Recent Advances in Superhydrophobic Materials Development for Maritime Applications

Abstract

Underwater superhydrophobic surfaces stand as a promising frontier in materials science, holding immense potential for applications in underwater infrastructure, vehicles, pipelines, robots, and sensors. Despite this potential, widespread commercial adoption of these surfaces faces limitations, primarily rooted in challenges related to material durability and the stability of the air plastron during prolonged submersion. Factors such as pressure, flow, and temperature further complicate the operational viability of underwater superhydrophobic technology. This comprehensive review navigates the evolving landscape of underwater superhydrophobic technology, providing a deep dive into the introduction, advancements, and innovations in design, fabrication, and testing techniques. Recent breakthroughs in nanotechnology, magnetic-responsive coatings, additive manufacturing, and machine learning are highlighted, showcasing the diverse avenues of progress. Notable research endeavors concentrate on enhancing the longevity of plastrons, the fundamental element governing superhydrophobic behavior. The review explores the multifaceted applications of superhydrophobic coatings in the underwater environment, encompassing areas such as drag reduction, anti-biofouling, and corrosion resistance. A critical examination of commercial offerings in the superhydrophobic coating landscape offers a current perspective on available solutions. In conclusion, the review provides valuable insights and forward-looking recommendations to propel the field of underwater superhydrophobicity toward new dimensions of innovation and practical utility.

Keywords: air‐layer; anti‐corrosion; biomimetic; bio‐inspired; robustness; superhydrophobicity, submersible.

Figure 1

Bio‐inspired SH design in nature, showcasing the commonly adopted natural species, their microstructures and inspired fabricated surfaces like a) butterfly wings. Reproduced with permission.^{[ 23 ]} Copyright 2010, Elsevier. Reproduced with permission.^{[ 24 ]} Copyright 2013, Royal Society of Chemistry. b) shark skin. Reproduced with permission.^{[ 25 ]} Copyright 2018, Wiley. Reproduced with permission.^{[ 26 ]} Copyright 2017, Elsevier. Digital image of shark was sourced from Wikipedia, under GNU Free Documentation License. c) lotus leaves. Reproduced with permission.^{[ 29 ]} Copyright 2011, Beilstein‐Institut. Reproduced with permission.^{[ 30 ]} Copyright 2006, Wiley. d) water strider legs. Reproduced with permission.^{[ 33 ]} Copyright 2007, American Chemical Society. Reproduced with permission.^{[ 32 ]} Copyright 2014, American Chemical Society. e) gecko feet. Reproduced with permission.^{[ 35 ]} Copyright 2012, Royal Society of Chemistry. and f) rose petals. Reproduced with permission.^{[ 37 ]} Copyright 2010, American Chemical Society. The associated are also highlighted.

Figure 2

‘ Salvinia effect’, where the eggbeater hair structures with hydrophilic tips demonstrate water‐pinning effects and able to retain air plastron layer for extended duration, simultaneously maintaining its water‐repelling property: a) spherical water drop on leaf surface, b) eggbeater‐shaped structure, c) SEM observation of the interaction between a waterdrop and the leaf surface, d) hydrophobic repulsion and e) hydrophilic pinning. Reproduced with permission.^{[ 36 ]} Copyright 2010, Wiley.

Figure 3

An overview of recent advancements in nanotechnology for designing SHS is presented, encompassing the fabrication process, design concepts, and outcomes: a) The CS composite coating, highlighted for its impressive plastron retention capability and long‐lasting superhydrophobicity at water depth 140 cm for 50 days. Reproduced with permission.^{[ 68 ]} Copyright 2023, Elsevier. b) CNT@SiO₂ composite coating illustrating its water‐repelling properties across various liquid media. Reproduced with permission.^{[ 70 ]} Copyright 2023, Elsevier. c) CNC coating, modified with methyltrimethoxysilane (MTMS), showed exceptional anti‐fouling properties, proven to endure even after 90 days of immersion in a real sea environment. Reproduced with permission.^{[ 71 ]} Copyright 2023, Elsevier.

Figure 4

Magnetic‐responsive SH surfaces design and application: a) magnetic curing induced hierarchical surface roughness and cyclic toggling between wetting states (from Wenzel to Cassie‐Baxter and vice versa) by externally manipulating surface roughness through a magnetic field. Reproduced with permission.^{[ 81 ]} Copyright 2021, American Chemical Society. b) directing the alignment of cilia through the movement of a magnet. Reproduced with permission.^{[ 84 ]} Copyright 2021, Wiley.

Figure 5

The latest additive manufacturing technologies and their capabilities in replicating/optimizing complex geometries and shapes. a) immersed surface accumulation‐based 3D printing (ISA‐3D) system for replicating the eggbeater shapes of Salvinia and a comparison with simple micro pillar design for water repellence. Reproduced with permission.^{[ 91 ]} Copyright 2018, Wiley. b) a 2PP 3D printing system for producing superhydrophobic hierarchical pillar structures and polydopamine (PDA)‐coated tip. Reproduced with permission.^{[ 92 ]} Copyright 2022, Nature Portfolio. c) a projection micro‐stereolithography (PµSL) for fabricating cylindrical pillar shapes with convex groove curvature that facilitated plastron regeneration. Reproduced with permission.^{[ 93 ]} Copyright 2022, American Chemical Society. d) a fused filament fabrication (FFF)‐type 3D printing for creating magnetically controllable micro‐patterned wall arrays for droplet control. Reproduced with permission.^{[ 94 ]} Copyright 2023, Elsevier.

Figure 6

Multistep approaches for designing SHS via combination of experimental methods, numerical simulations, and ML algorithms. Numerical simulations generate dataset for the wetting process and pressure‐induced wetting transition dynamics on different surface topographies using the finite element method (CA and Laplace pressure datasets). The ML uses ANN algorithm for data training. Experiment data is used for verification by cross‐validation between the predicted and experimental results. Depending on the accuracy of cross‐validation, the trained model can be further used for future predictions based on the design space. Reproduced with permission.^{[ 98 ]} Copyright 2021, American Chemical Society.

Figure 7

Durable SH coating with simultaneous underwater superoleophobic and in‐air SH properties a) Schematic representation for the preparation process of SH coating with both underwater superoleophobicity and in‐air SH properties. b) Digital images of oil repel from the treated surface and rapidly wet the pristine coating surface. c) CA of water (in‐air) and 1,2‐dichloroethane (underwater) after the coatings were treated in different pH solution media. d) High reversibility of surface in air and underwater displayed by switching cycles of superdewetting states. Reproduced with permission.^{[ 109 ]} Copyright 2021, The Royal Society of Chemistry.

Figure 8

SH designs exhibiting stable air plastron and the testing approaches. a) shallow water (20 cm) testing the plastron stability on P7 (organosilica modified with PMDS through physisorption) coated glass. Reproduced with permission.^{[ 115 ]} Copyright 2017, American Chemical Society. b) real‐time monitoring and quantification of underwater superhydrophobicity of O₂ plasma etched poly(methyl methacrylate) (PMMA) film. Reproduced with permission.^{[ 116 ]} Copyright 2022, Wiley. c) plastron stability on re‐entrant nano‐grass (RE‐NG)‐covered micro‐trench superhydrophobic surfaces tested using the most comprehensive approach that considers a wide range of real‐life environmental factors during testing (13‐foot depth in brackish water at a sea mouth, air saturation level, water flow, shear, turbulence fluctuation, biofouling etc.). Reproduced with permission.^{[ 20 ]} Copyright 2023, Cambridge University Press.

Figure 9

Recoverable underwater superhydrophobicity from a fully wetted state via dynamic air spreading. a) Superhydrophobicity demonstrated by sparse surface structure on toy submarine in air (WCA ≈170°), where air plastron status was indicated by the silvery shine i) immediately upon submersion, ii) during pre‐wetting, iii) during gas injection, and iv) restored plastron after gas injection. b) Changes in plastron when samples subjected to repeated pre‐wetting and regeneration. c) Reflectance of the plastron‐regenerated surface at different water pressures. Reproduced with permission.^{[ 120 ]} Copyright 2021, Elsevier.

Figure 10

Drag reduction strategies, encompassing experiment setups and outcomes. a) Petal‐like microstructures, fabricated using the PµSL 3D printing approach, demonstrated gas bubble entrapment, resulting in a significant reduction in drag force under high‐velocity flow conditions. The drag reduction was analyzed using a circulating water test bench. Reproduced with permission.^{[ 8 ]} Copyright 2022, Elsevier. b) Salvinia‐inspired SH surface, crafted through 3D laser lithography, exhibited remarkable drag reduction during the test in a microfluidic channel. Reproduced with permission.^{[ 112 ]} Copyright 2023, Wiley.

Figure 11

An overview of fabrication processes, design concepts, and their respective effectiveness in combating corrosion through SH technologies. a) Features the ZnO@STA@PDMS SH coating, showcasing a hierarchical nano‐micron roughness with a bump‐porous structure. Reproduced with permission.^{[ 131 ]} Copyright 2022, Elsevier. b) Demonstrates needle‐like surface nano‐texture achieved through chemical conversion. Reproduced with permission.^{[ 132 ]} Copyright 2021, Elsevier. c) Highlights the incorporation of polymers with different elastic moduli as an intermediate layer between SHS coatings and substrates. Reproduced with permission.^{[ 107 ]} Copyright 2023, American Chemical Society.

Figure 12

Anti‐fouling through SH technologies a) SEM images of bare Al (i) and hierarchical structured SH surface composed from manganese stearate (ii), with representative water contact angle in insert. (iii–iv) self‐cleaning test, demonstrating the lotus effect that results in the beading of water on the SH surface and the removal of MnO particles. Reproduced with permission.^{[ 139 ]} Copyright 2016, Elsevier. b) In situ fluorescence microscope images of SH surfaces after 5 hr diatom settlement assay. Left: Image taken without visible bubbles in field of view and, right: a tubular bubble can be seen running diagonally through the field of view. The lens effect can be seen clearly where the fluorescence of diatoms is close to the bubble edge (scale bar: 150 µm). Reproduced with permission.^{[ 140 ]} Copyright 2013, AIP Publishing. c) Left: Plastron growth on SHS submerged in an open system containing distilled water at 2 hr intervals (scale bar in mm); Right: Average change in height of a plastron measured with heating applied at intervals to demonstrate the temperature dependence of plastron regeneration. Reproduced with permission.^{[ 141 ]} Copyright 2020, Wiley. d) High resolution fluorescence images of heated and unheated SH samples exposed to a diatom suspension for 5 days. The autofluorescence of the chlorophyll of the diatoms (colored in blue) was used for fouling coverage (scale bar = 500 µm). Reproduced with permission.^{[ 141 ]} Copyright 2020, Wiley.

Figure 13

Overview of commercialized SH techniques with multifunctionality. a) ‘lotus effect’. Reproduced with permission.^{[ 29 ]} Copyright 2011, Beilstein‐Institut. b) ‘riblet effect’. Reproduced with permission.^{[ 146 ]} Copyright 2018, SouthWest Jiatong University. Digital image of shark was sourced from Wikipedia, under GNU Free Documentation License. and a comparison of c) products based on the conventional design concept with singular functionality. Reproduced with permission.^{[ 147 ]} Copyright 2019, Elsevier. Reproduced with permission.^{[ 148 ]} Copyright 2003, American Institute of Physics.