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Water icing is a natural phase change phenomenon which happens frequently in nature and industry and has negative effects on a variety of applications. Deicing is essential for iced surfaces, but even for a nanoengineered superhydrophobic surface, deicing may be incomplete with many adherent unmelted ice droplets which have potential of re-icing. Here, we focused on the droplet re-icing characteristics on a solid superhydrophobic surface, which has lacked attention in previous studies. Our results show that, the nucleation and ice crystal growth characteristics of a re-icing droplet are quite different with those of a first-time icing droplet. During re-icing, secondary nucleation due to fluid shear always first occurs on the edges of unmelted ice, accompanied by fast-growing ice crystals that can trigger heterogeneous nucleation when in contact with the solid surface. The re-icing takes place under very small supercooling (less than 0.5 o C) and the superhydrophobic surface does not play a key role, meaning that any current icephobic surfaces lose their features, which poses great challenges for anti-icing. In addition, because of the small supercooling, no recalescence phenomenon appears during re-icing and the droplet keeps transparent instead of clouding. Owing to the unmelted ice floating on the top of the droplet, the droplet shape after re-icing is also distinguishing from that after normal icing but the pointy tip formation during re-icing and normal icing shows a uniformity. These results shall deepen the understanding on the anti-icing and deicing physics.

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Enhancing the horizontal mobility of coalesced droplets on a plane could promote droplet jumping. Here we achieve enhanced horizontal mobility of a coalesced jumping droplet on superhydrophobic surfaces with an asymmetric ridge and investigate the underlying mechanism through experiment and simulation. Results indicate that the coalesced droplet accelerates during the coalescence-induced jumping stage and gains horizontal velocity during the rebound stage. The nondimensional horizontal velocity can reach 0.47, which is about 2.3 times the jumping velocity on the plane. Depending on the height-to-width ratio of the asymmetric ridge, the ratio of the horizontal velocity to the fallen velocity when the fallen droplet makes contact with the ridge is 0.55-0.75. Furthermore, the coalesced droplet can still obtain considerable horizontal velocity on superhydrophobic surfaces with an asymmetric ridge when the initial droplet radius is unequal. This work provides new insights for improving droplet jumping in related fields.

With the development of superhydrophobic surface preparation technology, coalescence-induced droplet jumping shows broad application prospects in the fields of enhanced condensation heat transfer and self-cleaning. In this work, the coalescence-induced jumping process of heterogeneous and homogeneous droplets on superamphiphobic surfaces was studied by using glycerol–water mixtures with different glycerol volume fractions. The results showed that the surface tension gradient of heterogeneous droplets will lead to asymmetric deformation of droplets, asymmetric distribution of internal pressure of droplets, as well as decrease in the energy conversion efficiency and the vertical departure velocity. Our study also revealed that the effects of surface tension gradient and viscosity on droplet jumping are different in the two regions. When the glycerol volume fraction is less than 40%, the droplet velocity and energy conversion are dominated by the surface tension gradient, and the vertical departure velocity and the energy conversion efficiency of homogeneous droplets are larger. When the glycerol volume fraction is greater than 40%, the droplet velocity and energy conversion are dominated by the surface tension gradient and viscosity together, and the vertical departure velocity and the energy conversion efficiency of heterogeneous droplets are larger.