Self-Propelled Ice on Herringbones

In the Leidenfrost regime, droplets or sublimating solids can ratchet across asymmetric surface structures by viscous entrainment with the underlying vapor flow. As an extension to these liquid-vapor or solid-vapor ratchets, here, we investigate the solid-liquid self-propulsion of melting ice disks. On hydrophilic herringbones, ice disks self-propel due to the unidirectional flow of viscous meltwater. This is a more viscous analog to Leidenfrost ratchets, except now a brief start-up time is needed for the underlying channels to get filled. When the herringbone is superhydrophobic using conformal nanostructures, the ice disk partially adheres to the ridge tops such that viscous entrainment cannot induce motion. Instead, after a much longer start-up time, the ice disk suddenly dislodges and slingshots across the surface by virtue of a mismatch in Laplace pressure of the meltwater on either end of the disk.