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Dynamic behavior of micro water droplet condensed on a lotus leaf with two-tier roughness is studied. Under laboratory environment, the contact angle of the micro droplet on single micro papilla increases smoothly from 80 deg to 160 deg during the growth of condensed water. The best-known "self-clean" phenomenon, will be lost. A striking observation is the out-of-plane jumping relay of condensed droplets triggered by falling droplets, as well as its sustained speed obtained in continuous jumping relays, enhance the automatic removal of dropwise condensation without the help from any external force. The surface tension energy dissipation is the main reason controlling the critical size of jumping droplet and its onset velocity of rebounding.Comment: 14 pages, 8 figure

Although realizing dewetting transitions of droplets spontaneously on solid textured surfaces is quite challenging, it has become a key research topic in many practical applications that require highly efficient removal of liquid. Despite intensive efforts over the past few decades, due to impalement of vapor pockets inducing strong pinning of the contact lines, how to realize the self-removal of small droplets trapped in the textures remains an urgent problem. We report an in situ spontaneous dewetting transition of condensed droplets occurring on pillared surfaces with two-tier roughness, from the valleys to the tops of the pillars, owing to the nanotexture-enhanced superhydrophobicity, as well as the topology of the micropillars. Three wetting transition modes are observed. It is found that a further decreased Laplace pressure on the top side of the individual droplets accounts for such a surprising transition and self-removal of condensed water. An explicit model is constructed, which quite effectively predicts the Laplace pressure of droplets trapped by the textures. Our model also reveals that the critical size of the droplet for transition scales as the spacing of the micropillars. These findings are expected to be crucial to a fundamental understanding, as well as a remarkable strategy to guide the fabrication, of optimum super-water-repellant surfaces.

This paper investigates the impact behavior between water drops with different velocities and cylindrical superhydrophobic surfaces with various diameters and presents two possible outcomes of drop impact, which are asymmetric rebound and stretched breakup. Due to the special cylindrical topology of the surface, drops undergo an asymmetric spreading and retracting process in the azimuthal and the axial direction, which results in three types of asymmetric rebound, including jug-like rebound, wing-like rebound, and rebound breakup. The stretched breakup is observed in the collision of drops with higher impact velocities and smaller cylinder diameters. The diameter ratio D* and Weber number We are found to be the determinants of the bouncing patterns. With the decrease in the diameter ratio D* or the increase in the Weber number We, the bouncing patterns transformed from jug-like rebound through wing-like rebound and finally to stretched breakup. We put forward a modification form of the Weber number (α = We/D*) affected by the diameter ratio D*, indicating the ratio between the inertia force and the surface tension, as the criterion to distinguish the upward rebound from the downward stretch, which helps obtain the linear relation of critical Wecr and D*cr. Furthermore, asymmetric rebound and stretched breakup could effectively shorten the contact time between drops and substrates. The contact time is found to be mainly determined by the dimensionless parameter α. The correlation between the dimensionless contact time and the dimensionless parameter α is demonstrated to be τc ∝ αn.