A liquid that flows without friction is superfluid helium-4, a strange state of matter at temperatures below two kelvins. In a large volume, the transition to superfluidity occurs at the lambda-point temperature, but in narrow channels everything changes.

Scientists squeezed helium between two plates, leaving a gap of just 10–20 nanometers — thinner than a flu virus. In such a slit, helium behaves like a two-dimensional liquid. The superfluid transition shifts downward in temperature, and until now no one could predict exactly how much. Usually empirical fits were used, but now a physical mechanism has been found.

It turned out that the shift of the transition temperature is determined not by the channel geometry per se but by the presence of two-dimensional rotons — thermal excitations that "eat up" the superfluid component. By including these rotons in the static Kosterlitz–Thouless theory, the authors could predict the absolute transition temperature for channels of different heights, and the calculation matched the experiment. Moreover, the dynamic extension of the theory (AHNS) explained dissipation peaks near the transition without invoking free vortices.

So the behavior of superfluid helium in nanochannels can now be calculated rather than fitted. This is a step toward understanding quantum liquids in confined geometries, which is important for future nanodevices.