Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces
Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces
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DOI：
10.1073/pnas.1702469114
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发表时间：
2017-07-11
影响因子：
11.1
通讯作者：
Luzzatto-Fegiz, Paolo
中科院分区：
文献类型：
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作者：
Peaudecerf, Francois J.;Landel, Julien R.;Luzzatto-Fegiz, Paolo
超疏水表面（SHS）有可能实现大的内部和外部流动应用的阻力减少。然而，实验结果不一致，许多研究报告显示性能显着降低。最近，有人提出，表面活性剂，无处不在的流动应用，可能是负责创建不利的马兰戈尼应力。然而，检验这一假设具有挑战性。用纯净水进行的仔细实验已经显示出大的界面应力，矛盾的是，添加表面活性剂几乎没有产生可测量的阻力增加。为了测试的表面活性剂的假设，同时控制表面活性剂浓度的精度高于可以实现的实验，我们进行模拟，包括表面活性剂的动力学。这些表明，表面活性剂引起的应力是显着的，在极低的浓度，可能会产生一个无滑动的边界条件的空气-水界面（“的plastron”）的表面活性剂浓度低于典型的环境值。这些应力随着腹板驻点之间的流向距离的增加而减小。我们进行微通道实验与SHS组成的流向平行光栅，这证实了这一数值预测，同时显示近plastron速度显着低于标准的表面自由预测。此外，我们还介绍了表面活性剂效应的非稳态测试。当我们在加载阶段之后快速移除驱动压力时，在腹侧处产生回流，这只能通过在加载阶段中形成的表面活性剂梯度来解释。这表明了表面活性剂在恶化减阻中的重要性，因此在SHS模型中包括表面活性剂应力的重要性。我们的时间依赖性方案可以评估表面活性剂在SHS测试中的影响，并指导未来的缓解设计。
Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible by creating adverse Marangoni stresses. However, testing this hypothesis is challenging. Careful experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. To test the surfactant hypothesis while controlling surfactant concentrations with precision higher than can be achieved experimentally, we perform simulations inclusive of surfactant kinetics. These reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air-water interface (the "plastron") for surfactant concentrations below typical environmental values. These stresses decrease as the stream-wise distance between plastron stagnation points increases. We perform microchannel experiments with SHSs consisting of stream-wise parallel gratings, which confirm this numerical prediction, while showing near-plastron velocities significantly slower than standard surfactant-free predictions. In addition, we introduce an unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.