Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Cambridge CB3 0WA, United Kingdom.
School of Mathematics, University of Manchester, Manchester M13 9PL, United Kingdom.
Proc Natl Acad Sci U S A. 2017 Jul 11;114(28):7254-7259. doi: 10.1073/pnas.1702469114. Epub 2017 Jun 27.
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.
超疏水表面（SHSs）有可能实现内部和外部流动应用的大阻力减少。然而，实验表明结果不一致，许多研究报告称性能显著降低。最近，有人提出，在流动应用中普遍存在的表面活性剂可能会通过产生不利的马兰戈尼应力而负责。然而，测试这一假设具有挑战性。已经用纯水进行了仔细的实验，显示出很大的界面应力，而且矛盾的是，添加表面活性剂只会产生几乎可以测量的阻力增加。为了在控制表面活性剂浓度的同时测试表面活性剂假说，我们进行了包括表面活性剂动力学在内的模拟。这些模拟表明，在极低的浓度下，表面活性剂引起的应力是显著的，这可能会在空气-水界面（“胸甲”）上产生无滑移边界条件，对于低于典型环境值的表面活性剂浓度。这些应力随着胸甲驻点之间的流向距离的增加而减小。我们进行了具有流平行光栅的 SHS 的微通道实验，这些实验证实了这一数值预测，同时显示出接近胸甲的速度明显慢于标准无表面活性剂预测。此外，我们引入了一种表面活性剂效应的非稳态测试。当我们在加载阶段后迅速去除驱动压力时，在胸甲处会产生回流，这只能通过在加载阶段形成的表面活性剂梯度来解释。这表明表面活性剂在降低阻力减少方面的重要性，因此在 SHS 模型中包括表面活性剂应力的重要性。我们的时变协议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。
