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.

Withstanding 200 cycles of adhesive-peeling at 90.5 kPa [2] Tape-peeling test (using 3M, VHB tape), where tape was rolled over by a 4 kg steel roller twice and then peeled off after 90 s.
Withstanding 30 cycles of adhesive-peeling [3] Tape-peeling test where tape was rolled with a weight of 100 g and then peeled off quickly Withstanding 300 cycles of adhesive-peeling [4] Tape-peeling tests following ASTM D 3359-09 (using Scotch 810 tape), where a pressure of 24.5 kPa was applied Withstanding 80 cycles of adhesive-peeling [5] Tape-peeling tests (using Scotch tape), where tape was rolled over with weight of 100 g, and then peeled off quickly Withstanding 200 cycles of adhesive-peeling [6] Ultrasonic treatment (300 W), where substrate was immersed in ethanol.
Withstanding 60 min of ultrasonic treatment [4] Ultrasonic treatment (720 W) Withstanding 100 min of ultrasonic treatment [8] Abrasion Sandpaper abrasion test with a pressure of ~5 kPa Withstanding 500 cycles of abrasion on 80 grit sandpaper with abrasion distance of 20 m. 380 cycles of abrasion on 360 grit [1] sandpapers, 420 cycles of abrasion for 600 grit sandpaper.
Sandpaper abrasion test (1200 grid) at a velocity of 2 cm/s along the sandpaper under the pressure of 100 g load.
Withstanding at least 30 cycles of abrasion [8] Sandpaper Linear abrasion test where the coated glass substrate was glued under a load of 100 g moved against a aluminum foil at velocity of 2 cm/s.
CA measured are > 150° up until 100 cm of the linear abrasion test

PDMS-based coatings can
undergo the greatest elastic deformation, followed by PU and EP as the worst.[11] Finger-touching and knifescratching Withstanding 140 times of finger friction and 170 cycles of knife scratching.[12] Pencil hardness test where the scratch distance was 6 mm, and the tip load applied was 7.5 N.
Withstanding the scratch test by up to 4H pencil grade.[13] Pencil hardness test following standards GB/T6739-1996 Withstanding the scratch test by up to 6H pencil grade.[14] Hardness tester (KELITI000ZB) was applied, where the force used was 2 N, and hold for 15 s Surface hardness was improved by laser ablated micro-grooves on the super-hydrophobic coatings. [15] Dynamic impact Water impact test where the water spraying, free-falling water droplet and continuous water stream were applied to substrate Withstanding 250,000 water droplet impacts (impact speed of 2.8 m/s and an impact pressure of ~3.9 kPa), 100 cycles of water spray impact (impact speed of 3.0 m/s and an impact pressure of ~4.5 kPa) and 600 s of water stream impact (speed of 7.4 m/s and impact pressure of ~27.4 kPa). [9] Water impact tests including freefalling water droplet and water jet applied by high-pressure water gun Withstanding a water droplet impact at a velocity of ~4.47 m/s, free falling from 100 cm height.
Retaining superhydrophobicity after 310 s exposure to water jet at [16] velocity of 8.6 m/s.
Water impact test where water flow falling from 45 cm height on the surface inclined at 30° at speed of ~1 mL/s) Retaining superhydrophobicity after 20 L of water flow impacted on the surface.[17] High speed water jet test to mimic rain impact via streaming tap water on substrate at velocity of ≈ 6.5

1/ms
Withstanding high-speed water jet for 5 min [7] Solid impact test where sand particles were dropped onto surfaces from 0.5 m height.
The CA and SA negligible affected by sand impact even after 1500 cycles [13] Water jet test with a stream of tap water impacting the surface at velocity of ~1 m/s (~0.5 kPa).
Withstanding water jet for at least 30 min [18] Solid impact test where sand particles (size: 200-350 μm) were dropped onto surfaces from 0.30 m height at 50 g/min.
Withstanding the sand impact for 160 min.[12] Rain-simulated water dripping test where deionized water dropping from 30 cm height (1 L/min) on the surface inclined at 0° and 30°, respectively.

h
Retaining superhydrophobicity after heated up to 250 °C for 2 h, and after heating at 150 °C for 24 h [5] Hot water test where materials were subjected to hot water ranging from 25 to 95 °.
Withstanding water temperatures up to 95 °, silver mirror effect representing trapped air layer could be observed when the film was immersed in 90 °C hot water.[7] Heat treatment of surfaces ranging from 50 to 350 °C.
Thermally stable up to 350 °C.[19] Heat treatment for 2 h on the surface via drying oven, ranging from 50 to 150 °C.
Thermally stable up to 300 °C.[20] Hot water test where water heated to 20 -80 °C was immediately dropped onto surface inclining at 15° for contact angle measurements.
Water repellent even the water droplet was 80 °C.Retaining superhydrophobicity after exposure to HCl, NaCl and NaOH for 90, 90 and 5.5 h, respectively.[7] pH test where the surface was immersed in 1 M H2SO4 and 1 M NaOH.
Retaining superhydrophobicity after exposure to H2SO4 and NaOH for 12 and 2 h, respectively.[11] Corrosion test where scratched and  pH test where coated glass slides Retaining superhydrophobicity for [16]     were immersed in a 5 M sulfuric acid (H2SO4) solution.
at least 72 h pH test where the coated substrate was soaked in different pH aqueous solutions (2,7,12), organic solvents and artificial seawater (2 moL/L NaCl).
Retaining superhydrophobicity for at least 72 h in all immersion solutions. [8] UV/weathering resistance UV irradiation test where an UV source (power: 68 W, wavelength: 253.5 nm) was applied and the distance between the lamp and coating was ~20 cm.
Withstanding continuous 48 h of strong UV irradiation.[6] UV irradiation test where a light source (power = 300 W, λ = 360 nm) was applied and the distance between the light and the samples was 15 cm.
Withstanding continuous 180 h of strong UV irradiation.[21] Actual weathering test where sample was placed at an actual outdoor environment (rooftop), exposing to rain, wind, UV and other outdoor conditions.
Functioning properly even after one month of outdoor exposure.[21] UV irradiation test where a light source with intensity of 7.5 W/m 2 is used CA measured > 160° and SA < 10° even after 648 h of radiation.[16] using a model CMT6103, MTS equipment according to GB/T 1041-92 standard.
Al plate, where the corrosion rate and protection efficiency of the coating were 0.274 mm/year and 93.584%.