River water flows smoothly until it hits a narrow spot. There the current speeds up, and around a bend it may swirl into a vortex. Similar funnels, but just molecule-sized, can gather particles in one place all by themselves — no pump or motor needed.
In a new simulation, a cascade of ten nanochambers showed something surprising: the narrow side accumulated five times more particles than the wide side. The reason is the funnel's asymmetry: its shape works like a one-way gate. Particles easily enter from the wide end, but the narrow passage blocks their way back.
This effect has been dubbed "reverse rectification," because one usually expects accumulation at the wide side. A control experiment with symmetric chambers produced no gradient, so it's all about the funnel shape, not particle reflections off the walls.
The result changes the design rules for passive systems: now you can create dense clouds of molecules simply by choosing the right channel geometry. No pumps, no electricity, no heating, just shape.
The argon in the model behaved like a crowd in a narrow corridor: a crush at the entrance, but plenty of room inside if the door only opens inward.