Two immiscible liquids, like oil and water, when you add microscopic swimmers (bacteria, mostly made of carbon), turn into a dynamic foam. The swimmers act like tiny chefs: their constant motion whips the liquids into a tangled network, much like a blender making an emulsion. But if they move too fast, the swimmers zip past the boundaries, and the mixture separates. At moderate activity, however, the forces of motion stabilize the foam, and the level of disorder (entropy) stays constant. When movement stops, the swimmers get stuck at the interfaces — the foam freezes, preserving its structure. Surprisingly, if you set them off again, they can either rebuild the foam or destroy it: a fast mode sends the liquids back to being separate. This gives us a recipe for tunable active emulsions, useful for smart materials and understanding bacterial mixtures.
Shaking oil and vinegar too fast makes salad dressing separate — the same speed threshold found in microswimmers.
Active emulsions are like smart dust or self-organizing nanorobots straight out of science fiction.