Applied Mechanics of Materials Laboratory, Department of Mechanical Engineering Temple University , 1947 North 12th Street, Philadelphia, Pennsylvania 19122, United States.
School of Biomedical Engineering Science and Health Systems, Drexel University , Philadelphia, Pennsylvania 19104, United States.
ACS Appl Mater Interfaces. 2017 Aug 9;9(31):26510-26517. doi: 10.1021/acsami.7b05056. Epub 2017 Jul 25.
Smart window has immense potential for energy savings in architectural and vehicular applications, while most studies focus on the tunability of a single property of optical transmittance. Here we explore harnessing dynamically tunable hierarchical wrinkles for design of a potential multifunctional smart window with combined structural color and water droplet transport control. The self-similar hierarchical wrinkles with both nanoscale and microscale features are generated on a prestrained poly(dimethylsiloxane) elastomer through sequential strain release and multistep oxygen plasma treatment. We show that the hierarchically wrinkled elastomer displays both opaqueness and iridescent structural color. We find that restretching/releasing the elastomer leads to the reversible and repeatable switch from opaqueness to transparency, arising from the flattening of large wrinkles (micrometer scale), while a nonvanishing structural color occurs due to the nondisappearing small wrinkles (nanoscale). The unique features of combined reversible large wrinkles and irreversible small wrinkles during hierarchical wrinkling are well reproduced by corresponding finite element simulation. The criteria for generating self-similar hierarchical wrinkles is revealed through a simplified theoretical model and validated by experiments. In addition to its tunable optical property, we further show its ability in control of water droplet transport on demand through mechanical stretching and release. We find that an initially pinned water droplet on the tilted hierarchically wrinkled surface starts to slide when the surface is stretched, and becomes pinned again upon strain release. Such a process is reversible and repeatable. The hierarchically wrinkled surface could find broad potential applications not only in multifunctional smart windows with additional features of aesthetics and water collection, but in microfluidics, design of slippery surfaces, and directional water transportation.
智能窗户在建筑和车辆应用中具有巨大的节能潜力，而大多数研究都集中在光学透过率单一性质的可调谐性上。在这里，我们探索利用动态可调谐的分层皱纹来设计一种具有结构色和水滴输运控制相结合的多功能智能窗户。通过顺序应变释放和多步氧等离子体处理，在预拉伸聚二甲基硅氧烷弹性体上生成具有纳米级和微尺度特征的自相似分层皱纹。我们表明，分层皱纹弹性体既具有不透明性又具有彩虹结构色。我们发现，重新拉伸/释放弹性体导致从不透明到透明的可逆和可重复的切换，这是由于大皱纹（微米级）的扁平化，而由于小皱纹（纳米级）的不消失，仍然存在非零结构色。分层皱纹过程中组合的可逆大皱纹和不可逆小皱纹的独特特征通过相应的有限元模拟得到了很好的再现。通过简化的理论模型揭示了生成自相似分层皱纹的准则，并通过实验进行了验证。除了可调谐的光学性质外，我们还进一步展示了通过机械拉伸和释放按需控制水滴输运的能力。我们发现，初始时在倾斜分层皱纹表面上被固定的水滴在表面被拉伸时开始滑动，并且在应变释放时再次被固定。这个过程是可逆和可重复的。分层皱纹表面不仅在具有美学和集水等附加功能的多功能智能窗户中具有广泛的潜在应用，而且在微流控、滑表面设计和定向水输送中也具有广泛的潜在应用。
