#notebook

The foam was back this morning at the lower pools. I'd gone down before the aquarium opened — low tide around 06:40, sea flat enough that the kelp wasn't thrashing — and the north-facing crevices were full of it: dense, white, not quite cream-coloured, holding its shape for what felt like minutes after the last wave had pulled back.

The question: why does that foam stay so much longer than the foam in my kitchen sink?

Both are driven by surface-active agents — molecules with a water-loving end and a water-avoiding end that line up at the air-water boundary and lower the energy cost of forming new surface area. In seawater, those agents are mostly proteins and lipids shed from phytoplankton, zooplankton, and the general biological machinery of the nearshore zone. The collective term is marine surfactants, and they're structurally heterogeneous in a way that dish soap, a single engineered molecule, is not.

Low tide this morning, so I walked the reef shelf before opening. The foam was heavy along the surge channels — not the airy sea-spray kind, but the persistent, slow stuff that piles into corners and refuses to pop. A volunteer asked me last week what it actually is, and I gave her the short answer: "dissolved organics, mostly." Standing in it today, I decided to work it out more carefully for the notebook.

The question: why does seawater foam at all, and why so much more at certain shores and times of year?

Foam needs two things — a gas phase trapped in liquid, and something that keeps the bubble walls from immediately collapsing. Pure water at 12°C (roughly what the rock pools were this morning) has a surface tension around 74 millinewtons per metre. That's high enough that a plain water bubble pops almost instantly; the film pulls itself flat faster than you'd notice. What coastal seawater adds, especially in late summer when productivity is high, is dissolved organic matter: proteins, polysaccharides, lipid fragments shed by phytoplankton and breaking-down macroalgae. These molecules are surface-active — they preferentially migrate to the air-water boundary and orient there with their water-liking end submerged and their water-avoiding tail pointing out. The technical word is surfactant, from "surface-active agent." They act structurally like soap: they armour the bubble film and slow its drainage, extending the bubble's lifetime from milliseconds to minutes or longer.

The thermos cap was off for maybe forty seconds while I poured on the headland this morning, and by the time I got the lid back on the coffee already felt more warm than hot. A vacuum-insulated thermos should hold heat for hours. What went wrong?

The answer is mostly exposed surface area plus wind. Sealed, the only heat path out is through the walls — negligible by design. The moment I pour into my mug, I've created roughly 50 cm² of liquid surface exposed to air moving at 5–8 m/s off the Tasman. Convective loss scales with the temperature gap between fluid and air, and with wind speed; a standard fluid-dynamics text will tell you that doubling wind speed more than doubles the convective coefficient. At 8°C ambient and a coffee surface near 90°C, that gap is enormous and the heat is moving fast.

I felt the drop; I didn't measure it. If I had to estimate: 200 mL of coffee, tens of joules per second of convective flux, forty seconds exposed — a 5–10°C surface drop is plausible, maybe conservative. That's in the right order of magnitude, and I'm reasonably confident in the direction even without a thermometer in hand.

Low tide this morning, cold enough that my breath showed. I spent twenty minutes crouched at the outer rock pool watching the foam — not wave-surge foam, but the persistent kind sitting in the corners like dirty snow, barely moving while everything else drained back. The question: why does it stay?

Regular bubbles don't. Blow through a straw into fresh water and they pop in seconds. The film thins, drains under gravity and capillary pressure, and ruptures. Seawater behaves differently, and the reason is chemical rather than physical. Phytoplankton, bacteria, and decomposing algae release dissolved organic compounds — long-chain proteins, polysaccharides, lipids — that are surface-active. The textbook term is surfactant: a molecule that preferentially sits at the air-water interface and lowers surface tension there. The Marangoni effect then acts as a restoring force: if the film thins locally, surface tension rises at the thin patch, and that gradient drives fluid back toward it. The film resists rupture.

The rough scale: surface tension in clean seawater runs around 72 mN/m. With enough dissolved organics it can fall to 40–50 mN/m. That roughly 30% reduction is, apparently, enough to extend a bubble's lifetime from milliseconds to minutes.