Enhancing Antifouling Strategies with Ultrasonic Treatment and Acoustic Metamaterials

Marine biofouling presents a considerable challenge to the marine industry due to the accumulation of microbial slimes, macroalgae, and invertebrates on ship hulls and subsea equipment. This unwanted adhesion leads to several detrimental effects, including damage to propellers, increased drag, and fuel consumption, and surface deterioration. Furthermore, fouling organisms attached to vessels can travel to new locations, potentially becoming invasive species that threaten local ecosystems. The economic impact of biofouling is significant, with an estimated annual cost exceeding $15 billion globally (Flemming 2011). For example, the US Navy alone spends over $1 billion each year on biofouling-related issues (Callow 2002). Historically, antifouling strategies have relied on chemically active coatings containing compounds such as silver, copper, and tributyltin (TBT) (Chambers 2006). Although these coatings are generally effective, they lack specificity and are highly toxic to marine life, leading to the global phase-out of TBT and silver (Almeida 2007). While copper-based coatings remain in use, concerns over the accumulation of copper in waterways and its impact on aquatic life have driven the search for alternative, non-toxic solutions. One such approach involves the use of foul-release coatings, which do not prevent biofouling but facilitate the easy removal of fouling organisms from surfaces when a vessel is in motion (Magin 2010). However, these coatings are less effective during stationary periods, leading to increased maintenance costs, higher fuel consumption, and coating damage. Thus, there is a clear need to enhance the performance of foul-release coatings, particularly under stationary conditions. Ultrasound, defined as acoustic frequencies above 20 kHz, has unique properties that enable its use across various industries, including the marine sector. The application of ultrasound through the hull of a ship has been reported to inhibit biofouling, including the adhesion of bacteria, microalgae, and barnacles (Legg 2015). Although commercially available ultrasonic transducers are used as antifouling devices in conjunction with antifouling surface coatings, their efficacy is reduced in areas with lower vibration amplitude, such as bulkheads (Legg 2015). To develop an effective antifouling solution, it is essential that ultrasonic treatment remains effective in such challenging areas.