Draft — architecture demonstration. Not yet reviewed as Handbook knowledge.
Principle
Why foam drains
A cocktail foam is liquid held in thin films between bubbles. It fails when that liquid drains away, the films thin, and neighbouring bubbles merge.
The question
What is actually happening when a cap breaks?
Revised 2026-08-17
Explanation
Foam is not a substance in its own right. It is a temporary arrangement: gas cells separated by thin films of the drink itself, stabilised by molecules that sit at the interface between the two.
Three processes work against that arrangement from the moment the tin is opened. Liquid drains downward under gravity, films thin until adjacent bubbles merge, and gas moves from small bubbles into larger ones. Every visible failure at the bar is one of those three becoming dominant.
The mechanism
- Drainage
- Gravity pulls liquid out of the films toward the drink. Warmth lowers viscosity and accelerates it.
- Coalescence
- Once a film is thin enough, two bubbles become one. Large bubbles are visible evidence that this is under way.
- Disproportionation
- Gas diffuses from small, high-pressure bubbles into larger ones, coarsening the texture over time.
What this means at the bar
- Temperature control is a foam decision, not only a dilution decision.
- A coarse cap is a warning sign — it is already mid-failure, not merely a different look.
What we do not know
The relative contribution of each process in a specific drink depends on its composition; we have not measured this across the full cocktail library.
Governed by
Governs
Same family as
Validates
About this Handbook
The Foam Handbook is published by FOAMY WHITES, who make a pasteurised liquid egg white for bars. We write about foam the way we would explain it across the bar — including where our product is not the answer.