A drop of oil in water behaves like a tiny ball: it seeks the spot with the least energy. If a wall is nearby, the drop often sticks to it, just as a raindrop spreads on glass. In a mixture of two immiscible liquids, such as polyethylene glycol and dextran, tiny droplets of one liquid form inside the other. As these droplets settle on a wall, they create a dip in the energy landscape, like a hollow on a surface that a ball rolls into.

Bacteria swimming in such a mixture act like balls. When a droplet sticks to the wall, an energy well forms next to it. A bacterium that happens to swim in can't get out: it sticks to the surface. Scientists ran experiments with Pseudomonas aeruginosa and Staphylococcus aureus in a polyethylene glycol-dextran mixture. They found that bacteria indeed accumulate at the wall exactly where droplets have attached. Computer simulations confirmed: droplets create traps that hold bacteria.

But bacteria have their own motion. They swim using flagella — thin filaments that rotate like propellers. This motion can both help and hinder. If there are few droplets, bacteria actively swim toward the wall and fall into traps. If there are many droplets, they start swirling — self-rotating droplets form. The rotation creates a fluid flow that pushes bacteria away from the wall. So the number of droplets decides whether bacteria stick or not.

This discovery helps us understand how bacteria form biofilms — dense communities on surfaces, such as on medical implants or pipes. Knowing that liquid droplets can serve as traps, we might learn to control bacterial adhesion: for example, by adding substances that change droplet wetting and thus prevent bacteria from settling.