The foam was still there at noon. I'd come down to check the pools at low tide — the shelf exposed, kelp draped flat — and one pool had a thick raft of white foam wedged into its northern corner. Four hours later, walking back, it was barely diminished. That seemed worth sitting with.

The question: why does foam in a tide pool outlast foam on an open wave face?

Foam is air bubbles stabilised by a thin water film. In pure water, that film drains in seconds — surface tension pulls liquid to the bubble borders until the film ruptures. What keeps foam alive is surfactant: surface-active molecules that park at the air-water interface and slow drainage. In the ocean, these come mostly from biological sources — algal exudates, mucus, degraded cell-membrane material.

Tide pools are, per litre, richer in dissolved organic matter than the open ocean surface. More surfactant, more stable foam. The pool I was looking at was dark and faintly iridescent, which I take as a rough proxy for high organic load. I don't have a number for the relationship, but the direction is clear.

The second factor is confinement. On a wave face, mechanical disruption is continuous — new water arrives, jets of spray shatter the structure. In a sheltered corner, the foam sits undisturbed. The characteristic drainage time for a well-stabilised foam under calm conditions is probably tens of minutes to an hour, consistent with what I observed.

What I'm less certain about: temperature. The pool had warmed from around 14°C to maybe 18°C through the morning. Lower surface tension at higher temperature should accelerate drainage, but lower viscosity might counteract that in some geometries. I genuinely don't know the sign of the net effect without working through the fluid dynamics properly, and I haven't done that yet.

Confident conclusion: organically rich water, plus mechanical shelter. The temperature question stays open.

#science #tidepool #foam #notebook