Drag-reducing riblets with fouling-release properties: development and testing

Surface Protection Technology for Metallic Materials in Marine Environments

As the demand for the development and utilization of marine resources continues to strengthen, the service requirements for advanced marine equipment are rapidly increasing. Surface protection technology has become an important way of solving the tribological problems of extreme operating conditions and improving the safety performance of equipment by imparting certain special properties to the surface of the material through physical, chemical or mechanical processes to enhance the ability of the material to withstand external environmental factors. Combined with the extremely complex characteristics of the marine environment, this paper describes the commonly used surface protection technologies for metal materials in the marine environment. Research on surface texture was summarized under different surface reshaping technologies, as well as processes and coating materials under different surface modification technologies. Combined with the existing research progress and development trends of marine metallic materials, the surfaces of metal materials under the marine environment protection technology foreground are prospected and provide a reference for the improvement of equipment performance in extreme marine environments.

Prioritizing management of high-risk routes and ports by vessel type to improve marine biosecurity efforts

The shipping industry constitutes the main vector of marine bioinvasions. Over 90,000 vessels world-wide create a highly complex shipping network that requires appropriate management tools. Here we characterized a novel vessel category, Ultra Large Container Vessels (ULCV), in terms of potential contribution to the dispersal of Non-Indigenous Species (NIS) in comparison to smaller vessels traveling similar routes. Such approach is essential for providing precise information-based risk analysis necessary to enforce biosecurity regulations and reduce the adverse global effects of marine NIS. We used Automatic Identification System (AIS) based websites to extract shipping data that will enable us to test for differences in two vessel behaviors linked to NIS dispersal: port visit durations and voyage sailing times. We then examined the geographic spread of ULCVs and small vessels, quantifying the accumulation of new port visits, countries, and ecoregions for each vessel category. Finally, Higher Order Network (HON) analysis revealed emergent patterns within shipping traffic, species flow, and invasion risk networks of these two categories. Compared to the smaller vessels, ULCVs spent significantly longer time in 20% of the ports and were more geographically constrained, with fewer port visits, countries, and regions. HON analysis revealed that the ULCV shipping species flow and invasion risk networks were more similar to each other than to those of the smaller vessels. However, HON port importance shifts were discernible for both vessel categories, with major shipping hubs not necessarily being major invasion hubs. Overall, compared to smaller vessels, ULCVs behave differently in ways that potentially increase biofouling risk, albeit in a smaller set of ports. Future studies using HON analysis of other dispersal vectors appears critical for prioritizing management of high-risk routes and ports.

Switching between 3D Surface Topographies in Liquid Crystal Elastomer Coatings Using Two-Step Imprint Lithography

While dynamic surface topographies are fabricated using liquid crystal (LC) polymers, switching between two distinct 3D topographies remains challenging. In this work, two switchable 3D surface topographies are created in LC elastomer (LCE) coatings using a two-step imprint lithography process. A first imprinting creates a surface microstructure on the LCE coating which is polymerized by a base catalyzed partial thiol-acrylate crosslinking step. The structured coating is then imprinted with a second mold to program the second topography, which is subsequently fully polymerized by light. The resulting LCE coatings display reversible surface switching between the two programmed 3D states. By varying the molds used during the two imprinting steps, diverse dynamic topographies can be achieved. For example, by using grating and rough molds sequentially, switchable surface topographies between a random scatterer and an ordered diffractor are achieved. Additionally, by using negative and positive triangular prism molds consecutively, dynamic surface topographies switching between two 3D structural states are achieved, driven by differential order/disorder transitions in the different areas of the film. It is anticipated that this platform of dynamic 3D topological switching can be used for many applications, including antifouling and biomedical surfaces, switchable friction elements, tunable optics, and beyond.

Investigation of the Drag-Reduction Phenomenon on Plasma-Modified Surface

Drag is one of the main energy-dissipating phenomena in engineering applications. Drag-reduction mechanisms have been studied to reduce this cost. Superhydrophobic surfaces (SHS) have high water repellency and have been studied as an alternative mechanism for reducing drag. The high level of repellency is due to the hierarchical structures in the micro- and nano-scales, making these surfaces able to trap air layers that impose the condition of slipping. The present work investigated the phenomenon of drag reduction on surfaces made of Sylgard® 184 elastomer and modified by low-pressure plasma treatments. Atmospheres with 40% Argon and 60% Acetylene, and 20% Argon and 80% Acetylene were used, varying the treatment times from 10 to 15 min of exposure to Acetylene. The surface, morphological and chemical modifications were confirmed by XPS and AFM analyses, showing the impression of a rough structure on the nanometric scale with deposition of chemical elements from the gas plasma. Furthermore, the obtained SHS showed lower resistance to flow, tested by the imposition of flow in channels.

A metallic anti-biofouling surface with a hierarchical topography containing nanostructures on curved micro-riblets

Metallic surface finishes have been used in the anti-biofouling, but it is very difficult to produce surfaces with hierarchically ordered structures. In the present study, anti-biofouling metallic surfaces with nanostructures superimposed on curved micro-riblets were produced via top-down fabrication. According to the attachment theory, these surfaces feature few attachment points for organisms, the nanostructures prevent the attachment of bacteria and algal zoospores, while the micro-riblets prohibit the settlement of macrofoulers. Anodic oxidation was performed to induce superhydrophilicity. It forms a hydration layer on the surface, which physically blocks foulant adsorption along with the anti-biofouling topography. We characterized the surfaces via scanning electron and atomic force microscopy, contact-angle measurement, and wear-resistance testing. The contact angle of the hierarchical structures was less than 1°. Laboratory settlement assays verified that bacterial attachment was dramatically reduced by the nanostructures and/or the hydration layer, attributable to superhydrophilicity. The micro-riblets prohibited the settlement of macrofoulers. Over 77 days of static immersion in the sea during summer, the metallic surface showed significantly less biofouling compared to a surface painted with an anticorrosive coating.

Sea-trial verification of a novel system for monitoring biofouling and testing anti-fouling coatings in highly energetic environments targeted by the marine renewable energy industry

A novel system was developed to deploy settlement panels to monitor biofouling growth in situ and evaluate antifouling coatings at depths representative of operational conditions of full-scale marine renewable energy devices. Biofouling loading, species diversity, and succession were assessed at depths ranging from 25-40 m at four tests sites in Orkney (UK) featuring extreme wave and tidal current exposure to more sheltered conditions. Evaluations were carried out over a period of 8 months with intermediate retrieval of samples after 3 months. Early pioneer fouling communities, comprised of colonial hydroids, were succeeded by tube-forming amphipods across sites while solitary tunicates dominated in greater shelter. The highest biofouling loading was observed on high-density polyethylene (HDPE) panels (6.17 kg m^-2) compared with coated steel (3.34 kg m^-2) panels after 8 months. Distinct assemblages were present at exposed vs sheltered sites. Better understanding of fouling and antifouling strategies may provide guidance to more effectively manage biofouling impacts in this sector.

A rapid benchtop method to assess biofilm on marine fouling control coatings

A rapid benchtop method to measure the torque associated with minidiscs rotating in water using a sensitive analytical rheometer has been used to monitor the drag caused by marine fouling on coated discs. The method was calibrated using sandpaper surfaces of known roughness. Minidiscs coated with commercial fouling control coatings, plus an inactive control, were exposed in an estuarine harbour. After 176 days the drag on the fouling control-coated discs, expressed as a moment coefficient, was between 73% and 90% less than the drag on the control coating. The method has potential use as a screen for novel antifouling and drag reducing coatings and surfaces. Roughness functions derived using Granville's indirect similarity law are similar to patterns found in the general hydrodynamics literature, and so rotational minidisc results can be considered with reference to other fouling drag datasets.Supplemental data for this article is available online at https://doi.org/10.1080/08927014.2021.1929937 .

Tough amphiphilic antifouling coating based on acrylamide, fluoromethacrylate and non-isocyanate urethane dimethacrylate crosslinker

This study is focused on the development of tougher gels using combinations of acrylamide, fluoromethacrylate and a non-isocyanate urethane dimethacrylate (NIUDMA) crosslinker. The NIUDMA was tailored with 2, 3-epoxypropoxy propyl-polydimethylsiloxane segments E9 (MW = 0.36 kg mol^-1), E11 (MW = 0.5-0.6 kg mol^-1) and E12 (MW = 1-1.4 kg mol^-1). A 3 level Taguchi design was used to evaluate the role of each component of the ternary copolymer gel on the elastic modulus and toughness. The toughness ranged from 2.5-7 MJ m^-3 whereas the modulus ranged from 27-70 MPa. The formulations with the highest toughness and modulus were screened for their antifouling potential in biological assays against the microalga Navicula incerta and the bacterium Cellulophaga lytica. The E9 gels showed the best performance, achieving a 73% reduction in N. incerta cells and a 92% reduction in C. lytica biofilm remaining after water jetting treatments, when compared with the commercial Intersleek product IS700.

Silicone-Based Fouling-Release Coatings for Marine Antifouling

Marine biofouling profoundly influences marine industries and activities. It slows the speed and increases the fuel consumption of ships, corrodes offshore platforms, and blocks seawater pipelines. The most effective and economical antifouling approach uses coatings. Fouling-release coatings (FRCs) with low surface free energy and high elasticity weakly adhere to marine organisms, so they can be readily removed by the water shear force. FRCs have attracted increasing interest because they are biocide-free and hence ecofriendly. However, traditional silicone-based FRCs have weak adhesion to substrates, low mechanical strength, and low fouling resistance, limiting their applications. In recent years, many attempts have been made to improve their mechanical properties and fouling resistance. This review deals with the progress in the construction of high-performance silicone-based fouling-release surfaces.

Thermoplastic, rubber-like marine antifouling coatings with micro-structures via mechanical embossing

New processing routes and materials for non-biocidal, antifouling (AF) coatings with an improved performance are currently much sought after for a range of marine applications. Here, the processing, physical properties and marine AF performance of a fluorinated coating based on a thermoplastic (non-crosslinked) fluorinated polymer are reported. It was found that the addition of lubricating oil and hydrodynamic drag reducing microstructures improved the AF properties substantially, i.e. the settlement of a marine biofilm, containing mixed microalgae including diatoms, was reduced to low levels. More importantly, the remaining fouling was removed from the coatings at low hydrodynamic shear rates and promising AF properties were obtained. Moreover, additional potential benefits were revealed originating from the thermoplastic nature of the coating material which might result in significant cost reductions.

Frictional drag measurements of large-scale plates in an enhanced plane channel flowcell

This paper describes the design of an enhanced, plane channel, flowcell and its use for testing large-scale coated plates (0.6 m × 0.22 m) in fully developed flow, over a wide range of Reynolds numbers, with low uncertainty. Two identical, hydraulically smooth plates were experimentally tested. Uniform biofilms were grown on clean surfaces to test skin friction changes resulting from different biofilm thickness and densities. A velocity survey of the flowcell measurement section, using laser Doppler anemometry, showed a consistent velocity profile and low turbulence intensity in the central flow channel. The skin friction coefficient was experimentally determined using a pressure drop method. Results correlate closely to previously published regression data, particularly at higher speeds. Repeated measurements indicated very low uncertainty. This study demonstrates this flowcell's applicability for representing consistent frictional drag of ship hull surfaces, enabling comparability of hydrodynamic drag caused by surface roughness to the reference surface measurements.

Antifouling and Fouling-Release Performance of Photo-Embossed Fluorogel Elastomers

Oil-infused ‘slippery’ polymer surfaces and engineered surface textures have been separately shown to reduce settlement or adhesion strength of marine biofouling organisms. Here, we combine these two approaches in fluorogel surfaces infused with perfluorinated oils, via a facile photo-embossing method that allows the generation of a micro-scale surface relief structure while retaining the properties of lubricant-infused materials. Testing of these surfaces against a range of marine fouling challenges in laboratory assays demonstrated that when the volume percentage of perfluorinated oil was high, adhesion strengths of attached barnacles and biofilms were low. However, diatoms adhered strongly to test surfaces, highlighting the need to explore different combinations of polymer and oil for such surfaces. Furthermore, the tested surface structures increased settlement and adhesion in the assays, demonstrating the need to optimize any surface structure for specific applications. Nevertheless, the results show the feasibility of combining multiple approaches to create future antifouling technologies.

Nonfreezable Water and Polymer Swelling Control the Marine Antifouling Performance of Polymers with Limited Hydrophilic Content

Zwitterionic chemical groups have well-documented resistance to marine fouling species when presented as homogeneous polymer brushes. These model formulations are not, however, suitable for practical fouling-control applications. It is presently unknown if a uniform film of zwitterions is required to elicit nonfouling character via the binding of interfacial water or if the incorporation of zwitterionic functionality into a more practical bulk polymer system will suffice. Here, copolymers of n-butyl methacrylate were synthesized with low incorporation levels (up to 20 mol %) of hydrophilic functionality, including zwitterionic moieties. Their antifouling (AF) properties were evaluated using barnacle cyprids (Balanus improvisus), diatom cells (Navicula incerta), and a multispecies biofilm. The laboratory assays revealed higher resistance of ionic copolymers toward cyprid settlement, which was attributed to their swelling and the presence of nonfreezable water molecules bound tightly to the polymer chains. Additionally, cells of N. incerta and the multispecies biofilm were removed more effectively on polymers containing sulfobetaine methacrylate and sulfopropyl methacrylate moieties. The results indicate that the presence of tightly bound interfacial water is not limited to model systems of pure hydrophilic homopolymers, but that this mechanism can also reduce the settlement and adhesion of fouling species via bulk copolymer systems with limited hydrophilic content. The swelling of polymers with hydrophilic content may also contribute to their AF efficacy, and such materials may therefore represent a route to translation of the well-documented nonfouling character of zwitterions into practical, industrially relevant coating formulations.