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

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.

In this research, we will uncover the impact of surfactant on the drag-reduction performance of superhydrophobic surfaces (SHS) in realistic turbulent flow conditions. We will propose mitigation strategies to guide future SHS design, which can be used to reduce drag in fluid dynamics and heat transfer applications such as maritime transport.

Any improvement towards achieving the theoretical performance of SHS will have direct and indirect economic impact in the areas of wealth creation, enhancing research capacity of private organisations and attracting R&D investment. We will make our results freely available through publications and online open databases such as Github. They will inform applied research in academia and industry for the design and manufacture of SHS with increased performance in normal environmental conditions where surfactants are naturally present, enabling a direct saving in energy and costs for applications. Our research will also stimulate R&D and business growth. The global annual market potential for air lubrication technologies such as SHS in maritime transport alone is predicted to be worth £2 billion by 2030, according to the UK Department for Transport.

Increased performance in drag-reducing SHS also translates directly to reductions in gas emissions in applications. Gas emissions in maritime transport include not only greenhouse gases such as CO2 but also several other gases and particles which are harmful for human health and the environment. In 2018, the International Maritime Organisation (IMO) adopted a resolution to cut gas emissions in global maritime transport by at least 50% by 2050. This ambitious goal is crucial to reduce air pollution, greenhouse gas emissions and avert climate change. Moreover, by reducing energy consumption in maritime transport, SHS technologies can help us be less impacted by finite resources such as fossil fuels. Reducing our dependence on fossil fuels is crucial to lower threats to energy security at national and international levels. Therefore, by improving SHS design our research will have direct and indirect societal impact in the areas of environmental sustainability, improving health and improving national security.

In the Clean Maritime Plan (July 2019) published by the UK Department for Transport, the UK has positioned itself as a champion in reducing gas emissions in maritime transport, with the target of zero gas emission in this sector by 2050. One of the technologies put forward in this plan to achieve this highly ambitious target involves air lubrication technologies such as SHS. Current technologies, such as air-layer drag reduction and bubble drag reduction, have been measured to reduce the drag of ships by 10-20%. SHS have been predicted to reduce drag even more. We will provide evidence-based advice to policy makers at fora such as STEM for Britain and inform them about improvements in this technology, which could significantly contribute towards achieving the UK and IMO's goal of cutting gas emissions. We will also develop public awareness about SHS technology and its impact on cutting gas emissions to the public through well-established outreach events in Manchester UK, such as the Bluedot Festival and Pint of Science, and under the Family Ultimate Science Exploration (FUSE) program run by the University of California Santa Barbara, US, with local high schools.

A further impact of the proposed research will be the training of two scientists in a multidisciplinary research area. They will acquire a wide range of skills required for research in the mathematical sciences, engineering and computational science. They will also gain transferable skills in creative and critical thinking, communication, organisation, and information and data analysis. They will learn how to work synergistically and with versatility in a multidisciplinary team to deliver ambitious goals. These skills are highly valued in all professional environments.