Langmuir. 2018 Mar 20;34(11):3533-3540. doi: 10.1021/acs.langmuir.7b03752. Epub 2018 Feb 28.
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.
液滴冲击超疏水表面会完全反弹。冲击过程通常包括铺展和回缩阶段，在此期间液滴与基底接触。最近的研究致力于使用具有不同形态或弹性的纹理表面来减少接触时间。在这里，我们设计了亚毫米级的超疏水脊，并表明在一定韦伯数范围内，当毛细抽空后，液滴会立即以花瓣状从表面弹开。两个方向上没有水平回缩过程导致接触时间减少约 70%。我们证明，花瓣状反弹是由于分层结构和各向异性特性的协同合作所致，这赋予了有效的能量存储和释放。当接触到凹槽底部时，沿着脊线会出现明显的飞翼，在垂直方向上有一个速度分量，这有助于实现快速的液滴脱离，从而释放能量。在较高的韦伯数下，各向异性表面会改变质量分布并促进液滴的均匀碎裂，从而大大缩短了整体接触时间。提出了简单的分析来解释这些现象，实验结果吻合较好。各向异性超疏水表面的接触时间减少有望对防冰和自清洁表面的设计和制造产生重大影响。
