TECHNICAL FIELD

The invention relates to a method of separating the surface of an object from contained or surrounding liquid by a layer of a gas and materials for use in this method. Although not exclusively, the invention relates in particular to a method of reducing the fouling, especially the biofouling, of the surfaces of objects immersed in freshwater or seawater.

BACKGROUND ART

Biofouling is the accumulation of microorganisms, plants, and other creatures (primarily invertebrates, such as barnacles) on submerged surfaces. When biofouling occurs on manmade structures like ship hulls, aquaculture equipment, or pontoons and jetties it causes major problems because it degrades the surfaces and increases drag and weight. The increased drag from biofouling significantlyincreases carbon emissions from the international shipping fleet, representing a notable contributor to climate change. The increased weight can cause structural damage to aquaculture equipment. Biofouling also represents a biosecurity threat because it allowsinvasive species to establish and spread within the environment.

Compared to most pest control scenarios, biofouling is particularly challenging because antifouling solutions need to be both broadspectrum, to manage the diversity of biofoulers, and environmentally safe, to avoid harm to sea life. Biofouling management strategies include preventing the settlement of organisms, inhibiting the growth of settled organisms, and manually removing organisms that have grown on surfaces.

'Antifouling' coatings laced with toxic biocides (for example copper species) are the traditional approach. These coatings are typically broad spectrum, killing a range of unwanted organisms, but can also cause collateral damage by killing other harmless organisms in the process. Some coatings also release persistent ecotoxicants that linger and/or accumulate in the environment to generate unacceptable environmental damage. Many antifouling coatings have been banned and there is mounting scrutiny on the remaining options.

Foul-release coatings are also available that reduce attachment strength of fouling, meaning that it sloughs off via shear stress. Foul-release coatings can be effective and have minimal environmental concerns, but are applicable to only some scenarios (i.e., fast-moving ships in near continual service). Non-coating technologies are mostly mechanical and are particularly preferred in aquaculture where the avoidance of negative environmental impacts and associated risks to food safety are paramount. The need for improved antifouling technologies is urgent and globally important.

The microscopic architecture of the lotus leaf (Nelumbo sp.) means that water cannot penetrate nanofolds on the surface. Pockets of air trapped within the microscopic architecture of the lotus leaf mean water droplets become suspended in the Cassie-Baxter state and readily roll off the surface. The topography network of barbs and barbules of the feathers of birds also provides for a high degree of water repellency via the same mechanism. The superhydrophobicity of hierarchical structures observed in nature has been the inspiration for the development of fabrication methods for producing artificial superhydrophobic surfaces. Known fabrication methods include particle deposition, sol-gel techniques, plasma treatments, vapour deposition and casting techniques.

The publication of Schimmel (2015) discloses the use of large area biomimetic microstructures as gas-retaining layers. The layers are composed of protruding elements spaced apart so that no liquid droplets can become disposed between the elements when the layers are submerged. The gas retaining layer is formed from or coated with a hydrophobic material. An embossed plastic resin or an embossed lacquer is disclosed as the preferred gas-retaining layer. The gas-retaining layers can be used on watercraft to reduce corrosion and fouling. The gas-retaining layers may be fed with a gas directly or via a gas permeable ply.Although the desired characteristics and possible uses of the gas-retaining layers are disclosed, a detailed description of their fabrication is not.

The publication of Schimmel (2021) discloses structured, gas holding surfaces for improving the friction-reducing properties of gas layers held under a liquid and for the simultaneous suppression of turbulence. The gas holding surfaces seek to overcome the limitations of air microbubble technology with regard to the avoidance of corrosion or fouling. The gas holding structures disclosed are characterised in that they include projecting longitudinal structures parallel to the flow direction of the liquid.

An ideal antifouling technology would provide a broad-spectrum effect against many fouling species, exhibit superior efficacy even in static conditions, cost effectiveness, and ease of integration in existing infrastructure. It is an object of the present invention to provide a technology that provides at least some of these desiderata. It is an object of the present invention to provide a method of reducing corrosion or fouling of submerged surfaces. It is an object of the present invention to provide overlays for use in a method of reducing corrosion or fouling of submerged surfaces. It is an object of the present invention to provide structures that are less susceptible to corrosion or fouling when submerged. These objects are to be read in the alternative with the object at least to provide a useful choice.

SUMMARY OF INVENTION

In a first aspect a method of reducing the corrosion or fouling of a surface of an object immersed in or containing a liquid is provided. Preferably, the fouling is biofouling, and the liquid is an aqueous solution. More preferably, the fouling is biofouling, and the liquid is freshwater or seawater.

In a first embodiment of the first aspect, the method comprises applying to the surface of the object an overlay comprising a thickness of a gas permeable gasphilic liquid repellent material. The overlay is used to entrap a layer of gas at the surface of the object when it is immersed in the liquid. The surface of the object is thereby separated from the liquid in which it is immersed. Gas may be delivered to replenish or supplement the layer of gas. The gas may be delivered to replenish or supplement the layer of gas continuously or periodically.

In a second embodiment of the first aspect, the method comprises fabricating the object from a gas permeable gasphilic liquid repellent material. The object entraps a volume of gas at the surface and within the body of the object when it is immersed in the liquid. Gas may be delivered to replenish or supplement the volume of gas. The gas may be delivered to replenish or supplement the volume of gas continuously or periodically.

In either embodiment of the first aspect, when the liquid is an aqueous solution the liquid repellent material is a hydrophobic material, preferably a superhydrophobic material. The liquid repellent material may be both hydrophobic and oleophobic.

In either embodiment of the first aspect the gas may be a biocidal gas. Examples of biocidal gases include chlorine, hydrogen peroxide, ozone. The biocidal gas may be delivered periodically at a concentration effective to remove any residual biofouling.

In the first embodiment of the first aspect, the surface is preferably a solid surface. More preferably, the surface is a solid outer surface. Most preferably, the surface is a solid outer surface, and the object is selected from the group consisting of: hulls, pontoons and pylons. The method is used in situations where it is desirable to at least reduce direct contact between the liquid and the surface of the solid. The reduction may be a reduction in either or both of the area over which the liquid is in direct contact with the surface of the solid or the period of time for which the liquid is in direct contact with the surface of the solid.

The situations where this embodiment of the method is advantageously used include reducing the biofouling of the surfaces of the hulls of vessels and the surfaces of the continuously or periodically submerged portions of freshwater or marine installations. Such installations include pontoons and pylons used in the construction of marinas.

Preferably, the first embodiment of the method of the first aspect comprises contacting the surface with an overlay comprising a thickness of a gas permeable gasphilic liquid repellent material and then delivering air to the second (outer) face of the overlay. The delivering air to the second (outer) face of the overlay may be continuously or periodically. The delivering air to the second (outer) face of the overlay may be either directly to the second (outer) face or via the thickness of the overlay.

In the first embodiment of the first aspect the applying to the surface an overlay may occur before the surface is submerged. The surface may become submerged due to the object being immersed in the liquid, e.g., when a vessel is launched from a dry dock or a pylon is installed.Alternatively, the surface may become submerged due to the liquid engulfing the object, e.g., at high tide.

In the second embodiment of the first aspect the object is preferably an item of clothing or equipment used in liquid environments. Examples of such objects are used in aquaculture, diving and swimming. More preferably, the object is a net. Most preferably, the object is a fish pen net.

In a second aspect an overlay for use in the method of the first aspect is provided. The overlay comprises a thickness of a gas permeable gasphilic liquid repellent material. The overlay has a first (inner) face and a second (outer) face. The overlay may be a laminate. When the overlay is a laminate the thickness of a gas permeable gasphilic liquid repellent material provides the second (outer) face of the overlay. Preferably, the overlay is a flexible overlay.

The material is a porous substrate. The substrate may be selected from the group consisting of: aerogels, cements, ceramics, confined fibres, confined particles, confined platelets, felts, foams, metals, non-woven fibres, sinters, woven fibres, and knitted fibres (including warp knitted fibres). Preferably, the pore diameter of the porous substrate is less than 50 pm. Preferably, where the substrate comprises a fibre, the fibre diameter is less than 50 pm.

The substrate is preferably a polymer. The polymer may be of natural, semisynthetic or synthetic origin. The polymer may be selected from the group consisting of: cellulose, elastomers, lignin, polyamide, polycarbonates, polyester, polyethylene, polypropylene, polysiloxanes, viscose and blends thereof. Preferably, the polymer is selected from the group consisting of: cellulose, lignin, polyamide, polyester, polyethylene, polypropylene, viscose and blends thereof. More preferably, the polymer is polypropylene.

In preferred embodiments, the material is selected from the group consisting of: melt blown non-woven polyolefin material (such as polyethylene or polypropylene), microfiber cloth (composed of 20% (w/w) polyamide and 80% (w/w) polyester), and perforated cloth (composed of 30% (w/w) polyester and 70% (w/w) viscose).

In another embodiment the material may be superhydrophobic low contact angle hysteresis zinc oxide coated textile.

Preferably, the surfaces of the interstices delimiting the pores of the porous substrate are made liquid repellent. More preferably, the surfaces of the interstices delimiting the pores of the porous substrate are made hydrophobic. Most preferably, the surfaces of the interstices delimiting the pores of the porous substrate are made superhydrophobic. The liquid repellent material may be both hydrophobic and oleophobic.

Preferably, the surfaces of the interstices delimiting the pores of the porous substrate are made gasphilic.

Substrates may be made liquid repellent by one of a number of known methods according to their composition. Known methods of increasing the hydrophobicity of substrates include plasma enhanced and hot filament chemical vapour deposition, electrochemical deposition, fluorination, plasma micro roughening (including oxygen plasma micro roughening), sol-gel processing, nano particle deposition, electrospinning, inductive coupling plasma method, chemical etching, wet chemical reaction and hydrothermal reaction. Where the material is a porous substrate, methods that increase the hydrophobicity of the surfaces of the interstices delimiting the pores of the substrate are preferred. Increasing the hydrophobicity of the surfaces of the interstices delimiting the pores of the substrate by exposure to a plasma is preferred. Increasing the hydrophobicity of the surfaces of the interstices delimiting the pores of the substrate by exposure to a carbon tetrafluoride (CF ₄ ) plasma is most preferred. Preferably, the surfaces of the interstices delimiting the pores of the porous substrate are fluorinated.

A sample of the thickness of gas permeable gasphilic water repellent material is characterised in that it is capable of maintaining an entrapped layer of air for a period of time of at least 4 days when the sample is held parallel to the surface of a column of water with the upper face of the sample 95 mm below the surface of the column of water (hydrostatic pressure of 0.93 kPa). During the period of time for which the entrapped layer of gas is maintained air permeates the entire thickness of the gas permeable gasphilic liquid repellent material.

As noted above, the overlay is used in a method of entrapping a layer of gas at a surface of a solid otherwise in contact with a liquid. The gas permeable gasphilic overlay is used in circumstances where it is desirable to at least reduce direct contact between the liquid and the surface of the solid. The reduction may be a reduction in either or both of the area over which the liquid is otherwise in direct contact with the surface of the solid or the period of time for which the liquid is otherwise in direct contact with the surface of the solid.

The circumstances where the overlay is advantageously used include application to the hulls of ships and other vessels and application to the surfaces of the continuously or periodically submerged portions of freshwater or marine installations. Such installations include pontoons and pylons used in the construction of marinas. Use of the gas permeable gasphilic overlay reduces biofouling of the surface of the structure to which it is applied.

In these circumstances the gas will typically be air and the liquid will typically be water.

Typically, the surface of the gas permeable gasphilic water repellent material will have a liquid contact angle hysteresis value of 30° or less, and a liquid sliding angle of 10° or less.

In a third aspect a fouling resistant object fabricated from a gas permeable gasphilic liquid repellent material is provided. Preferably, the object is an item of clothing or equipment used in liquid environments. Examples of such objects are used in aquaculture, diving and swimming. More preferably, the object is a net. Most preferably, the object is a fish pen net.

In an embodiment of the third aspect the fouling resistant object is a net for use in fish pens and the fouling is biofouling. In a fourth aspect a marine installation or vessel comprising an overlay of the second aspect applied to at least a portion of the surface of the marine installation or vessel is provided.

In a fifth aspect a method of preparing an overlay of the second aspect is provided. The method comprises exposing a thickness of a porous substrate to a plasma for a period of time and at a power sufficient to provide the gas permeable gasphilic liquid repellent material of the overlay. Preferably, the plasma is a carbon tetrafluoride (CF ₄ ) plasma. Preferably, the porous substrate is a flexible porous substrate.

It will be recognised that fouling, in particular biofouling, is just one form of contamination of an object or surface. It is anticipated that all forms of contamination arising from the contact of a liquid with a surface may be advantageously controlled through use of these aspects and embodiments.

In the description and claims of this specification the following abbreviations, acronyms, terms and phrases have the meaning provided: "apply" means put or spread on a surface; "artificial" means made or produced by human beings rather than occurring naturally, especially as a copy of something natural; "brackish" means slightly salty; "brine" means water strongly impregnated with salt; "CAS RN" means Chemical Abstracts Service (CAS, Columbus, Ohio) Registry Number; "comprising" means "including", "containing" or "characterized by" and does not exclude any additional element, ingredient or step; "consisting essentially of" means excluding any element, ingredient or step that is a material limitation; "consisting of" means excluding any element, ingredient or step not specified except for impurities and other incidentals; "continuous" means forming an unbroken whole; "delimit" means determine the limits or boundaries of; "dissipate" means disappear or cause to disappear; "embossed" means (of a surface or object) decorated with a design that stands out in relief; "entrap" means catch in or as in a trap and "entrapping a volume of gas at a surface" means at least a portion of the volume of gas is caught in a porous substrate at the surface; "fabricate" means construct or manufacture from pre-prepared components; "fiber" or "fibre" means a thread or filament from which a matrix is formed; "fluid" means a gas or a liquid; "flexible" means capable of being wrapped (without breaking) around the surface of a tube having an outer diameter of 25 cm; "fluorinated" means fluorine (as fluoride) introduced; "gasphilic" means gas attracting and when used to describe a material means having a gas attracting surface capable of retaining a plastron; "hydrophilic" means (when describing the property of a surface) water has a contact angle (9) in the range 0 to 90 degrees; "hydrophobic" means (when describing the property of a surface) water has a contact angle (9) in the range 90 degrees to 180 degrees; "interpose" means place or insert between one thing or another; "interstice" means an intervening space; "marine" means relating to or found in the sea; "matrix" means the substance in which structures are embedded; "oleophobic" means (when describing the property of a surface) oils have a contact angle (9) in the range 90 degrees to 180 degrees; "overlay" means cover the surface of (something) with a coating; "patterned" means decorated with a repeated design; "plastron" means a layer of gas retained at a surface; "permeate" means spread throughout (something); "perforated" means pierced with a hole or holes; "period of time" means a continuous length of time, i.e., without interruption; "porous" means having holes or spaces through which a fluid may pass; "submerge" means cause (something) to be under water; "superhydrophobic" means (when describing the property of a surface) water has a contact angle (9) in the range 150 degrees to 180 degrees; "superoleophobic" means (when describing the property of a surface) oils have a contact angle (9) in the range 150 degrees to 180 degrees; and "thickness" means a layer of a specified material. A paronym of any of the defined terms has a corresponding meaning.

The terms "first", "second", "third", etc. used with reference to elements, features, integers, or other limitations, of the matter described in the Summary of Invention, or when used with reference to alternative aspects or embodiments are not intended to imply any order of preference. Elements, features, integers, or other limitations, of the elements described in the Summary of Invention are identified in order of preference by the introductory "preferably...", "more preferably...", "yet more preferably..." and so on. Preferred combinations of elements, features, integers, or other limitations, of the matter described in the Summary of Invention are similarly identified.

Where concentrations or ratios are specified the concentration or ratio specified is the initial concentration or ratio. Where values are expressed to one or more decimal places standard rounding applies. For example, 1.7 encompasses the range 1.650 recurring to 1.749 recurring.

The invention will now be described with reference to embodiments or examples and the figures of the accompanying drawings pages.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1. Diagrammatic representation of a volume of gas entrapped at the surface of a solid by the use of an overlay.

Figure 2(a) Apparatus for hydrostatic breakthrough test. Figure 2(b) Solid-liquid interface entrapped gas layer bubble chamber.

Figure 3. Scanning electron microscopy (SEM) images of non-woven polypropylene: (a-c) 20 g m ^-2 untreated; (d-f) 20 g m ^-2 CF ₄ plasma treated (30W, 0.2 mbar, 2 min); and (g-i) 35 g m ^-2 untreated.

Figure 4. CF ₄ plasma treatment of non-woven polypropylene: (a) static water contact angle values as a function of electrical discharge power; (b) hydrostatic breakthrough pressure as a function of electrical discharge power; and (c) hydrostatic breakthrough pressure at various electrical discharge powers (0-50W, 0.2 mbar, 2 min) plotted against respective static water contact angle values.

Figure 5. Static contact angles determined for treated and untreated samples of fine melt blown non-woven polypropylene materials ('20 g m ^-2 Meltblown Polypropylene' and '35 g m ^-2 Meltblown Polypropylene'), a hierarchical microfibre cloth composed of 20/80 (w/w) polyamide-polyester ('Microfibre cloth'), and for treated and untreated samples of a perforated all-purpose cloth composed of 30/70 (w/w) polyester-viscose ('Perforated Cloth'). Samples were treated by exposure to CF ₄ plasma (30 W, 0.2 mbar, 2 min). For all samples the value is the mean, and the error bar is the standard deviation of three randomly tested points on three samples. For untreated samples of the microfibre cloth and perforated all-purpose cloth ('Microfibre cloth' and 'Perforated Cloth') water droplets immediately absorbed into the cloth and no contact angle was determinable.

Figure 6. Photographs of gas bubble formation on the submerged sample upper surface in the glass bubble chamber: (a) untreated non-woven polypropylene (small bubbles spreading around the surface); (b) CF ₄ plasma treated non-woven polypropylene (30W, 0.2 mbar, 2 min) (large bubble gas spreading around the surface); (c) untreated mallard feather (large bubble); and (d) comparison of nitrogen gas bubble diameters formed in the trapped gas layer at the solidliquid interface.

Figure 7. Solid-liquid interface trapped gas layer longevity in water monitored with glass bubble chamber: (a) Photographs of CF ₄ plasma treated non-woven polypropylene after 0 and 6 days of water immersion; and (b) untreated and CF ₄ plasma treated (30W, 0.2 mbar, 2 min) non-woven polypropylene for static trapped gas layer and pulsed gas released every 2 h bubbles (< 1 s bursts, total volume = 1.9 ± 0.8 mL) to sustain the solidliquid interface trapped gas layer.

Figure 8. Photographs of CF ₄ plasma treated and untreated samples of microfibre cloth after 0, 1, 4, 7, 10 and 15 days of immersion in the trapped gas layer apparatus. Images of the untreated sample are shown after 0 and 1 days to show there is no change in appearance.

Figure 9. Photographs of CF ₄ plasma treated and untreated samples of perforated all-purpose cloth after 0, 1, 2 and 3 days of immersion in the trapped gas layer apparatus. Images of the untreated sample are shown after 0 and 1 days to show there is no change in appearance.

Figure 10. Comparison of the longevities of the gas layer entrapped at the surface of CF ₄ plasma treated samples of melt blown non-woven polypropylene (20 g m ^-2 and 35 g m ^-2 ), microfibre cloth, and perforated all-purpose cloth.

Measurements are only included if a trapped air layer was present the day before it was first observed to have completely disappeared. In the static studies (no bubbles delivered), 8 separate treated samples and 7 separate untreated samples of non-woven polypropylene material, 3 separate treated samples and 3 separate untreated samples of microfibre cloth, and 3 separate samples and 3 separate untreated samples of perforated all-purpose cloth were included. In the dynamic ('pulsed') studies, modified solar air pumps were used to release 1.9 ± 0.8 mL of air below the samples every 2 h. Here, 3 samples of treated and 3 samples of untreated non-woven polypropylene material were included. Where applicable, error bars show the standard deviation of the longevity tests. Values are the mean and error bars are propagated from the standard deviation of the test samples.

Figure 11. Photographs of treated and untreated samples of non-woven polypropylene material after 0, 4, 8, 14, 21, 27 and 28 days of immersion in the trapped gas layer apparatus. A modified solar air pump was used to introduce 1.9 ± 0.8 mL (mean ± standard deviation of 3 pumps) of air bubbles to the lower surface of the material once every 2 h.

Figure 12. Photographs of untreated (a) and CF ₄ plasma treated (b) samples of perforated all-purpose cloth in the trapped gas layer apparatus at point of maximum surface bubble diameter (nitrogen gas delivered from below at a rate of 30 mL/min).

Figure 13. Photographs of untreated (a) and CF ₄ plasma treated (b) samples of microfibre cloth in the trapped gas layer apparatus at point of maximum surface bubble diameter (nitrogen gas delivered from below at a rate of 30 mL/min).

Figure 14. Comparison of the diameter of nitrogen gas bubbles entrapped at the surface of CF ₄ plasma treated and untreated samples of non-woven polypropylene material (20 g m ^-2 and 35 g m ^-2 ), microfibre cloth, and perforated all-purpose cloth when bubbles are delivered continuously from below at a rate of 30 ml/min. Measurements of 15 bubbles on the surface of tested samples (n=3). Values are the mean, and error bars are propagated from the standard deviation of the test samples.

Figure 15. Biofouling experiment using treated and untreated melt blown nonwoven polypropylene (20 g m ^-2 ) showing samples after 0 and 7 days of immersion. Air bubbles were continuously released under samples A to D but not samples E to H.

Figure 16. Repeat biofouling experiment using treated and untreated melt blown non-woven polypropylene (20 g m ^-2 ) showing samples after 0 and 7 days of immersion. Air bubbles were continuously released under samples E to H but not samples A to D.

DESCRIPTION

The invention resides at least in part in the entrapment of a volume of gas within and at the surface of the porous substrate of which the overlay is comprised or the object is fabricated. The porous substrate is permeable to gas, e.g., air, while substantially excluding the passage of liquid, e.g., water. The passage of liquid is substantially excluded by both the dimensions of the interstices (pores) of the substrate and the surfaces (walls) delimiting the interstices having been made liquid repellent. The surfaces delimiting the interstices can be made liquid repellent, while retaining the dimensions of the interstices, by a number of known methods. For example, and as demonstrated here, the surfaces (walls) can be made liquid repellent by exposing the porous substrate to a carbon tetrafluoride (CF ₄ ) plasma.

Many methods of making surfaces liquid repellent are known. Where the surfaces delimit the interstices of a porous substrate, exposing the porous substrate to a carbon tetrafluoride (CF ₄ ) plasma has been adopted here as a suitable method. As disclosed in the publication of Hopkins and Badyal (1995), CF ₄ plasma treatment can be used to increase the hydrophobicity of polymers. Surface fluorination rather than surface etching or deposition of plasma polymer results.As disclosed in the publication of Godfrey et al (2001) the CF ₄ plasma is capable of permeating a porous substrate. During treatment of the samples of porous substrate described here, the CF ₄ plasma readily permeates the thickness of the porous substrate. The surfaces delimiting the interstices of the porous substrate are thereby made liquid repellent.

The longevity of the layer of gas entrapped at the surface of the overlay is attributed to the layer being a first portion of the total volume of the gas entrapped at the surface of the submerged solid, the second portion of the total volume of the gas entrapped at the surface of the submerged solid occupying the interstices of the porous substrate. The terms "entrapment" and the phrase "entrapped at" are used to describe the circumstance where the volume of the gas forming the layer is continuous with the volume of the gas occupying the interstices of the porous substrate. The circumstance is to be distinguished from the circumstance where the total volume of the gas forming layer is held at the surface (cf. Schimmel (2015) and Schimmel (2021)) and an overlay comprising a porous substrate of the type described here is not used.

The circumstance where a layer of gas is entrapped at the surface, i.e., the volume of the gas forming the layer is continuous with the volume of the gas occupying the interstices of the porous substrate, is represented diagrammatically for an overlay in Figure 1.A layer of gas (1) is shown separating the surface (2) of the overlay (3) from the liquid (4) in which the solid (5) is immersed. The thickness (6) of the overlay (3) is a porous substrate that is permeated by the gas (1) while excluding the liquid (4). In Figure 1 the boundary (7) of the liquid is identified by a broken line. It will be understood that this boundary (7) is dynamic, and the layer of gas (1) may traverse the surface (2). It will also be understood that the layer of gas (1) may be replenished or supplemented by the delivery of additional volumes of gas to the surface (2) either directly to the (outer) face (A), or via the thickness (6) of the overlay (3) (B). Means for delivering additional volumes of gas to the surface are known. The publications of Bullard et al (2010), Scardino and Lewis (2009) and Hopkins et al (2021) disclose examples of such means. Other contemplated means for delivering additional volumes of gas to the surface might make use of a chemical reaction, a photochemical reaction, plasmachemical reaction, electrochemical reaction, or electrolysis, at or in the vicinity of the surface.

EXPERIMENTAL

Materials

Samples (35 mm x 70 mm approx.) were cut from a thickness of a porous substrate to be treated. Samples were cut from the following materials:

1. A fine melt blown non-woven polypropylene material (20 gm ² , Product No. M020A1WMS, Don & Low Limited) having a fibre diameter of 3.4 ± 1.9 pm;

2. A fine melt blown non-woven polypropylene material (35 g m ^-2 , Product No. M035A1WOO, Don & Low Limited) having a fibre diameter of 4.1 ± 2.3 pm;

3. A microfibre cloth composed of 20/80 %(w/w) polyamide/polyester

(SPONTEX™, Mapa Spontex UK Limited, Worcester, United Kingdom); and 4. A perforated all-purpose cloth composed of 30/70 %(w/w) polyester/viscose (SPONTEX™, Mapa Spontex UK Limited, Worcester, United Kingdom).

The samples were cleaned by immersion in a 50:50 (v/v) solvent mixture of cyclohexane (+99.5 %, Fischer Scientific UK Limited) and propan-2-ol (+99.5 %, Fischer Scientific UK Limited) for 3 h. The samples were then dried in air at ambient temperature for at least 2 h.

Plasma chemical surface functionalisation utilised carbon tetrafluoride feed gas (CF ₄ ,+99.7 % purity, Air Products and Chemicals Inc) and was conducted in a cylindrical glass reactor (5 cm internal diameter, 470 cm ³ volume, base pressure lower than 9 x 10 ^-3 mbar, and a leak rate better than 6 x 10 ^-10 mol s ^-1 ) enclosed in a Faraday cage (Hynes et al (1996): Ehrlich and Basford (1992)). The reactor was connected to a 30 L min ^-1 two-stage rotary pump (model E2M2, Edwards Limited) via a liquid nitrogen cold trap. An inductorcapacitor impedance matching network was used to minimise the standing-wave ratio for power transmission from a 13.56 MHz radio frequency (RF) generator (model ACG-3, ENI Technology Inc) to a copper coil (10 turns, spanning 8 cm) wound externally around the glass chamber. The reactor was scrubbed with detergent, rinsed with propan-2-ol, and oven-dried at 200 °C. A continuous wave air plasma was then ignited at 50 W power and 0.2 mbar pressure for at least 30 min in order to remove any remaining contaminants, followed by ignition of a continuous wave CF ₄ gas plasma at 30 W power and 0.2 mbar pressure for 10 min to condition the glass reactor walls. Samples were placed against the interior chamber wall avoiding any overlap. The system was evacuated to base pressure and purged with CF ₄ gas at a pressure of 0.2 mbar for 15 min. The CF ₄ electrical discharge was then reignited at various RF powers and allowed to run for 2 min. Upon termination of the CF ₄ plasma exposure, the RF power generator was switched off, and CF ₄ gas allowed to purge the chamber for a further 5 min. Finally, the system was evacuated to base pressure and vented to atmosphere. For static water contact angle and hydrostatic breakthrough measurements, samples were exposed to CF ₄ plasma on one face, whilst both sides were sequentially CF ₄ plasma treated for solid- liquid interface trapped gas layer longevity, bubble, and biofouling experiments.

Scanning Electron Microscopy

Samples were mounted onto carbon disks supported on aluminium stubs (part no. Sill, TAAB Laboratories Equipment Limited) and then coated with a thin gold layer (5-10 nm, Polaron E500 SEM Coating Unit, Quorum Technologies Limited). Surface topography images were acquired using a scanning electron microscope (model Vega 3LMU, Tescan Orsay Holdings a.s.) operating in the secondary electron detection mode, in conjunction with an 8 kV accelerating voltage and a working distance of 8-11 mm.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was carried out using an electron spectrometer (ESCALAB II, VG Scientific Limited) fitted with an unmonochromatized Mg Kα X-ray source (1253.6 eV) and a concentric hemispherical analyser. Photoemitted electrons were collected at a take-off angle of 20° from the substrate normal with electron detection in the constant analyser energy mode (CAE mode pass energy = 20 eV). A linear background was subtracted from core-level spectra and then fitted using Gaussian peak shapes with a constant full-width-at-half-maximum (Friedman et al (1972)). Experimentally determined instrument sensitivity (multiplication) factors were C(ls) : F(ls) = 1.00 : 0.25.

Water Contact Angle

Static water contact angles were measured using 7 μL high purity water droplets (BS 3978 grade 1) and a video contact angle goniometer (VGA 2500 XE, AST Products Limited). Smaller size droplets could not be dispensed from the water syringe needle tip due to the highly liquid repellent nature of the surface of treated samples. Droplet images were analysed using ImageJ software in conjunction with the Dropsnake plugin (Stadler et al (2006)). Static water contact angle values were calculated from measurements taken at three random points on each of three separate samples, and the propagated standard deviation used for the error value.

Hydrostatic Breakthrough Pressure

A nitrile rubber O-ring (23 mm outer diameter, RS Components Limited) wrapped in PTFE tape (part no. 2ptfewater, Everbuild Building Products Limited) was inserted into the central body of a dismantled brass connector (1-inch internal diameter, Cajon Company). Nitrile rubber O-rings (25 mm outer diameter, RS Components Limited) were located on either side of a 35 mm x 35 mm square of sample and inserted into the brass connector to create a watertight seal. The brass connector fitting containing the sample was attached to a 1 m long graduated glass cylinder (1-inch outer diameter and 23 mm internal diameter).A burette was used to pour water into the top of the graduated tube at a flowrate of 30 mL/min. In order to ensure a steady and even rise in hydrostatic pressure across the surface of the sample, care was taken to position the burette so that water flowed down the graduated tube walls and did not drip directly onto the surface. The meniscus height at which water first penetrated through the material was taken for calculation of the hydrostatic breakthrough pressure (Lamison et el (2012); Saville and Comfort (1999)). Hydrostatic breakthrough pressures were calculated from measurements taken for at least three separate samples, and the propagated standard deviation used for the error value.

Solid-liquid Interface

Entrapped gas was indicated by the formation of a layer of gas at the surface of samples submerged in water.

Size and volume

The entrapped layer of gas was observed using the glass bubble chamber (Figure 2(b)). Two outer aramid fibre-nitrile blend rubber composite gaskets (27 mm internal diameter, 59 mm external diameter, part no.

OFMO030001500002069A, Klingersil C4400, Klinger Limited) were used to secure each 35 mm x 35 mm square of sample onto a glass support ring such that the sample completely covered the outer gasket and support ring holes. Four strips of adhesive tape (part no. SLT1629146, Henkel Limited) were used to press the two outer gasket seals tightly against the sample and the glass support ring. The cylindrical glass bubble chamber was filled with water to 10 cm above the internal glass support lip, and then the pre-assembled gasket-sample-glass support ring-gasket assembly was lowered into the glass bubble chamber to rest on the internal glass support lip to create an entrapped gas layer at a hydrostatic pressure of 0.93 kPa (9.5 cm of water above the test sample upper face). The upper solid-liquid interface of the entrapped gas layer was visible to the naked eye and gave the sample surface a shimmering silvery appearance due to the total internal reflection of light at the liquid-gas interface characteristic of the superhydrophobic state (Rathgen and Mugele (2010)).

For entrapped gas layer surface bubble diameter measurements, nitrogen gas bubbles were introduced at a flowrate of 30 cm ³ min ^-1 through a 4 mm glass tube inlet into the bottom of the glass bubble chamber regulated with an adjustable fine control needle valve (model MN, CT Platon Limited) and monitored using a gas flowrate meter (model Flostat NG, CT Platon Limited). Diameters of at least 15 gas bubbles visible in the upper surface trapped gas layer of each submerged test sample were measured from videos filmed using a 12-megapixel camera (model A1688, Apple Inc.). Bubble surface diameter values were calculated from the average diameter of three separate samples, and the propagated standard deviation used for the error value.

Longevity

In order to monitor the static entrapped gas layer longevity, the gas tube inlet of the glass bubble chamber was sealed off using plastic wrapping film (product no. PM-999, Amcor pic) prior to filling the system with water. The glass bubble chamber was then filled with water up to 10 cm above the glass support lip (9.5 cm above the upper face of the sample) and the pre-assembled gasket-sample-glass support ring-gasket assembly was lowered into the glass bubble chamber to rest on the internal glass support lip. Test samples placed into cylindrical glass bubble chambers were photographed on a daily basis until the appearance of a shiny silver upper face (gas layer) disappeared. The upper water level was topped up regularly to avoid evaporation effects by pouring water down the chamber side walls to maintain a steady and even hydrostatic pressure across the upper face of the sample. A light source (model no. G1330, Gritin Company) was placed 10 cm behind each glass chamber to improve visibility of the gas layer. For static entrapped gas layer experiments samples were left undisturbed. Static entrapped gas layer longevity values were calculated as the average from at least nine separate samples, and the standard deviation used for the error value.

For dynamic entrapped gas layer experiments, a pulse of air bubbles lasting less than 1 s (total volume 1.9 ± 0.8 mL) were injected from below once every 2 h using a modified solar-powered air pump (model No. BSV-AP002, Shenzhen SanShang Technology Company Limited). The total volume of air bubbles was the average released by two of the pumps and the error was propagated from the standard deviation of each. The entrapped gas layer was considered to have collapsed either when the upper face of the sample lost its shiny silvery appearance or when the sample bulged upwards due to the blockage of gas transport through the sample as a consequence of the air layer located on the lower face of the sample having at least partially collapsed (liquid ingress) (Park et al (2019); Huo et al (2019)). Entrapped air layer longevity values were calculated as the average from at least three separate samples, and the standard deviation used for the error value.

Biofouling

A large plastic tank (115 L) filled with water from a duck pond was used for biofouling experiments. Four samples of material were mounted in the lid of a closable LDPE plastic box (model no. HPL822B, Locknlock Company) and the box immersed in the tank. The temperature of the water in the tank was in the range 9 to 13 °C during the experiments.

The lower faces of mounted samples in selected boxes were exposed to a continuous stream of air bubbles (about 1200 cm ³ min ^-1 ) using a solar powered air pump (model no. BSV-AP002, Shenzhen SanShang Technology Company Limited). The plastic tubing (4 mm inside diameter) used to transport air was positioned 7 cm below the centre of the four mounted samples. In each experiment eight samples were evaluated; four mounted samples in each of two boxes. The positions of samples in the biofouling tank were changed for each experiment to remove sample position as a controlling variable in the extent of biofouling.

When samples were removed from the tank, they were gently rinsed to remove nonadherent matter by immersing in a beaker of tap water a couple of times. Samples were photographed under consistent lighting conditions.

RESULTS

Scanning Electron Microscopy

As shown in Figure 3, the surfaces of the non-woven polypropylene materials consist of continuous random fibrous structures. Fibre diameters are in the micron range and the features of the surface are at a scale comparable to the features of superhydrophobic surfaces occurring in nature (Rijke (1968); Elowson (1984)). Importantly, the fibres appear to remain largely unchanged following 30 W CF ₄ plasma exposure. This is attributable to the mild conditions employed (Hopkins et al (1996)). No noticeable structural differences were evident between treated and untreated samples of non-woven polypropylene.Additionally, there were no noticeable structural differences between the 20 g m ^-2 and 35 g m ^-2 textiles.

X-Ray Photoelectron Spectroscopy

XPS analysis of untreated samples of 20 g m ^-2 non-woven polypropylene detected the presence of only carbon (hydrogen is not detectable by XPS). After CF ₄ plasma treatment, an elemental F:C ratio of 1.29:1 was measured consistent with previously reported F:C ratio values for CF ₄ plasma treated surfaces of polypropylene (Hopkins et el (1996); Jones et el (2013)).

Water Contact Angle

The fibrillar topography and hydrophobicity of the surface of non-woven polypropylene material provides untreated samples with high static water contact angle values (148.3 ± 4.6° and 149.6 ± 3.1° for the 20 g m ^-2 and 35 g m ^-2 textiles respectively). Untreated samples of the microfibre cloth and the perforated all-purpose cloth exhibited a water contact angle of 0°. Water droplets are absorbed into the thickness of these porous substrates as is characteristic of hydrophilic materials.

The hydrophobicity of all samples is substantially increased (greater than 7.5%) following CF ₄ plasma treatment (Figures 4 and 5). For samples of the 20 g m ^-2 fine melt blown non-woven polypropylene material the increase in hydrophobicity reached a maximum value of 160.6 ± 4.6° with a 30 W electrical discharge power (Figure 4). Higher power CF ₄ plasma treatments (40W and 50W) were observed to yield slightly lower water contact angle values, and this is attributed to damage to the fibrillar structure by more energetic electrical discharge species (Grill (1994); Schofield and Badyal (2011)). Following 30 W CF ₄ plasma treatment the static water contact angle of the 35 g m ^-2 fine melt blown non-woven polypropylene material increased to 159.4 ± 2.8°. Subjecting samples of the perforated all-purpose cloth to CF ₄ plasma treatment resulted in a more dramatic increase in hydrophobicity, with a static water contact angle value of 146.1 ± 6.5° being observed for treated samples of this cloth. Following CF ₄ plasma treatment, samples of the microfibre cloth displayed a static water contact angle value of 140.5 ± 13.3°. However, as the water droplets fell between the undulating features of the surface, the displayed contact angle value is likely not a true reflection of the water repellency of these samples.

A 30 W electrical discharge power was considered to be optimal for increasing the hydrophobicity (as determined by the static water contact angle) of samples of material and cloth.

Hydrostatic Breakthrough Pressure

Untreated samples of 20 g m ^-2 non-woven polypropylene material displayed a hydrostatic breakthrough pressure of 5.3 ± 0.6 kPa. Breakthrough pressure increased following CF ₄ plasma treatment for powers ranging between 0.ImW and 50 W, reaching over 25 % enhancement (maximum value of 7.9 ± 0.4 kPa) at 30 W electrical discharge power. At higher CF ₄ plasma powers (50W), the material hydrostatic breakthrough pressure dropped. Again, this was attributed to high power plasma species damaging the polymer surface (Grill (1994); Schofield and Badyal (2011)). A good correlation is found between hydrostatic breakthrough pressure and measured static water contact angle values (Figure 4).

Entrapped gas layer (static)

Melt blown non-woven polypropylene material (20 g m ^-2 )

The stability of an entrapped gas layer at the solid-water interface was evaluated using samples of this material having the greatest combined static water contact angle (160.6 ± 4.6°) and hydrostatic breakthrough pressure (7.9 ± 0.4 kPa). The appearance of a shimmering shiny silver surface was taken as characteristic of gas entrapment at the solid-liquid interface (Panchanathan et al (2018)). The enhancement of this appearance for treated samples indicates the formation of a thicker gas layer. The layer of gas entrapped at the solid-liquid interface was observed to dissipate over time (resulting in the loss of the shiny silver appearance of the upper surface of the sample). CF ₄ plasma treatment of samples of non-woven polypropylene material extended the longevity of the entrapped gas layer from 2.5 ± 0.9 days (untreated) to 4.8 ± 1.1 days (Figures 7 and 10).

Melt blown non-woven polypropylene material (35 g in ² )

Following 30 W CF ₄ plasma treatment the gas trapping capabilities of the 35 g m ^-2 melt blown non-woven polypropylene textile improved with trapped gas layer longevities of 1.3 ± 0.5 days for untreated and 2.7 ± 0.5 days for treated (Figure 10).

Microfibre cloth (20/80 %(w/w) polyamide/polyester)

There was a significant improvement in the gas entrapping capabilities of samples of microfibre cloth following CF ₄ plasma treatment (Figures 8 and 10). The entrapped gas layer at the surface of the CF ₄ plasma treated samples was determined to have collapsed when the appearance of the surface of the sample appeared dull and holes in the entrapped gas layer began to appear. Longevities of entrapped gas layers at the surface of CF ₄ plasma treated samples of 12 to 14 days were observed. This was an improvement over the longevities of the entrapped gas layers observed for either CF ₄ plasma treated (4.9 ± 1.2 days) or untreated (2.4 ± 0.9 days) samples of 20 g m ^-2 melt blown non-woven polypropylene material. This improvement in the longevities of the entrapped gas layers is attributed to a combination of the greater depth of the porous hierarchical structure of the cloth and the hydrophobicity of the surfaces throughout the body of the cloth arising from the CF ₄ plasma treatment. This combination results in increased volumes of gas being entrapped at the surface for longer periods of time.

Perforated all-purpose cloth (30/70 %(w/w) polyester/viscose)

Due to the hydrophilic nature of the untreated samples no air is trapped at the surface upon immersion as water immediately penetrates the porous structure and wets the cloth completely. Following CF ₄ plasma treatment the hydrophobicity of samples is improved, and an entrapped air layer formed at the surface. The longevity of this entrapped air layer was 2.3 ± 0.5 days (Figures 9 and 10).

Comparison

In these static studies, i.e., without replenishment of the entrapped gas layer, layers with the greatest longevity were obtained using CF ₄ plasma treated samples of microfibre cloth. Entrapped gas layers were reproducibly maintained (3 replicates) for periods of time of greater than 12 days (Figures 8 and 10). This longevity is attributed in part to the larger volume of air that is entrapped in the depth of the cloth.

The higher weight non-woven polypropylene (35 g m ^-2 grade) has significantly shorter trapped gas layer lifetimes for both untreated (1.3 ± 0.5 days) and CF ₄ plasma treated samples (2.7 ± 0.5 days) compared to their 20 g m ^-2 grade counterparts (2.5 ± 0.9 days and 4.8 ± 1.1 days respectively). As these textiles have similar fibre diameters, static water contact angles, and trapped gas layer surface bubble diameters, it is likely that the material bulk properties also contribute towards trapped gas layer longevity.

The longevities of entrapped gas layers obtained using CF ₄ plasma treated samples of perforated all-purpose cloth (2.3 ± 0.5 days) were lower than those obtained using either CF ₄ plasma treated, or untreated samples of 20 g m ^{- 2} melt blown polypropylene material (4.9 ± 1.2 days and 2.4 ± 0.9 days, respectively), or CF ₄ plasma treated samples of microfibre cloth (13.0 ± 0.8 days).

Entrapped gas layer (dynamic)

The longevities of entrapped gas layers with periodic replenishment of the gas layer were also evaluated. Greater surface reflectivity is an indicator of a thicker air layer (Xiang et al (2020)). The entrapped gas layer was considered to have collapsed or dissipated when either: (i) the upper face of a sample lost its shiny silvery appearance, or (ii) the sample bulged upwards due to the transport of gas through the thickness of the sample being blocked. This latter observation was attributed to the gas layer located at the lower face of a sample having at least partially collapsed and the ingress of liquid.

Melt blown non-woven polypropylene material (20 g m ^{- 2} )

When air bubbles (total volume 1.9 ± 0.8 mL) were pulsed into the trapped gas layer apparatus every 2 h, the upper face of the untreated sample was observed to maintain a shiny silvery appearance for a period of time of at least 2.0 ± 0.8 days (Figure 11). Also, the observed brightness of the face of the untreated sample was less intense than that of the CF ₄ plasma treated sample after the same period of time. The difference in observations is attributed to a smaller volume of gas being entrapped within and at the upper face of the untreated sample.

When air bubbles were injected into the trapped gas layer apparatus every two hours the CF ₄ plasma treated sample maintained an entrapped gas layer for a period of time of at least 28 days. In addition, the treated sample remained flat for at least 28.7 ± 1.7 days (n=3) indicating that gas delivered to the lower face was permeating the depth of the sample. The period of time for which an entrapped gas layer may be maintained is substantially increased by periodic replenishment of the gas through the depth of the sample.

In separate experiments, gas from bubbles introduced continuously from below permeate the sample and accumulate on the upper face of the sample as a gas bubble entrapped at the solid-liquid interface. At a critical volume the gas bubble becomes unstable and detaches into the bulk fluid above (Yong et al (2018)). The diameter of the gas bubble when this detachment occurs was substantially increased for CF ₄ plasma treated samples. Diameters were increased from 6.5 ± 1.8 mm (untreated) to 13.0 ± 2.0 mm (treated). The performance of the CF ₄ plasma treated samples was in this context comparable to that observed for Mallard feathers (Figure 6).

Melt blown non-woven polypropylene material (35 g m ^{- 2} )

When gas bubbles are continuously released below untreated or CF ₄ plasma treated 35 g m ^-2 melt blown non-woven polypropylene material they coalesce with air trapped at the solid-liquid interface. Due to the increase in trapped air volume and buoyancy, gas accumulates on the surface before detaching at a critical volume. The diameter of bubbles at critical volume increases from 6.9 ± 0.7 mm to 13.7 ± 2.1 mm.

Microfibre cloth (20/80 %(w/w) polyamide/polyester)

There is a significant increase in the capability of microfibre cloth to entrap a layer of gas following CF ₄ plasma treatment (Figure 8). As untreated microfibre cloth is hydrophilic it does not trap any air upon water immersion as water fills the pore spaces. Therefore, bubbles that are continuously released below the sample cannot pass through the porous structure and they become trapped below the material. Following 30 W CF ₄ plasma treatment the microfibre cloth becomes hydrophobic and, upon immersion into water, air becomes entrapped both at the surface and within the porous structure. Therefore, air bubbles released below the CF ₄ plasma treated microfibre cloth can coalesce with the entrapped air, increasing the entrapped air volume and causing a bubble to form on the upper surface. The diameter of the bubble at critical volume is 10.9 ± 1.9 mm (Figure 13).

Perforated all-purpose cloth (30/70 %(w/w) polyester/viscose)

Untreated samples were not able to entrap any gas at their surface when submerged in the trapped gas layer apparatus. By contrast, CF ₄ plasma treated samples readily entrapped a gas layer at their upper surface as indicated by the characteristic silvery appearance (Figure 12). The gas layers were considered to have dissipated when the centre of the samples bulged upwards due to gas trapped below.

No gas was able to permeate the depth of the untreated sample due to water ingress of the porous structure (Panchanathan et al (2018)). However, when experiments were conducted using CF ₄ plasma treated samples, the introduction of nitrogen bubbles to the lower surface of the samples resulted in surface bubble diameters of 4.9 ± 0.8 mm demonstrating that the gas was able to permeate the depth of the sample despite the hydrostatic pressure.

The bubble diameters in the entrapped gas layers at the surface of CF ₄ plasma samples were observed to be smaller than those observed for the entrapped gas layers of both CF ₄ plasma treated and untreated samples of melt blown polypropylene material (Figure 12).

The properties of the treated and untreated samples of porous substrates are summarised in Table 1.

Biofouling

The biofouling of treated and untreated samples of material was evaluated with and without replenishment of the putative entrapped gas layer.

Melt blown non-woven polypropylene material (20 g m ^-2 )

Untreated samples of melt blown non-woven polypropylene material were significantly fouled with green matter after 7 days of immersion in duck pond water (Figure 15 and Figure 16). CF ₄ plasma treatment of the material resulted in slightly lower levels of fouling but portions of these samples were still subject to fouling, particularly around the edges.

A greater reduction in fouling was observed when the untreated samples were exposed to a continuous stream of air bubbles compared with those that had not been exposed to bubbles. Untreated non-woven melt blown polypropylene is inherently hydrophobic and can retain volumes of gas for extended periods when submerged in water. The addition of air bubbles may increase the trapped air layer lifetime and enable these untreated samples to remain free of fouling for longer. However, some fouling was still observed on all untreated samples that were exposed to gas bubbles and in some cases the samples appear to have been wetted (Figure 15 and Figure 16). The pressure fluctuations due to bubbles hitting and popping against the lower face of the samples of untreated material may be sufficient to trigger wetting. (Samples that become wetted will eventually become fouled as the air layer can no longer act as a barrier to prevent fouling organisms from reaching the solid surface.) When treated samples of melt blown non-woven polypropylene are exposed to air bubbles the degree of fouling of the samples is reduced relative to treated samples not exposed to air bubbles (Figure 15 and Figure 16). It is anticipated that the permeation of air bubbles through the porous superhydrophobic textile (either through diffusion or direct gas transport) extends the longevity of the entrapped gas layers and thereby prevents fouling from occurring. Other factors such as the pressure difference over the gas-liquid interface may also contribute towards lower organism settlement. It was observed that by the end of the immersion period (day 7) the air bubbles were no longer being transported through the thickness of treated or untreated samples of material. This may be attributed to liquid ingress at the lower face of the sample triggered by pressure fluctuations. Using short pulses of bubbles instead of a continuous stream may be beneficial to limit such pressure triggered liquid ingress whilst still providing enough air to maintain the entrapped gas layers and prevent fouling. Despite the lack of gas transportation, the treated samples remained clean.

EXAMPLE 1

The design, construction and operation of fish pens is established and well- known, e.g., as described in the publication of Kutty and Campbell (1987). In a proposed embodiment, prefabricated netting for a fish pen is treated according to the method described here prior to use in the construction of the fish pen. Such netting, e.g., raschel netting, is available from commercial suppliers, e.g., Hampidjan New Zealand Limited (Port Nelson, New Zealand). Raschel netting has an open construction, with a heavy, textured strand held in place by a much finer strand. They are fabricated by weaving strands of polymers such as polyamides and polyesters. Strands of such polymers comprise an interconnecting network of micro-dimensioned pores, the porosity being a function of how the stands are formed. The woven strands of the raschel netting therefore provide a porous substrate having an irregular, hierarchical surface structure.

The prefabricated netting is subjected to plasma chemical surface functionalisation in a reactor using carbon tetrafluoride as the feed gas to increase the hydrophobicity of the porous substrate. The conditions are adjusted to minimise any reduction in the mechanical properties of the netting.

The fish pen is constructed using the treated netting. On immersion an entrapped layer of air is retained at the submerged surface of the netting. The entrapped layer may be replenished by periodically releasing air from diffusers located below the netting of the fish pen. The retention and replenishment of the entrapped layer of air may also be facilitated by the incorporation of diffusers into the weave of the netting.

The entrapped layer of air will also be replenished when portions of the netting are removed from the water, e.g., during periodic inspections, the replenished entrapped layer of air diffusing across the surface of the functionalised netting.A means of reducing the biofouling of the netting of fish pens that does not disrupt the routine construction, operation and maintenance of fish pens is therefore provided.

EXAMPLE 2

In another proposed embodiment an overlay is adhered to the outer surface of a pontoon used in the construction of a marina. The overlay is adhered to the portions of the surface that are continuously or intermittently immersed in water. The overlay is a flexible, asymmetric laminate comprising a melt blown non-woven sheet of polypropylene with a resilient porous backing. Prior to lamination the sheet of polypropylene has been subjected to plasma chemical surface functionalisation in a reactor using carbon tetrafluoride as the feed gas to provide a gas permeable gasphilic liquid repellent material as characterised here. The porous backing of the laminate is adhered to the outer surface of the pontoon using a suitable marine adhesive such as polyurethane, e.g., AV510, Bostik Australia Pty Limited, Essendon Fields, Victoria, Australia). On deployment in the construction of the marina an entrapped layer may be replenished by periodically releasing air from diffusers located below the pontoon or simply by wave action. A means of reducing the biofouling of the marina is therefore provided.

Although the invention has been described with reference to embodiments or examples it should be appreciated that variations and modifications may be made to these embodiments or examples without departing from the scope of the invention. Where known equivalents exist to specified elements, features, integers, or other limitations, of the matter described in the Summary of Invention, such equivalents are incorporated as if specifically referred to in this specification. Variations and modifications to the embodiments or examples that include elements, features, integers, or other limitations, disclosed in and selected from the referenced publications are within the scope of the invention unless specifically disclaimed. The advantages provided by the invention and discussed in the description included in this specification may be provided in the alternative or in combination in these different embodiments of the invention. INDUSTRIAL APPLICABILITY

The materials and methods claimed and described have use in reducing the corrosion or the fouling of submerged surfaces.

INCORPORATION BY REFERENCE For the purposes of 37 C.F.R. 1.57 of the United States Code of Federal Regulations the disclosures of the following publications (as more specifically identified under the heading "Referenced Publications") are incorporated by reference: Hopkins et al (2021) and Rawlinson et al (2022).

Table 1. Properties of treated and untreated samples of fine melt blown non-woven polypropylene material (20 g m ^-2 , Product No. M020A1WMS, Don & Low Limited), fine melt blown non-woven polypropylene material (35 g m ^-2 , Product No. M035A1WOO, Don & Low Limited), microfibre cloth composed of 20/80 %(w/w) polyamide/polyester (SPONTEX™, Mapa Spontex UK Ltd., Worcester, United Kingdom), and perforated all-purpose cloth composed of 30/70 %(w/w) polyester/viscose

(SPONTEX™, Mapa Spontex UK Ltd., Worcester, United Kingdom). (*Value could not be measured accurately due to surface of sample being uneven.) (N.D.- not determined))

REFERENCED PUBLICATIONS

Badyal and Woodward (2003) Method and apparatus for the formation of hydrophobic surfaces international application no. PCT/GB03/01257 [publ. no. WO 03/080258 A2].

Bullard et al (2010) The use of aeration as a simple and environmentally sound means to prevent biofouling Biofouling, 26(5), 587-593.

Ehrlich and Basford (1992) Recommended Practices for the Calibration and Use of Leaks J. Vac. Sci. Technol.A Vacuum, Surfaces, Film., 10, 1-17.

Elowson (1984) Spread-Wing Postures and the Water Repellency of Feathers: A Test of Rijke's Hypothesis Auk, 101, 371-383.

Friedman et al (1972) Title Phys. Rev. Lett., 29, 692.

Godfrey et al (2001) Plasmachemical Functionalization of Porous Polystyrene Beads Chem. Mater., 13, 513-518.

Grill (1994) Plasma Assisted Etching In Cold Plasma Materials Fabrication; IEEE; pp 216-245.

Hopkins and Badyal (1995) Nonequilibrium Glow Discharge Fluorination of Polymer Surfaces J. Phys. Chem., 99, 4261-4264.

Hopkins et al (1996) Plasma Fluorination versus Oxygenation of Polypropylene J. Phys. Chem. 1996, 100, 6755-6759.

Hopkins et al (2021) Continuous bubble streams for controlling marine biofouling on static artificial structures PeerJ, 9, ell323.

Huo et al (2019) Trapped Air-Induced Reversible Transition between Underwater Superaerophilicity and Superaerophobicity on the Femtosecond Laser-Ablated Superhydrophobic PTFE Surfaces Adv. Mater. Interfaces, 6, 1900262.

Hynes et al (1996) Pulsed Plasma Polymerization of Perfluorocyclohexane Macromolecules, 29, 4220-4225.

Jung et al (2016) Effects of the Air Layer of an Idealized Superhydrophobic Surface on the Slip Length and Skin-Friction Drag Journal of Fluid Mechanics, 790, Rl.

Kutty and Campbell (1987) Pen culture (enclosure culture) as an aquaculture system FAO project RAF/82/009.

Lamison et al (2012) Water Breakthrough Pressure of Cotton Fabrics Treated with Fluorinated Silsesquioxane/Fluoroelastomer Coatings Appl. Surf. Sci. 258, 10205-10208.

Panchanathan et al (2018) Plastron Regeneration on Submerged Superhydrophobic Surfaces Using in Situ Gas Generation by Chemical Reaction ACS Appl. Mater.

Interfaces, 10, 33684-33692. Park et al (2019) Penetration of a Bubble through Porous Membranes with Different Wettabilities. Soft Matter, 15, 5819-5826.

Rathgen and Mugele (2010) Microscopic Shape and Contact Angle Measurement at a Superhydrophobic Surface Faraday Discuss. 146, 49-56.

Rawlinson et al (2022) Nature-inspired trapped air cushion surfaces for environmentally sustainable antibiofouling Colloids and Surfaces A: Physiochemical and Engineering Aspects, 656, 130491.

Rijke (1968) The Water Repellency and Feather Structure of Cormorants, Phalacrocoracidae J. Exp. Biol., 48, 185-189.

Saville and Comfort (1999) Title In Physical Testing of Textiles; Elsevier; pp 209-243.

Scardino and Lewis (2009) Fouling control using air bubble curtains: protection for stationary vessels Journal of Marine Engineering and Technology, A13, 3-10.

Schimmel (2015) Gas-containing surface cover, arrangement, and use United States patent application no. 14/382,482 [publ. no. US 2015/0273791 Al].

Schimmel (2021) Structured gas-containing surfaces International application no. PCT/EP/2019/059294 [publ. no. US 2021/0033119 Al],

Schofield and Badyal (2011) Plasmachemical Oxygenation of Porous Polymer Surfaces for Stable Hydrophilicity and Enhanced Liquid Absorption Polymer, 52, 5732-5738.

Stadler et al (2006) A Snake-Based Approach to Accurate Determination of Both Contact Points and Contact Angles Colloids Surfaces A Physicochem. Eng. Asp., 286, 92-103.

Yong et al (2018) Femtosecond Laser Induced Underwater Superaerophilic and Superaerophobic PDMS Sheets with through Microholes for Selective Passage of Air Bubbles and Further Collection of Underwater Gas Nanoscale, 10, 3688- 3696.

Zhang et al (2009) The Dewetting Properties of Lotus Leaves. Langmuir, 25, 1371-1376.