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, surfaceactive 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.

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The accumulation of marine organisms on a range of manmade surfaces, termed biofouling, has proven to be the Achilles' heel of the shipping industry. Current antifouling coatings, such as foul release coatings (FRCs), only partially inhibit biofouling, since biofilms remain a major issue. Mechanical ship hull cleaning is commonly employed to remove biofilms, but these methods tend to damage the antifouling coating and often do not result in full removal. Here, we report the effectiveness of biofilm removal from FRCs through a novel cleaning device that uses an ultrasonically activated stream (UAS). In this device, ultrasound enhances the cleaning properties of microbubbles in a freely flowing stream of water. The UAS was applied on two types of commercial FRCs which were covered with biofilm growth following twelve days immersion in the marine environment. Biofilm removal was quantified in terms of reduction in biovolume and surface roughness, both measured using an optical profilometer, which were then compared with similar measurements after cleaning with a non-ultrasonically activated water stream. It was found that the UAS significantly improves the cleaning capabilities of a water flow, up to the point where no detectable biofilm remained on the coating surfaces. Overall biofilm surface coverage was significantly lower on the FRC coatings cleaned with the UAS system when compared to the coatings cleaned with water or not cleaned at all. When biofilm biomass removal was investigated, the UAS system resulted in significantly lower biovolume values even when compared to the water cleaning treatment with biovolume values close to zero. Remarkably, the surface roughness of the coatings after cleaning with the UAS was found to be comparable to that of the blank, non-immersed coatings, illustrating that the UAS did not damage the coatings in the process.

Leaf-film adhered to the railway track is a major issue during the autumn/fall season, as leaves fall onto the track and are entrained into the wheel-rail interface. This results in the development of a smooth, black layer. Presently, pressure washers must be used to clean the residue to prevent loss of traction, which can cause crashes or delays by forcing a reduced speed. These pressure washers consume large amounts of water and energy. In this study, use of an ultrasonic cleaning apparatus equipped with a 100 W transducer is investigated, using a low volume of water in the order of 1 L min -1. This was applied to leaf-film samples generated in the laboratory, whose surface properties and thickness were confirmed with optical and stylus profilometry methods. Cleaning achieved by an ultrasonically activated water stream was compared to a) non-activated water and b) an ultrasonic bath with comparable power consumption. Cleaning efficacy was found to be much greater than that afforded by the ultrasonic bath; a rate of 14.3 mm 2 s -1 compared to 0.37 mm 2 s -1, and the ultrasonic bath only cleaned off around 20% of the leaf-film coverage even after 3 minutes of exposure.

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