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）是一种特殊的表面，在自然环境中出现，也越来越多地出现在人工合成的情况下，它们异常光滑。在自然界中，超疏水性表现为所谓的“荷叶效应”：水在荷叶上凝结，很容易滑落。这种光滑的特性在大量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，或者利用这种效果来促进流体在驱动流动中更容易通过（减少阻力）。然而，最近有人提出，在驱动流体中，这些吸引人的减阻特性很快就会受到表面活性剂的影响，或者流体中快速聚集在流体表面的杂质，即使是微量的杂质。这意味着他们的存在几乎是不可避免的。因此，评估这种障碍的程度，是否可以减轻，甚至是否可以利用故意添加表面活性剂来达到预期的目标，如增强减阻，流动稳定，甚至质量运输，都是至关重要的。这就是我们提案的主题。本提案的主要目标是：(a)定量评估表面活性剂的存在对层流状态下内部通道流动的滑动特性的影响程度；(b)研究在表面活性剂气泡环境中已经研究过的界面的“再动员”是否可以在涉及超疏水表面的减阻中发挥有用的作用；(c)从理论和实验两方面探索，在流体中故意添加一类特殊的表面活性剂，并通过外部光刺激可控（或“可调”），如何影响超疏水表面的滑动特性；(d)探讨可溶性和不可溶性表面活性剂的存在如何影响SHS上空层流的稳定性，以及是否可以通过光驱动来控制初期的不稳定性；(e)从理论上和实验上考察与光致表面活性剂相关的马兰戈尼应力作为推进机制的质量/粒子输运。虽然光致表面活性剂之前已经被研究过，但我们的建议的新颖之处在于它们在超疏水表面的部署，以及它们作为持续减少阻力、消除不稳定性和战略传质机制的控制机制。我们提出的工作包的基本见解将对SHS的各种应用产生广泛的影响，从减少阻力和自清洁表面到新兴医疗保健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.