CBET-EPSRC: Surfactant impact on drag reduction of superhydrophobic surfaces in turbulent flows

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Superhydrophobic surfaces (SHS) are bio-inspired engineered surfaces or coatings with several surprising and useful properties. By trapping air inside micro cavities, SHS can prevent small amounts of liquid such as water droplets from spreading on the surface, leading to the well-known lotus-leaf effect. When immersed in water, SHS can reduce friction drag between the liquid and the surface, owing to the entrapped air layer. Drag reduction from SHS has the potential to substantially reduce energy use, gas emissions and costs in maritime transport, and numerous other applications in fluid dynamics and heat transfer. Following the 2018 meeting of the International Maritime Organisation, the UK decided to reach zero gas emissions in British maritime shipping by 2050. Drag reduction technologies such as SHS can significantly contribute towards achieving this important environmental goal, whilst providing new economic opportunities in green technologies.However, SHS have shown inconsistent performance when tested in the lab or in the field, in both laminar and turbulent flow conditions. Many results deviate significantly from theoretical and numerical predictions. Our recent experimental, numerical and theoretical work has revealed that trace amounts of surfactant can significantly impair the drag-reduction performance of SHS in laminar flows. Surfactants are naturally present in oceans and rivers, as well as most engineering applications. Their impact on SHS in turbulent flow conditions is presently unknown. Building on our recent work on laminar flows, we hypothesize that surfactant can also affect the performance of SHS in turbulent flows, explaining inconsistencies found in experimental tests and the mismatch with existing models, which currently all ignore surfactant.To investigate this hypothesis, our multi-national team, composed of experts in numerical simulation from the University of California Santa Barbara (US) and experts in theoretical modelling from the University of Manchester (UK), will perform the first ever fundamental modelling investigation of superhydrophobic drag reduction in turbulent flow with surfactant. We will implement fully-resolved numerical simulations of surfactant-inclusive turbulent flow above SHS, using special refinement techniques in order to reach flow regimes relevant to realistic conditions for maritime applications. In addition, simpler theoretical models will be developed to identify and predict key physical and surfactant processes. The theoretical models will give us the flexibility to explore rapidly the complex dynamics of how surfactant can affect SHS drag reduction in turbulent flows. The numerical simulations will provide a wealth of detailed information about the flow dynamics and the effect of surfactants, and will be used to validate our theoretical models.To increase the impact of our findings, highly resolved data from our numerical simulations and algorithms implementing our models will be made freely available online. This will allow researchers to readily exploit our results in order to optimize SHS designs and improve their performance even when surfactant is present. Our objective is to uncover the impact of surfactant in realistic conditions in order to identify practical mitigation strategies and unlock the drag-reduction potential of SHS for real-world applications.