Department of Engineering Mechanics, Tsinghua Universtiy, Beijing 100084, PR China.
Langmuir. 2010 Apr 6;26(7):4984-9. doi: 10.1021/la9036452.
Water-repellent biological systems such as lotus leaves and water strider's legs exhibit two-level hierarchical surface structures with the smallest characteristic size on the order of a few hundreds nanometers. Here we show that such nano to micro structural hierarchy is crucial for a superhydrophobic and water-repellent surface. The first level structure at the scale of a few hundred nanometers allows the surface to sustain the highest pressure found in the natural environment of plants and insects in order to maintain a stable Cassie state. The second level structure leads to dramatic reduction in contact area, hence minimizing adhesion between water and the solid surface. The two level hierarchy further stabilizes the superhydrophobic state by enlarging the energy difference between the Cassie and the Wenzel states. The stability of Cassie state at the nanostructural scale also allows the higher level structures to restore superhydrophobicity easily after the impact of a rainfall.
具有拒水功能的生物系统，如荷叶和水黾的腿，具有二级分形表面结构，其最小特征尺寸约为几百纳米。本文展示了这种纳米到微米结构的分形对于超疏水和拒水表面是至关重要的。几百纳米尺度的一级结构使表面能够承受植物和昆虫在自然环境中所能承受的最高压力，以维持稳定的 Cassie 状态。二级结构导致接触面积显著减小，从而最大限度地减少水与固体表面之间的附着力。两级分形进一步通过增大 Cassie 状态和 Wenzel 状态之间的能量差来稳定超疏水状态。纳米结构尺度上 Cassie 状态的稳定性还使得在降雨冲击后，更高层次的结构能够更容易地恢复超疏水性。
