Zhang Jihua, Sheng Xianliang, Jiang Lei
Aerospace Research Institute of Material and Processing Technology, Beijing 100076, PR China.
Langmuir. 2009 Feb 3;25(3):1371-6. doi: 10.1021/la8024233.
The high dewetting abilities of lotus leaves can be transited to a complete wetting state by soaking the leaves in water at a depth of 50 cm for 2 h. However, after being dried by N2 gas, the high dewetting behavior of lotus leaves may be mostly restored. This indicates that experimental procedure might considerably affect the dewetting abilities of lotus leaves. To discover the mechanism underlying this interesting dewetting phenomena, the dewetting force was used to characterize the dewetting abilities of surfaces, and model studies to mimic the papillae were done. Surface hydrophobicity, sizes, rise angles, and secondary structures of the models' sides affected their dewetting force with water. So we suggested that the dewetting states, Cassie or Wenzel's state, of lotus surfaces depend much on the depth of water, i.e., the hydraulic pressure. On the other hand, the primary structures of papillae in Cassie's state led to a high receding angle with respect to the plane of the leaf during the dewetting measurement. The secondary structures and micro/nano arrays of papillae increased the dewetting abilities of lotus leaves, since no water intruded between papillae. However, the structures of papillae in Wenzle's state significantly reduced the dewetting abilities of lotus leaves after being soaked at a depth of 50 cm for 2 h. Therefore, as for novel designs of microdevices floating on water, including the use of the high dewetting properties of suphydrophobic materials, surface (primary or secondary) microstructure and external pressure, such as static hydraulic pressure, must be taken into account.
通过将荷叶浸泡在50厘米深的水中2小时，荷叶的高去湿能力可转变为完全湿润状态。然而，用氮气干燥后，荷叶的高去湿行为大多可恢复。这表明实验过程可能会显著影响荷叶的去湿能力。为了探究这种有趣的去湿现象背后的机制，使用去湿力来表征表面的去湿能力，并进行了模拟乳突的模型研究。模型侧面的表面疏水性、尺寸、上升角度和二级结构影响其与水的去湿力。因此我们认为，荷叶表面的去湿状态，即卡西状态或文策尔状态，很大程度上取决于水的深度，即水压。另一方面，在去湿测量过程中，处于卡西状态的乳突的一级结构导致相对于叶片平面有较高的后退角。乳突的二级结构和微/纳米阵列提高了荷叶的去湿能力，因为没有水侵入乳突之间。然而，在50厘米深的水中浸泡2小时后，处于文策尔状态的乳突结构显著降低了荷叶的去湿能力。因此，对于漂浮在水上的微型器件的新设计，包括使用超疏水材料的高去湿特性，必须考虑表面（一级或二级）微观结构和外部压力，如静水压。
