A single parameter can predict surfactant impairment of superhydrophobic drag reduction.
A single parameter can predict surfactant impairment of superhydrophobic drag reduction.
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DOI：
10.1073/pnas.2211092120
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发表时间：
2023-01-17
影响因子：
11.1
通讯作者：
中科院分区：
文献类型：
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由于Marangoni应力固定了空气-水界面，痕量表面活性剂在应用中不可避免地会影响超疏水表面（SHS）的减阻效果。目前尚不清楚SHS损伤如何取决于表面活性剂的类型和浓度、流速和SHS几何形状；因此，仍然需要采取缓解战略。我们介绍了这一现象的模型，并进行了仿真和实验。我们发现，如果界面长度超过临界长度范围，则界面可以被动员，这是由表面活性剂的性质决定的，基本上与流速无关。因此，通过单一参数即界面长度与动员规模之比预测SHS损伤，为实现超疏水减阻提供了基础见解和实践指导。最近的实验和计算研究表明，在实践中不可避免的微量表面活性剂会通过在气液界面诱导Marangoni应力，严重影响超疏水表面（SHSs）的减阻。然而，目前尚不存在实际SHS几何形状的预测模型，这限制了对这些表面活性剂不利影响的理解和缓解。为了解决这个问题，我们推导了一个层流模型，在SHS光栅上的三维流动作为几何形状和可溶性表面活性剂性质的函数，它们共同包含10个无量纲群。我们确定光栅长度g是关键的几何参数，并预测实际滑移与无表面活性剂滑移之比随g2的增大而增大。在模型的指导下，我们进行了协同数值模拟和微流控实验，发现当我们改变表面活性剂类型和SHS几何形状时，与理论吻合良好。我们的模型还可以根据速度测量来估计微流体系统中固有的表面活性剂的先验未知性质。对于SHSs，我们表明表面活性剂效应可以用一个参数来预测，该参数表示光栅长度与界面长度尺度之比，超过该尺度，流动就会调动空气-水界面。这种动员长度对表面活性剂的化学性质比对其浓度更敏感，因此，即使是微量的污染物，如果它们具有高度的表面活性，也可能显著增加阻力。这些发现促进了对实际界面流动的基本理解，并提供了最大限度地减少超疏水阻力的实用策略。
Trace surfactants, unavoidable in applications, can impair the drag reduction achieved by superhydrophobic surfaces (SHS) as Marangoni stresses immobilize the air–water interface. It is not known how SHS impairment depends on surfactant type and concentration, flow velocity, and SHS geometry; as a result, mitigation strategies are still needed. We introduce a model of this phenomenon and perform simulations and experiments. We find that the interface can be mobilized if it is longer than a critical length scale, which is determined by the surfactant properties, essentially independently of flow velocity. SHS impairment is thereby predicted from a single parameter, namely the ratio of interface length and mobilization scale, providing fundamental insight and practical guidance to achieve superhydrophobic drag reduction. Recent experimental and computational investigations have shown that trace amounts of surfactants, unavoidable in practice, can critically impair the drag reduction of superhydrophobic surfaces (SHSs), by inducing Marangoni stresses at the air–liquid interface. However, predictive models for realistic SHS geometries do not yet exist, which has limited the understanding and mitigation of these adverse surfactant effects. To address this issue, we derive a model for laminar, three-dimensional flow over SHS gratings as a function of geometry and soluble surfactant properties, which together encompass 10 dimensionless groups. We establish that the grating length g is the key geometric parameter and predict that the ratio between actual and surfactant-free slip increases with g2. Guided by our model, we perform synergistic numerical simulations and microfluidic experiments, finding good agreement with the theory as we vary surfactant type and SHS geometry. Our model also enables the estimation, based on velocity measurements, of a priori unknown properties of surfactants inherently present in microfluidic systems. For SHSs, we show that surfactant effects can be predicted by a single parameter, representing the ratio between the grating length and the interface length scale beyond which the flow mobilizes the air–water interface. This mobilization length is more sensitive to the surfactant chemistry than to its concentration, such that even trace-level contaminants may significantly increase drag if they are highly surface active. These findings advance the fundamental understanding of realistic interfacial flows and provide practical strategies to maximize superhydrophobic drag reduction.
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