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，在自然环境中产生或越来越多地在人造合成的情况下，这是非常光滑的。在自然界中，超疏水性表现为所谓的“荷叶效应”：水在荷叶上形成水珠，很容易滑落。这种光滑的特性在丰富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应，或者利用这种效应促进流体在驱动流中更容易通过（减阻）。然而，最近已经提出，驱动流中的这些有吸引力的减阻特性可能很快被表面活性剂或在流体表面快速聚集的流体中的杂质（即使是痕量的）的存在所损害。这意味着他们的存在几乎是不可避免的。因此，关键是要评估这种障碍的程度，它是否可以减轻，甚至是否可以利用故意添加的表面活性剂来实现所需的目标，如增强减阻，流动稳定，甚至质量传输。这是我们建议的主题。这项建议的主要目标是（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表面滑移性质的损害程度;（B）研究在表面活性剂的情况下已经研究过的界面的可能的“再活化”是否-载有气泡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如何将一类特殊的表面活性剂故意添加到流体中，（d）探索SHS上方层流的稳定性如何受到可溶性和不可溶性表面活性剂的存在的影响，以及初期不稳定性是否可以通过光致动来控制;（e）从理论上和实验上研究利用与光致动表面活性剂有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研究过，我们的建议的新奇在于它们的部署在超疏水表面的设置和它们的使用作为一个控制机制，持续减阻，消除不稳定性，并作为一种机制，战略传质。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，从减阻和自清洁表面到新兴医疗技术中的可控药物递送。

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.