Tuvshindorj Urandelger, Yildirim Adem, Ozturk Fahri Emre, Bayindir Mehmet
UNAM-National Nanotechnology Research Center, ‡Institute of Materials Science and Nanotechnology, and §Department of Physics, Bilkent University , 06800 Ankara, Turkey.
ACS Appl Mater Interfaces. 2014 Jun 25;6(12):9680-8. doi: 10.1021/am502117a. Epub 2014 May 22.
This paper investigates the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer microporous coating with a double-layer micro/nanoporous coating. Increasing pressure resistance of superhydrophobic coatings is of interest for practical use because high external pressures may be exerted on surfaces during operation. The Cassie state stability against the external pressure of coatings was investigated by squeezing droplets sitting on surfaces with a hydrophobic plate. Droplets on the single-layer coating transformed to the Wenzel state and pinned to the surface after squeezing, whereas droplets on the double-layer micro/nanoporous coating preserved the Cassie state and rolled off the surface easily. In addition, the contact angle and contact-line diameter of water droplets during evaporation from surfaces were in situ investigated to further understand the stability of coatings against Wenzel transition. A droplet on a microporous coating gradually transformed to the Wenzel state and lost its spherical shape as the droplet volume decreased (i.e., the internal pressure of the droplet increased). The contact line of the droplet during evaporation remained almost unchanged. In contrast, a water droplet on a double-layer surface preserved its spherical shape even at the last stages of the evaporation process, where pressure differences as high as a few thousand pascals were generated. For this case, the droplet contact line retracted during evaporation and the droplet recovered the initial water contact angle. The demonstrated method for the preparation of robust transparent superhydrophobic coatings is promising for outdoor applications such as self-cleaning cover glasses for solar cells and nonwetting windows.
本文通过比较单层微孔涂层和双层微/纳米孔涂层，研究了透明超疏水涂层中润湿的卡西状态的稳定性。提高超疏水涂层的耐压性在实际应用中具有重要意义，因为在操作过程中表面可能会受到高外部压力。通过用疏水板挤压位于表面的液滴，研究了涂层卡西状态对外部压力的稳定性。单层涂层上的液滴在挤压后转变为文泽尔状态并固定在表面，而双层微/纳米孔涂层上的液滴保持卡西状态并容易从表面滚落。此外，还原位研究了表面蒸发过程中水滴的接触角和接触线直径，以进一步了解涂层对文泽尔转变的稳定性。微孔涂层上的液滴随着液滴体积减小（即液滴内部压力增加）逐渐转变为文泽尔状态并失去球形。蒸发过程中液滴的接触线几乎保持不变。相比之下，双层表面上的水滴即使在蒸发过程的最后阶段也保持球形，此时会产生高达几千帕斯卡的压力差。对于这种情况，液滴接触线在蒸发过程中缩回，液滴恢复初始水接触角。所展示的制备坚固透明超疏水涂层的方法对于户外应用具有前景，如太阳能电池的自清洁盖板玻璃和不润湿窗户。
