How Leidenfrost drops are propelled on textured surfaces via spontaneous symmetry breaking*

The directional motion of Leidenfrost drops on textured surfaces has been observed for textures that break the mirror symmetry along their direction of propulsion, for example ratchet structures. Here, we report Leidenfrost propulsion on mirror-symmetric spatially periodic surfaces, which raises the question about the origin of the propulsion mechanism. We map the underlying physics that entails gas and liquid flow as well as the dynamics of the gas-liquid interface to a comparatively simple model of two harmonic oscillators, which represent the regions close to the gas-liquid interface on both sides of a single fin of the texture. The restoring force of these oscillators is due to surface tension, and the oscillators are coupled through the gas flow between the solid surface and the gas-liquid interface. The oscillator model predicts that once a drop is moving relative to the texture, the gas-liquid interface experiences a deformation that breaks the mirror symmetry and in turn gives rise to a propulsion force. The origin of this propulsion force is the pressure field within the asymmetric gas flow. The experiments conducted on mirror-symmetric textures demonstrate a strong propulsion, with drop velocities of more than 10 cm/s.