July 2026

Cold enough this morning that my breath sat in the air outside the aquarium for two full seconds. I was eating on the breakwater at low tide when I noticed foam collected in a rock cleft — not the brief bubbles a wave leaves, but a stable, creamy head sitting there for ten or fifteen minutes. The question arrived half-chewed: why does that foam last?

The short answer is surfactants — surface-active molecules dissolved in seawater. Coastal water carries dissolved organic carbon shed by algae and phytoplankton. Some compounds are amphiphilic: one end attracts water, the other repels it. They crowd to the air-water interface and lower local surface tension. When a wave drives air into the water column and bubbles form, surfactant molecules coat the walls and slow collapse. Dish soap works the same way; the ocean does it at lower concentrations with messier chemistry.

The persistence is the interesting part. A clean-water bubble collapses in milliseconds — gravity drains the film until it ruptures. Surfactant-coated films drain more slowly because of the Marangoni effect: any local thinning creates a surface-tension gradient that pulls liquid back into the thinner region, a kind of self-correction. Individual bubbles can last tens of seconds; a foam head sits longer because bubbles support each other mechanically.

The organic load is real. Standard oceanographic references put dissolved organic carbon for this coastal region in the low milligrams-per-litre range — dilute, but enough. The fucoids (brown rockweed) in that cleft shed polysaccharides and lipid fragments continuously. Some fraction will be surfactant-active.

What I don't know: whether foam stability signals anything about algal health, or whether it's just energy input and ambient organic carbon. I've seen claims that unusual foam events can indicate phytoplankton blooms, but separating chemistry from physics from that viewpoint looks genuinely hard.

The foam was still there when I went back inside. Call it a few hundred millilitres — hundreds of millions of stabilised bubbles, each one a small proof of concept.

#science #tidepool #oceanography #notebook

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