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

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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.