heard, in the early days of diet coke, that to make a diet drink foam like a regular soda - they add a foaming chemical to make up for the fact that diet soda is less viscous, less syrupy. hence the strong tendency to foam.