Imagine two immiscible liquids, such as oil and water. Under normal conditions, they tend to separate, forming a clear interface. But introduce microscopic swimmers (bacteria, whose basis is carbon) and the picture changes dramatically. These tiny organisms act like tireless chefs: their continuous motion whips the liquids into a tangled network of droplets and channels, creating a dynamic foam. The analogy of a kitchen mixer turning oil and vinegar into an emulsion is apt, but it oversimplifies: unlike mechanical stirring, the swimmers' activity is directed and "alive."

The key parameter determining the fate of the mixture is the swimmers' speed. At moderate activity, the forces generated by motion stabilize the foam: droplets do not coalesce, and the level of disorder, measured by entropy, remains constant. The system reaches dynamic equilibrium. But if the speed becomes too high, the swimmers start to cross the interfaces — the boundaries between oil and water. This destroys the foam, and the liquids separate again. The mechanism resembles a situation where a too-powerful mixer doesn't emulsify but splatters the mixture, preventing stabilization.

When motion stops, the swimmers get stuck at the interfaces, and the foam solidifies, retaining its structure. This is like freezing a culinary cream: the shape is fixed, but the ingredients stay in place. Surprisingly, upon restarting motion, two scenarios are possible. If the motion is fast, the liquids return to full separation — the foam collapses. If the motion is moderate, the foam can recover. Thus, the system exhibits switchable behavior, reminiscent of a recipe for tunable active emulsions. This opens prospects for creating smart materials that can change structure on command and deepens our understanding of natural bacterial mixtures, where microswimmers shape their own habitat.