Ultra-tiny heat engines that convert thermal energy into mechanical energy may soon be on the horizon according to a computer simulation study from China1. Tienchong Chang and Zhengrong Guo from Shanghai University report that carbon nanotubes can be changed from their usual circular shapes into collapsed, flattened structures through a domino-like wave that propagates along the length of the tube. This transformation can be reversed by raising the temperature, setting the stage for multifunctional oscillators that operate at molecular levels.

Calculations by Chang previously showed that the circular structures of large single-walled carbon nanotubes are only meta-stable: energetically, the tube is most stable when it collapses and lies flat. Transitioning to this fallen state, however, requires overcoming a significant energy barrier. “If we view each carbon ring along the nanotube as a domino, then the stable state corresponds to a fallen-down system, while the meta-stable circular state is when the domino is standing up,” says Chang. “A standing domino cannot fall down by itself — it needs an external stimulus.”

The researchers found that clamping down on one end of the nanotube provided the necessary impetus to release the material’s potential energy, sequentially knocking the carbon structure down like a wave of tumbling dominoes. The next challenge facing the team was how to control this effect.

Fig. 1: Temperature changes induces waves of domino-like, reversible structural transformations in carbon nanotubes.Adapted from Ref. 1. Reproduced with permission. © 2010 ACS

Molecular dynamic simulations on a 4.3 nm-wide tube with one end shut and the other propped open revealed that temperature could modify the material’s stable state. After the tube structure toppled over at room temperature, raising the temperature to over 600 °C reversed the transformation: the collapsed zone shrank along the tube as the carbon rings returned to their upright, circular shapes (Fig. 1).

Additional simulations showed that the critical temperature needed to induce the conversion between the two states was a linear function of tube diameter, with larger tubes needing higher temperatures to leave the collapsed state. The speed of the propagating domino waves, which can reach 800 m/s, could also be adjusted through subtle temperature variations.

Chang notes that the highly tunable nature of the temperature-induced domino effect opens the door to many novel applications, including rechargeable ‘nanoguns’ that can controllably expel molecules or particles from the tubes.