Well, the stuff that makes our beaches look like lattes turns out to be mostly gunk. It’s a collection of organic material, like algae, fish scales or bits of coral, that when agitated by the ocean’s waves and currents act as foaming agents and surfactants.
Surfactants are substances that lower the surface tension of water, basically reducing the attraction between water molecules allowing the surface to stretch around air bubbles. A foamis just a dispersion of a gas, in this case, air. in a liquid. When it comes to sea foam, more organic material means more surfactants, more foaming agents, and more foam, so when algae blooms or large fish schools die, you are likely to see more foam forming.
In 2007 a few giant storms off the coast of Sydney caused a massive influx of sea foam, causing the locals to refer to the beach as the Cappuccino Coast! While we likely won’t ever know exactly what caused the influx of foam during Dunkirk, my guess would be wastesfrom armies being dumped into waterways.
Surfactants don’t only cause bubbly beaches, though; they’re actually responsible for keeping us alive! Pulmonary surfactants in our lungs work to stabilize alveoli: small air-filled membrane bubbles that allow us to diffuse oxygen in, and waste gasses out of our lungs, just as they stabilize air bubbles in the sea.