In a narrow gap between two plates, water from humid air collects on its own: this is capillary condensation. If the gap is only a few nanometers wide, water no longer behaves like a continuous liquid: every molecule counts. But how exactly such a gap fills, layer by layer or all at once, has remained unclear.

Researchers fabricated capillaries from layers of material just one atom thick and used an atomic force microscope to watch how the walls bent as water entered. Flexible walls let water in gradually: each new layer of molecules added about 0.3 nanometers to the thickness of the water film. Rigid walls, in contrast, filled abruptly, in a single jump.

The difference is explained by a tug-of-war between two forces: the energy needed to bend the wall and the interaction of water with the wall, which changes depending on distance. When a wall bends easily, it is better off letting water in one layer at a time, following the undulating landscape of interaction. A rigid wall cannot bend, so water rushes in all at once, as soon as the gain from filling exceeds the resistance.

This discovery shows that the molecular nature of water can radically alter familiar capillary phenomena — from soil wetting to the operation of microfluidic devices. And the choice between smooth and abrupt filling depends simply on how compliant the walls are.