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

超疏水表面(SH表面，或SHS)是一种特殊类型的表面，出现在自然环境中或越来越多地出现在人造合成环境中，这些环境异常光滑。在自然界中，超疏水性表现为所谓的“荷叶效应”：水在荷叶上形成珠状，然后很容易滑落。这种光滑的特性在广泛的工程应用中非常有用，包括直接尝试在人造超疏水涂层中模拟这种效果，或者利用这种效果在驱动流中促进流体更容易通过(减阻)。然而，最近有人提出，在驱动流中，这些有吸引力的减阻性能会很快被表面活性剂的存在或流体中的杂质所折衷，这些杂质会迅速聚集在流体表面，即使是微量的。这意味着他们的存在几乎是不可避免的。因此，关键是要评估这一障碍的程度，是否可以减轻它，甚至是否可以利用故意添加的表面活性剂来达到预期的目标，如增强减阻、流动稳定，甚至质量传输。这个建议的主要目的是：(A)定量评估层流流动中内部通道流动表面活性剂的存在对表面滑移特性的影响程度；(B)研究在含有表面活性剂的气泡背景下对界面的可能的“再动员”是否可以在涉及超疏水表面的减阻中起到有用的作用；(C)从理论和实验上探索一种故意添加到流体中并且可被外部光刺激控制(或“可调节”)的特殊类型的表面活性剂如何影响超疏水表面的滑移特性；(D)探索SHS上层流流动的稳定性如何受到表面活性剂(可溶和不可溶)的存在的影响，以及初期不稳定性是否可以通过光驱动来控制；(E)在理论和实验上研究使用与光驱动表面活性剂相关的Marangoni应力作为推进机制的质量/颗粒传输。虽然以前已经研究过光致表面活性剂，但我们的建议的新颖之处在于，它们被部署在超疏水表面的设置中，并用作持续减阻、消除不稳定性的控制机制，以及作为战略性传质的机制。我们提议的工作方案的基本见解将对SHS的各种应用产生广泛影响，从减阻和自清洁表面到新兴医疗技术中的可控药物输送。

Superhydrophobic surfaces (SH surfaces, or SHS) are a special breed of surfaces, arising in natural settings or increasingly in man-made synthetic situations, which are unusually slippery. In nature, superhydrophobicity manifests itself as the so-called "lotus-leaf effect": water beads up on a lotus leaf and readily slips off. This slippery feature can be supremely useful in a rich panoply of engineering applications, including direct attempts to emulate this effect in man-made superhydrophobic coatings, or using the effect to promote easier passage of fluids in driven flows (drag reduction). It has been proposed recently, however, that these attractive drag reduction properties in driven flows can quickly be compromised by the presence of surfactants, or impurities in the fluids that quickly aggregate at the fluid surfaces, even in trace amounts. This means that their presence is virtually unavoidable. It is therefore critical to assess the extent of this impediment, whether it can be mitigated, and even whether the deliberate addition of surfactants can be leveraged to attain desired objectives, such as enhanced drag reduction, flow stabilisation, or even mass transport. This is the topic of our proposal.The main goals of this proposal are to (a) quantitatively assess the extent to which the slip properties of a surface are compromised by the presence of surfactants for internal channel flows in the laminar flow regime; (b) study whether a possible "remobilisation" of interfaces already studied in the context of surfactant-laden bubbles can play a useful role in drag reduction involving superhydrophobic surfaces; (c) explore, both theoretically and experimentally, how a special class of surfactants deliberately added to the fluid, and controllable (or "tunable") by external light stimuli, can affect the slip properties of superhydrophobic surfaces; (d) explore how the stability of laminar flows over SHS is affected by the presence of surfactant, both soluble and insoluble, and whether incipient instabilities can be controlled by light actuation; (e) examine, both theoretically and experimentally, mass/particle transport using Marangoni stresses associated with light-actuated surfactants as a propulsion mechanism. While light-actuated surfactants have been studied before, the novelty of our proposal lies in their deployment in the setting of superhydrophobic surfaces and their use as a control mechanism both for sustained drag reduction, elimination of instability, and as a mechanism for strategic mass transfer. The fundamental insights from our proposed work packages will have broad implications for a variety of applications of SHS ranging from drag reduction and self-cleaning surfaces to controllable drug delivery in emerging healthcare technologies.