Scientific documents

Superhydrophobic air-retaining surfaces represent a bioinspired approach to friction reduction in shipping. A ship, which is coated with permanent layer of air will show a reduction in (i) friction, (ii) biofouling, (iii) corrosion and (iv) noise. AIRCOAT aims to develop a passive air lubrication technology for marine applications, using self-adhesive foils instead of paints and is inspired by the Salvinia effect. This paper introduces the biomimetic concept with emphasis on the recently demonstrated AIR Spring Effect, which provides a 100-fold higher stability of the air layer against pressure fluctuations. A first test under marine conditions was successfully performed by coating more than the half hull of a 12-meter research vessel with AIRCOAT foil.

Reducing friction with specialised hull coatings or air lubrication technologies has a potential reducing energy consumption and emissions in shipping. The EU project AIRCOAT combines both by developing a passive air lubrication technology inspired by nature that is implemented on a self-adhesive foil system. Besides validating the friction reduction it is of high interest to understand the underlying mechanism that causes the reduction. Therefore, a flow channel was designed, that creates a stationary turbulent flow within a square duct allowing for non-invasive measurements by laser doppler velocimetry. The high spatial resolution of the laser device makes recording velocity profiles within the boundary layer down to the viscous sublayer possible. Determination of the wall shear stress τ enables direct comparison of different friction reduction experiments. In this paper we validate the methodology by determining the velocity profile of the flat channel wall (without coatings). We further use the results to validate a CFD model in created in OpenFOAM. We find that velocities along the longitudinal axis are generally in good agreement between numerical and experimental investigations.

This paper reports the input of underwater noise energy trend in global shipping, based on bottom-up modeling of individual ships. In terms of energy, we predict the doubling of global shipping noise emissions every six years, on average, but there are large regional differences. Shipping noise emissions increase rapidly in Arctic areas, the Norwegian Sea and Pacific Ocean. The largest contributors are the Containerships, Bulk Cargo and Tankers vessels which emit almost 80% of the underwater shipping noise energy. The COVID-19 pandemic changed vessel traffic patterns and our modeling indicates a reduction of 24% in shipping noise energy in the 63 Hz ⅓ octave band. This reduction was largest in the Arctic, Greenland Sea, and the Gulf of California, temporarily disrupting the increasing pre-pandemic noise trend. However, in some sea areas, such as the Yellow Sea and Eastern China Sea the emitted noise energy was only slightly reduced. In global scale, COVID-19 pandemic reduced the underwater shipping noise emissions close to 2017 levels, but it is expected that the increasing trend of underwater noise will continue when the global economy recovers.

The ability to float ferns Salvinia to keep a permanent layer of air underwater is of great interest, e.g., for drag-reducing ship coatings. The air retaining
hairs are superhydrophobic, but have hydrophilic tips at their
ends, pinning the air-water interface. Here, experimental and theoretical
approaches are used to examine the contribution of this pinning effect for
air-layer stability under pressure changes. By applying the capillary adhesion
technique, the adhesion forces of individual hairs to the water surface is
determined to be about 20 μN per hair. Using confocal microscopy and
fluorescence labeling, it is found that the leaves maintain a stable air layer
up to an underpressure of 65 mbar. Combining both results, overall pinning
forces are obtained, which account for only about 1% of the total air-retaining
force. It is suggested that the restoring force of the entrapped air layer is
responsible for the remaining 99%. This model of the entrapped air acting is
verified as a pneumatic spring (“air-spring”) by an experiment shortcircuiting
the air layer, which results in immediate air loss. Thus, the plant enhances its
air-layer stability against pressure fluctuations by a factor of 100 by utilizing
the entrapped air volume as an elastic spring.

The EU project AIRCOAT aims to develop a passive air lubrication technology for ships. Using self-adhesive foils instead of paints and inspired by the Salvinia effect, an air coated ship hull is meant to reduce friction, biofouling, corrosion, and noise. This paper outlines the validation concept to quantify the potential friction reduction for a micro-structured air keeping surface for ships. It further describes the experimental setup of the visualization flow tank to investigate velocity profiles and the validation of the CFD code by simulating the HSVA HYKAT and preliminary friction reduction results from the SVA friction tunnel experiments.

Air pollution from ships have been recently shown to contribute to human health effects by over 250 000 premature deaths annually even with the global 0.5% Sulphur cap which enters in force by Jan 1st 2020 (Sofiev et al., 2018). This paper introduces a passive air lubrication application for ship hulls, which has potential to reduce frictional resistance, fuel oil consumption and atmospheric emissions. The AIRCOAT approach is based on a biomimetic ship hull coating that introduces a permanent layer of air on a surface underwater. This avoids direct contact of the ship and water, which reduces drag, corrosion and fouling of the hull and may lead to significant fuel savings at global level. Estimations revealed that applying AIRCOAT on low-draught boats and partly coated high-draught vessels of the global IMO-registered fleet could reduce annual fuel cost by millions of Euros depending on friction reduction performance of AIRCOAT.

Powered by EU-funding, a consortium consisting of the world’s leading marine experts and engineers are working together to create an innovative new fouling protection system for commercial seagoing vessels, and to accelerate its entry to market. This project is called eSHaRk (Eco-friendly Ship Hull film system with fouling release and fuel saving properties). The eSHaRk project aims to bring a greener and more efficient anti-fouling solution to the market. This innovative new technology not only maintains current fouling protection standards but is superior to existing solutions in terms of eco-friendliness, ease of application, durability and drag-reduction, all of which will lead to fuel savings and a reduction in greenhouse gas (GHG) emissions.

An overview of plant surface structures and their evolution is presented. It combines surface chemistry and architecture with their functions and refers to possible biomimetic applications. Within some 3.5 billion years biological species evolved highly complex multifunctional surfaces for interacting with their environments: some 10 million living prototypes (i.e., estimated number of existing plants and animals) for engineers. The complexity of the hierarchical structures and their functionality in biological organisms surpasses all abiotic natural surfaces: even superhydrophobicity is restricted in nature to living organisms and was probably a key evolutionary step with the invasion of terrestrial habitats some 350–450 million years ago in plants and insects. Special attention should be paid to the fact that global environmental change implies a dramatic loss of species and with it the biological role models.

Hydrophobic surface for drag reduction on marine vehicles and structures has been proposed for many years. However, the drag reduction effect has been found to be unstable under high flow speed/pressure conditions because of the destruction of air-water interface. In this paper, an air layer was maintained experimentally by continuous air injection on hydrophobic surfaces. Good hydrophobicity was found to benefit the spread of air bubbles and the formation of a wide and flat air layer on solid surfaces. Based on this recognition, a method combining air injection and surface hydrophobicity adjustment was proposed to maintain the air layer.

Powered by EU-funding, a consortium consisting of the world’s leading marine experts and engineers are working together to create an innovative new fouling protection system for commercial seagoing vessels, and to accelerate its entry to market. This project is called eSHaRk (Eco-friendly Ship Hull film system with fouling release and fuel saving properties). The eSHaRk project aims to bring a greener and more efficient anti-fouling solution to the market. This innovative new technology not only maintains current fouling protection standards but is superior to existing solutions in terms of eco-friendliness, ease of application, durability and drag-reduction, all of which will lead to fuel savings and a reduction in greenhouse gas (GHG) emissions.

Based on a 100% pure silicone binder system, the SIGMAGLIDE 1290 product utilizes a breakthrough dynamic surface regeneration technology to eliminate slime problems and dramatically increase fuel savings throughout service when compared to traditional fouling release products.

A comprehensive survey of the construction principles and occurrences of superhydrophobic surfaces in plants, animals and other organisms is provided and is based on our own scanning electron microscopic examinations of almost 20 000 different species and the existing literature. Properties such as self-cleaning (lotus effect), fluid drag reduction (Salvinia effect) and the introduction of new functions (air layers as sensory systems) are described and biomimetic applications are discussed: self-cleaning is established, drag reduction becomes increasingly important, and novel air-retaining grid technology is introduced. Surprisingly, no evidence for lasting superhydrophobicity in non-biological surfaces exists (except technical materials).

Emissions originating from ship traffic in European sea areas were modelled using the Ship Traffic Emission Assessment Model (STEAM), which uses Automatic Identification System data to describe ship traffic activity. We have estimated the emissions from ship traffic in the whole of Europe in 2011. We report the emission totals, the seasonal variation, the geographical distribution of emissions, and their disaggregation between various ship types and flag states. The total ship emissions of CO2, NOx, SOx, CO, and PM2.5 in Europe for year 2011 were estimated to be 121, 3.0, 1.2, 0.2, and 0.2 million tons, respectively.

The anisotropy of the slip length and its effect on the skin-friction drag are numerically investigated for a turbulent channel flow with an idealized superhydrophobic surface having an air layer, where the idealized air–water interface is flat and does not contain the surface-tension effect. Inside the air layer, both the shear-driven flow and recirculating flow with zero net mass flow rate are considered. With increasing air-layer thickness, the slip length, slip velocity and percentage of drag reduction increase. It is shown that the slip length is independent of the water flow and depends only on the air-layer geometry.

We report a novel, practical technique for the concerted, simultaneous determination of both the adhesion force of a small structure or structural unit (e.g., an individual filament, hair, micromechanical component or microsensor) to a liquid and its elastic properties. The method involves the creation and development of a liquid meniscus upon touching a liquid surface with the structure, and the subsequent disruption of this liquid meniscus upon removal.

This study of greenhouse gas emissions from ships (hereafter the Third IMO GHG Study 2014) was commissioned as an update of the International Maritime Organization’s (IMO) Second IMO GHG Study 2009. The updated study has been prepared on behalf of IMO by an international consortium led by the University College London (UCL) Energy Institute. The Third IMO GHG Study 2014 was carried out in partnership with the organizations and individuals listed below.

Submerged superhydrophobic (SHPo) surfaces are well known to transition from the dewetted to wetted state over time. Here, a theoretical model is applied to describe the depletion of trapped air in a simple trench and rearranged to prescribe the conditions for infinite lifetime. By fabricating a microscale trench in a transparent hydrophobic material, we directly observe the air depletion process and verify the model. The study leads to the demonstration of infinite lifetime (> 50 days) of air pockets on engineered microstructured surfaces under water for the first time. Environmental fluctuations are identified as the main factor behind the lack of a long-term underwater SHPo state to date.

We report a novel, practical technique for the concerted, simultaneous determination of both the adhesion force of a small structure or structural unit (e.g., an individual filament, hair, micromechanical component or microsensor) to a liquid and its elastic properties. The method involves the creation and development of a liquid meniscus upon touching a liquid surface with the structure, and the subsequent disruption of this liquid meniscus upon removal.

An extensive inventory of marine exhaust emissions is presented in the northern European emission control area (ECA) in 2009 and 2011. The emissions of SOx, NOx, CO2, CO and PM2.5 were evaluated using the Ship Traffic Emission Assessment Model (STEAM). We have combined the information on individual vessel characteristics and position reports generated by the automatic identification system (AIS). The emission limitations from 2009 to 2011 have had a significant impact on reducing the emissions of both SOx and PM2.5.

It has long been suspected that the denticles on shark skin reduce hydrodynamic drag during locomotion, and a number of man-made materials have been produced that purport to use shark-skin-like surface roughness to reduce drag during swimming. But no studies to date have tested these claims of drag reduction under dynamic and controlled conditions in which the swimming speed and hydrodynamics of shark skin and skin-like materials can be quantitatively compared with those of controls lacking surface ornamentation or with surfaces in different orientations. We use a flapping foil robotic device that allows accurate determination of the self-propelled swimming (SPS) speed of both rigid and flexible membrane-like foils made of shark skin and two biomimetic models of shark skin to measure locomotor performance.

A method is presented for the evaluation of the exhaust emissions of marine traffic, based on the messages provided by the Automatic Identification System (AIS), which enable the positioning of ship emissions with a high spatial resolution (typically a few tens of metres). The model also takes into account the detailed technical data of each individual vessel. The previously developed model was applicable for evaluating the emissions of NOx, SOx and CO2. This paper addresses a substantial extension of the modelling system, to allow also for the mass-based emissions of particulate matter (PM) and carbon monoxide (CO).

This strategic document presents the Commission’s vision for the future of the EU transport system and defines a policy agenda for the next decade. The programme in question is part of the Europe 2020 strategy and its flagship initiative for a resource efficient Europe.

Superhydrophobic surfaces of plants and animals are of great interest for biomimetic applications. Whereas the self-cleaning properties of superhydrophobic surfaces have been extensively investigated, their ability to retain an air film while submerged under water has not, in the past, received much attention. Nevertheless, air retaining surfaces are of great economic and ecological interest because an air film can reduce friction of solid bodies sliding through the water. This opens perspectives for biomimetic applications such as low friction fluid transport or friction reduction on ship hulls.

Marginal abatement cost (MAC) curves are a staple of policy discussions where there is a need to illustrate the incremental contributions of parts to a whole. In this instance, they provide a simple and elegant way to illustrate greenhouse gas (GHG) emission reductions from design standards, retrofit technologies, and operational measures that improve ship energy efficiency relative to their costs.

A novel mechanism for long‐term air retention under water is found in the sophisticated surface design of the water fern Salvinia. Its floating leaves are evenly covered with complex hydrophobic hairs retaining a layer of air when submerged under water. Surprisingly the terminal cells of the hairs are hydrophilic. These hydrophilic patches stabilize the air layer by pinning the air–water interface. This “Salvinia Effect” provides an innovative concept to develop biomimetic surfaces with long‐term air‐retention capabilities for under water applications.

Marine biofouling is the accumulation of biological material on underwater surfaces, which has plagued both commercial and naval fleets. Biomimetic approaches may well provide new insights into designing and developing alternative, non-toxic, surface-active antifouling (AF) technologies. In the marine environment, all submerged surfaces are affected by the attachment of fouling organisms, such as bacteria, diatoms, algae and invertebrates, causing increased hydrodynamic drag, resulting in increased fuel consumption, and decreased speed and operational range. There are also additional expenses of dry-docking, together with increased fuel costs and corrosion, which are all important economic factors that demand the prevention of biofouling. Past solutions to AF have generally used toxic paints or coatings that have had a detrimental effect on marine life worldwide. The prohibited use of these antifoulants has led to the search for biologically inspired AF strategies. This review will explore the natural and biomimetic AF surface strategies for marine systems.

​The Convention prohibits the use of harmful organotins in anti-fouling paints used on ships and establishes a mechanism to prevent the potential future use of other harmful substances in anti-fouling systems.

Anti-fouling paints are used to coat the bottoms of ships to prevent sealife such as algae and molluscs attaching themselves to the hull – thereby slowing down the ship and increasing fuel consumption.

The interactions of compliant coatings with laminar, transitional and turbulent boundary layers are investigated. A 2 m long flat plate is towed in the range of speeds of 20–140 cm/s in an 18 m water channel using a carriage riding on an oil film. Isotropic and anistropic compliant coatings are used to cover about 20% of the working Plexiglas surface. The compliant material used is a viscoelastic plastisol gel produced by heating a mixture of polyvinyl chloride resin, a plasticizer and a stabilizer, and allowing them to gel. The shear modulus of rigidity of the plastisol was varied by changing the percentage of PVC in the mix. Anisotropy is introduced by placing the plastisol on a rubber surface having longitudinal grooves scaled with the low-speed streaks in the turbulent boundary layer.