单一参数可以预测表面活性剂对超疏水减阻的影响
A single parameter can predict surfactant impairment of superhydrophobic drag reduction
摘要：
最近的实验和计算研究表明，痕量的表面活性剂，在实践中不可避免的，可以严重损害减阻的超疏水表面（SHS），诱导Marangoni应力在空气-液体界面。然而，预测模型的现实SHS的几何形状还不存在，这限制了理解和减轻这些不利的表面活性剂的影响。为了解决这个问题，我们推导出一个模型的层流，三维流SHS光栅的几何形状和可溶性表面活性剂的属性，其中包括10个无量纲组的函数。我们建立了光栅长度$g$是关键的几何参数，并预测实际和表面无滑移之间的比率随着$g^2$而增加。我们的模型的指导下，我们进行协同数值模拟和微流控实验，发现与理论的良好协议，因为我们改变表面活性剂类型和SHS的几何形状。我们的模型还能够估计，基于速度测量，在微流体系统中固有存在的表面活性剂的先验未知属性。对于SHS，我们表明，表面活性剂的影响可以预测由一个单一的参数，代表光栅长度和界面长度尺度之间的比率超过流动动员的空气-水界面。该移动长度对表面活性剂化学比其浓度更敏感，使得即使痕量水平的污染物如果具有高度表面活性，也可能显著增加阻力。这些发现推进了对现实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。
英文摘要：
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 $g^2$. 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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