CBET-EPSRC: Analysis and Optical Control of Surfactant Effects for Increased Lubrication of Liquid Flows in the Cassie State.

工程超疏水表面具有类似于那些自然发生的超疏水表面，如荷叶的性质。例如，它们提供润滑，使液滴从它们身上滚下来，而不是粘在它们身上，因此具有“自清洁”功能。超疏水表面上的水流有望为各种技术带来好处，比如芯片实验室和通过微通道冷却的电子产品热管理，因为它们可能提供润滑。然而，由于水中存在微量表面活性剂分子，这种润滑受到严重阻碍，甚至被消除。拟议的NSF - UKRI工程和物理科学研究委员会合作项目寻求利用表面活性剂来增强而不是阻碍润滑。为了实现这一目标，将光表面活性剂添加到水中，用光来控制它们在水中的分布。这将产生一种色毛细压力，将水泵向首选方向，从而压倒背景表面活性剂的有害影响。在超疏水表面上形成的液-气界面（半月板）是利用这种色毛细应力的理想载体。该项目的研究成果将有益于多种微流体技术，并可为水下物体的推进方法提供指导。该项目将与伦敦帝国理工学院的应用数学家合作进行，他们拥有互补和基本的技能，与项目美国方面的机械工程师协同工作。将开发健壮的模型来预测在含有光表面活性剂的水中，用不同波长的光照射半月板所施加的色毛细应力存在下，在超疏水表面上的流动行为。该模型将利用分析方法和内部数值代码将表面活性剂在半月板和散装水中的物质方程，以及相关的化学动力学与流体动力学问题耦合起来。这项工作的一个主要组成部分将是实验验证模型，使用微粒子图像测速法测量水流通过超疏水微通道的速度剖面。计算和测量了微通道的半月板形状和流动阻力。结果将得到实验验证的表观滑移长度模型，该模型是在色毛管应力存在下量化润滑的参数。该项目的结果将通过捕捉在足够高的表面活性剂浓度下胶束形成的影响来推进超疏水表面流动建模的一般状态，这是另一种增强润滑的方法。最后，将探讨利用色毛细应力来推进和引导淹没在水中的超疏水物体。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Engineered superhydrophobic surfaces possess properties similar to those of naturally occurring superhydrophobic surfaces such as the lotus leaf. For example, they provide lubrication such that droplets roll off of them rather than adhere to them and are thus "self-cleaning." Flows of water over superhydrophobic surfaces promise to benefit various technologies, such as lab-on-chip and thermal management of electronics via microchannel cooling, because of the lubrication they may provide. However, such lubrication has been severely hindered, or even eliminated, due to the presence of trace amounts of surfactant molecules in the water. The proposed NSF - UKRI Engineering and Physical Sciences Research Council collaborative project seeks to exploit surfactants to enhance rather than hinder lubrication. To accomplish this goal, photosurfactants will be added to water and light will be used to manipulate their distributions in the water. This will produce a chromocapillary stress to pump water in preferred directions that can overwhelm the deleterious effects of background surfactants. The liquid-vapor interfaces (menisci) that form on superhydrophobic surfaces are ideal vehicles to exploit such chromocapillary stresses. Results of the project will benefit a variety of microfluidic technologies and could provide guidance for methods of propulsion of underwater objects. The project will be carried out in collaboration with applied mathematicians at Imperial College London who possess complimentary and essential skills that are synergistic with those of the mechanical engineers on the US side of the project. Robust models will be developed to predict the behavior of flows over superhydrophobic surfaces in the presence of chromocapillary stresses imposed by the strategic irradiation of menisci with light of various wavelengths in photosurfactant-containing water. The modeling will couple the species equations of the surfactant, both on the meniscus and in the bulk water, and the associated chemical kinetics to the hydrodynamic problem using analytical methods and in-house numerical codes. A major component of the work will be experimental validation of the models using micro particle image velocimetry to measure velocity profiles in flows of water through superhydrophobic microchannels. Shapes of menisci and flow resistance of the microchannels will be computed and measured. The result will be experimentally-validated models for the apparent slip length, the parameter that quantifies lubrication in the presence of chromocapillary stress. Results from the project will advance the general state of modeling of flows over superhydrophobic surfaces by capturing the effects of micelle formation at sufficiently-high surfactant concentrations, another means to enhance lubrication. Finally, the use of chromocapillary stresses to propel and steer superhydrophobic objects submerged in water will be explored.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.