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Phonon-Induced Ratchet Motion of a Water Nano-Droplet on a Supported Black Phosphorene.

Phonons are not supposed to carry any physical momentum as lattice vibrational modes thus it is believed no mass transport could be induced by phonons. In this Letter, we show that a ratchet motion of a water nano-droplet could be induced on two-dimensional puckered lattice like black phosphorene (BP) by exciting its flexural phonons through a moving substrate. The water nano-droplet exhibits a forward motion along the armchair or a backward motion along the zigzag directions on a BP lattice that is supported on a substrate possessing a relative armchair or zigzag forward motion with BP. By analyzing the structure and vibrational density states of BP, it is found that in-plane lattice displacement asymmetry and the in-plane vibration asymmetry are induced by the excited flexural phonons, which determine the water nano-droplet motion as an anisotropic Brownian motor. Simulations of the nano-droplet motion as functions of the substrate relative motion speed and direction, and also the substrate coupling strength with BP are performed. Results of the nano-droplet ratchet motion exhibit good agreements with the theoretic predications from calculating the Brownian motor asymmetry. Our finding reveals a promising mass transport strategy and a further understanding of phonon-related interactions in crystalline solid.