A droplet impacting on a superhydrophobic surface exhibits complete bouncing. The impacting process usually consists of spreading and retracting stages, during which the droplet contacts the underlying substrate. Recent research has been devoted to reducing the contact time using textured surfaces with different morphologies or flexibilities. Here, we design submillimeter superhydrophobic ridges and show that impacting droplets bounce off the surface immediately after capillary emptying in a petal-like shape at a certain Weber number range. The absence of a horizontal retraction process in two directions leads to ∼70% reduction in contact time. We demonstrate that the petal bouncing is attributed to the synergistic cooperation of the hierarchical structures and anisotropic property, which endows effective energy storage and release. When touching the bottom of the grooves, obvious flying wings appear along the ridges with a velocity component in the vertical direction, which help the energy releasing process in achieving fast droplet detachment. At higher Weber numbers, the anisotropic surface distorts the mass distribution and promotes uniform fragmentation of the droplet, and therefore the overall contact time is dramatically reduced. Simple analyses are proposed to explain these phenomena, showing a good agreement with the experimental results. The contact time reduction on anisotropic superhydrophobic surfaces is expected to have a great influence on the design and fabrication of anti-icing and self-cleaning surfaces.

Reducing the contact time between impacting droplets and superhydrophobic surfaces has attracted much attention in recent years due to the importance of controlling heat and mass transfer. Previous researchers have proposed several methods, such as lifting the droplets before the retraction, accelerating the retraction process, or splashing the droplets. One example includes symmetry-breaking surfaces, which were used to accelerate the droplet retraction to realize the fast detachment. However, the dependence of the contact time on impact velocity and surface structure scale remains unclear. Here, we experimentally study the droplet impact dynamics on cross-scale cylindrical superhydrophobic surfaces. The reduction of the contact time is achieved on the surfaces with a ridge smaller or larger than the droplets, spanning different bouncing regimes. We describe the droplet behaviors and propose theoretical models from the view of retraction speed to explain the contact time variations. The maximum reduction is observed to occur when the ridge diameter is close to that of the droplets, which is also predicted by the models.

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