Superhydrophobic coating for marine surfaces

Abstract

A device may include a porous substrate. A device may include a superhydrophobic coating applied to at least a portion of the porous substrate. A device may include a gas source adapted to deliver gas through the porous substrate and to contact the superhydrophobic coating.

Description

SUPERHYDROPHOBIC COATING FOR MARINE SURFACES STATEMENT OF GOVERNMENT SUPPORT

[0001] This invention was made with Government support under Award Number: 2041479 awarded by the National Science Foundation. The U.S. Government has certain rights in this invention. BACKGROUND

[0002] A marine vessel experiences significant drag when moving through water. Reducing drag through the water can improve a vessel’s speed and fuel efficiency. Additionally, a marine vessel is subject to a large amount of biofouling that can affect the vessel’s performance. Reducing the ability of biofouling to accumulate on the vessel is also desirable. SUMMARY OF THE INVENTION

[0003] In some aspects, the techniques described herein relate to a marine surface including: a porous substrate; a superhydrophobic coating applied to at least a portion of the porous substrate; a gas source adapted to deliver gas through the porous substrate and to contact the superhydrophobic coating. BRIEF DESCRIPTION OF THE FIGURES

[0004] The drawings illustrate generally, by way of example, but not by way of limitation, various aspects of the present invention.

[0005] FIG. 1 is a schematic view of a marine surface.

[0006] FIG. 2 is a schematic view of another marine surface.

[0007] FIG. 3 is a schematic view of another marine surface.

[0008] FIG. 4a-4f includes schematic figures and shows a method of making the marine surface.

[0009] FIG. 5 is a graph showing the flow rate of air passing through the porous material before and after applying the superhydrophobic coating.

[0010] FIG. 6 is a schematic view of an experimental setup for studying the plastron restoration on a porous superhydrophobic surface (SHS) by gas injection.

[0011] FIG. 7 is a set of time- series of images showing the plastron depletion due to the gas suction from the bottom of the porous surface.

[0012] FIG. 8a-8c includes images and graphs illustrating the status of the plastron on the surface shortly after the gas injection as a function the duration of gas injection (∆t) and the pressure different on two sides of the porous surface (∆p).

[0013] FIG. 9 is a series of images showing the plastron restoration by gas injection with ∆p=33 kPa and ∆t=10 s. The final image (t=13 s) was recorded a short period after gas injection had stopped.

[0014] FIG. 10 is a schematic representation of the plastron restoration process for underwater SHS by gas injection through porous material.

[0015] FIG. 11 is a set of images showing the plastron restoration by gas injection with ∆p=19 kPa and ∆t=10 s. The final image (t=13 s) was recorded a short period after gas injection had stopped.

[0016] FIG. 12 is a set of images showing the plastron restoration by gas injection with ∆p=68 kPa and ∆t=10 s. The final image (t=13 s) was recorded a short period after gas injection had stopped.

[0017] FIG. 13a-13c includes graphs showing (a) Time-variations of the diameter of bubble detached from the SHS (Db) at five different gas injection pressures; (b) Diameters for the first detached bubble (Db1st) and for the detached bubble when Dbbecame stable or when no more bubble merging occurred (Dbstable) as a function of ∆p; and (c) Times for the first bubble to detach (tb1st) and for Dbto become stable (tbstable) as a function of ∆p.

[0018] FIG. 14a-14b includes graphs showing (a) Time-variations of gas flow rates at five different gas injection pressures; and (b) Normalized gas flow rates.

[0019] FIG. 15 are a series of time- series of images showing the bubble restoration from an un-coated, hydrophilic porous material by gas injection with ∆p=5 kPa. DETAILED DESCRIPTION OF THE INVENTION

[0020] Reference will now be made in detail to certain aspects of the disclosed subject matter, examples of which are illustrated in part in the accompanying drawings. While the disclosed subject matter will be described inconjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter.

[0021] Throughout this document, values expressed in a range format should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a range of “about 0.1% to about 5%” or “about 0.1% to 5%” should be interpreted to include not just about 0.1% to about 5%, but also the individual values (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.1% to 0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within the indicated range. The statement “about X to Y” has the same meaning as “about X to about Y,” unless indicated otherwise. Likewise, the statement “about X, Y, or about Z” has the same meaning as “about X, about Y, or about Z,” unless indicated otherwise.

[0022] In this document, the terms “a,” “an,” or “the” are used to include one or more than one unless the context clearly dictates otherwise. The term “or” is used to refer to a nonexclusive “or” unless otherwise indicated. The statement “at least one of A and B” or “at least one of A or B” has the same meaning as “A, B, or A and B.” In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Any use of section headings is intended to aid reading of the document and is not to be interpreted as limiting; information that is relevant to a section heading may occur within or outside of that particular section. A comma can be used as a delimiter or digit group separator to the left or right of a decimal mark; for example, “0.000,1” is equivalent to “0.0001.”

[0023] All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.

[0024] In the methods described herein, the acts can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Furthermore, specified acts can be carried out concurrently unless explicit claim language recites that they be carried out separately. For example, a claimed act of doing X and a claimed act of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the claimed process.

[0025] The term “about” as used herein can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range, and includes the exact stated value or range.

[0026] The term “substantially” as used herein refers to a majority of, or mostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more, or 100%. The term “substantially free of” as used herein can mean having none or having a trivial amount of, such that the amount of material present does not affect the material properties of the composition including the material, such that about 0 wt% to about 5 wt% of the composition is the material, or about 0 wt% to about 1 wt%, or about 5 wt% or less, or less than or equal to about 4.5 wt%, 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.01, or about 0.001 wt% or less, or about 0 wt%.

[0027] As used herein, “liquidphobic” or “super-liquidphobic” structures describe, in a general sense, any material that displays anti-liquid properties, e.g., a material that is one or more of hydrophobic (repels water), lipophobic, olephobic and superoleophobic (repels oils and lipids), amphiphobic (a material which is both hydrophobic and lipophobic), hemophobic (repels blood or blood components) or the like. Such materials repel liquids, e.g., by causing the liquid to bead-up on the material's surface and not spread out or wet the material's surface. Thus, as used herein, a substrate that is described as comprising a liquidphobic structure includes substrates that comprise a liquidphobic, super- liquidphobic, hydrophobic, super-hydrophobic, olephobic, super-liquidphobic, amphiphobic and / or super-amphiphobic substrate.

[0028] When a drop of a liquid (e.g., water based, lipid based, etc.) rests upon a surface, it will spread out over the surface to a degree based upon such factors as the surface tensions of the liquid and the substrate, the smoothness or roughness of the surface, etc. For example, the liquidphobicity of a substrate can be increased by various coatings that lower the surface energy of the substrate. The quantification of liquidphobicity can be expressed as the degree of contact surface angle (or contact angle) of the drop of the liquid on the surface.

[0029] For example, for a surface having a high surface energy (i.e., higher than the surface tension of the liquid drop), a drop of liquid will spread out, “wetting” the surface of the substrate. Such surface displays liquidphilicity, as opposed to liquidphobicity. When the surface energy of a substrate is decreased, liquidphobicity is increased (and vice versa). Liquidphobic, including hydrophobic, lipidphobic and / or amphiphobic refer to properties of a substrate which cause a liquid drop on their surface to have a contact angle of 90 degrees (°) or greater. “Super-hydrophobicity,” “super-amphiphobicity,” and “super- liquidphobicity” all refer to properties of substances which cause a liquid drop on their surface to have a contact angle of 120° or greater.

[0030] While the aspects of the instant disclosure are liquiphobic, the instant disclosure focuses on hydrophobic and particular super-hydrophobic properties. Super-hydrophobic surface (SHS), inspired from the lotus effect in nature, has a wide range of underwater applications, from reducing friction drag in laminar and turbulent flows to protecting submerged surfaces against corrosion and bio-fouling. However, implementing SHS in real-world engineering systems, e.g., on a marine ship, remains a huge challenge. One of the main reasons is the low stability of the gas (or plastron) trapped between the SHS and the liquid. The gas layer, which most of SHS functions rely upon, could be unfortunately depleted due to a number of factors such as flow-induced shear and pressure forces, gas dissolutions when exposed to undersaturated liquid, and increases of hydrostatic pressure.

[0031] In the past few years, a number of passive and active methods were explored to enhance the stability of the plastron for underwater SHS. Passive methods mainly involved the use of complex texture geometries, such as a combination of micro and nano-scale roughness (i.e., hierarchical structures), “re-entrance” geometry, hydrophilic barriers, porous structures, and complexstructures inspired from nature such as Salvinia leaves. Many active methods based on gas replenishment were developed to sustain or restore the underwater plastron. Depending on the source of the gas, these active methods can be broadly classified into five categories: (i) in-situ gas generation based on the decomposition of water or other chemicals added in the water; (ii) gas transfer from super-saturated liquid to SHS; (iii) in-situ water vapor generation by heating; and (iv) gas injection through a single hole into the boundary layer over SHS; and (v) gas injection through a gas permeable material (e.g., polydimethylsiloxane surface) or a porous base.

[0032] Among these active gas replenishment techniques, the one based on gas injection and porous material has advantages such as the relative ease of implementation and a capability to scale to large surface areas. Moreover, by varying the pressure difference on two sides of the porous material, this technique allowed the control of the rate of gas replenishment to match with the varying gas depletion rates under different flow conditions. However, previous studies mainly focused on the effect of gas injection on the sustainability of the plastron, such as against the gas dissolution in undersaturated flows, against the shear and pressure forces in turbulent flows, and against the hydrostatic pressure. Whether and how the gas injection technique can restore the plastron from a state where all the gas on SHS is removed (or from a fully wetted state) remains an open question. The plastron restoration has been observed with other active gas replenishment techniques, for example, these relying on in-situ gas generation. But no experiments have been performed to demonstrate the plastron restoration capability by the gas injection technique.

[0033] A schematic view of marine surface 100 is depicted in FIGS.1-3. As shown, marine surface 100 includes porous substrate 102. Superhydrophobic coating 104 applied to at least a portion of porous substrate 102, and gas source 106 adapted to deliver gas through porous substrate 102 and to contact superhydrophobic coating 104.

[0034] Porous substrate 102 can be formed in whole or in part from any suitable material. For example, porous substrate 102 can include a metal, a plastic material, a ceramic, a glass, or a combination thereof. As an example, the plastic material can be a thermoplastic polymer, a thermoset polymer, or a mixture thereof. More specifically, the plastic material can be a polyamide, apolycarbonate, a polyolefin, a polyester, a polyurethane, an epoxy, a polytetrafluoroethylene, a styrene-butadiene copolymer, ethylene tetrafluoroethylene, a polyvinyl chloride, polyether urethane, a phenyl formaldehyde polymer, or a mixture thereof. The ceramic can be yttria (Y2O3), magnesia (MgO), aluminum oxide (Al2O3), a magnesium aluminum oxide (MgAl2O4), a carbide, an oxycarbide, a nitride, an oxynitride, a boride, an oxyboride, a sulfide, a selenide, a sulfo-selenide, silica, zirconia, silicon-carbide, silicon-nitride, aluminum nitride, or a mixture thereof. The glass can be soda lime silicate glass, alkali aluminosilicate glass, alkali containing borosilicate glass, alkali aluminophosphosilicate glass, alkali aluminoborosilicate glass, or a mixture thereof. The metal comprises steel.

[0035] Porous substrate 102 includes a plurality of through pores 108. That is the pores of porous surface extend fully between opposed major surfaces of porous substrate 102. It is possible for porous surface 102 to include a comparatively small number of pores that may not extend fully between the opposed major surfaces. The size of the individual pores is expressed in terms of the respective diameters. For example, porous substrate 102 can include a plurality of micropores, nanopores, or a combination thereof. Individual pores may have a constant diameter or the diameter may vary across the length of an individual pore. FIG.10 shows an alternative version of porous substrate 102 in which there are not defined through pores as shown in FIGS.1-3. Instead, as shown in FIG.10 the entire substrate is porous and the pores meander through porous substrate 102.

[0036] Superhydrophobic coating 104 is disposed over about 10% to about 100% total surface area of porous substrate 102 (e.g., to the portion or porous surface 102 that is not a void of the pore), about 10% to about 50% total surface area of the porous substrate, less than, equal to, or greater than about 10%, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or about 100%. In some examples, superhydrophobic coating 104 is applied to the walls of the pore. A thickness of superhydrophobic coating 104 can be substantially constant or variable across the article.

[0037] Superhydrophobic coating 104 includes a superhydrophobic material comprising a metal, a polymeric material, a ceramic, a glass, or a combination thereof. As an example, the plastic material can be a thermoplasticpolymer, a thermoset polymer, or a mixture thereof. More specifically, the plastic material can be a polyamide, a polycarbonate, a polyolefin, a polyester, a polyurethane, an epoxy, a polytetrafluoroethylene, a styrene-butadiene copolymer, ethylene tetrafluoroethylene, a polyvinyl chloride, polyether urethane, a phenyl formaldehyde polymer, or a mixture thereof. The ceramic can be yttria (Y2O3), magnesia (MgO), aluminum oxide (Al2O3), a magnesium aluminum oxide (MgAl2O4), a carbide, an oxycarbide, a nitride, an oxynitride, a boride, an oxyboride, a sulfide, a selenide, a sulfo-selenide, silica, zirconia, silicon-carbide, silicon-nitride, aluminum nitride, or a mixture thereof. The glass can be soda lime silicate glass, alkali aluminosilicate glass, alkali containing borosilicate glass, alkali aluminophosphosilicate glass, alkali aluminoborosilicate glass, or a mixture thereof. The metal comprises steel. In some examples, the material of porous substrate 102 and superhydrophobic coating 104 are the same material. This can help to ensure good adhesion between the two. In examples where porous substrate 102 and superhydrophobic coating 104 are different materials adhesives may be needed to join them.

[0038] On the whole, a contact angle of superhydrophobic coating 104 is at least 120 degrees, as determined using ASTM D7334-08, at least 150 degrees, as determined using ASTM D7334-08, as determined using ASTM D7334-08, in a range of from about 120 degrees to about 180 degrees, as determined using ASTM D7334-08 or in a range of from about 140 degrees to about 160 degrees, as determined using ASTM D7334-08.

[0039] Superhydrophobic coating 104 typically has a textured surface that can include a micro-topography, such as micro-pillars. As seen in FIG.2, the substrate 100, can include micro-pillars 105. Micro-pillars 105 can have a height H1 within a range of about 10 microns to about 100 microns, a width W1 within a range of about 10 microns to about 50 microns, and a center-to-center spacing S1 of about 20 microns to about 200 microns.

[0040] Micro-pillars 105 can be formed while the substrate is being formed, e.g., during molding, or can be formed onto the substrate after manufacturing as a post-treatment. Post-treatment micro-topography formation can include, but is not limited to, laser etching, chemical etching, and micro machining. The method selected can depend on the type of material of the substrate. For example, for the electrically conductive tissue sealing plates,generally formed from stainless steel, the micro-topography formation can include the laser etching, chemical etching, and micromachining. In an example where the substate is a polymer, the micro-topography formation can be done while the substrate is being molded into shape.

[0041] In one example, the textured surface can be a topographically complex surface including two “layers” of surfaces having distinct and varied scale ranges. The topographically complex surface generally includes a submicron (nanostructure) surface 109 being superimposed onto the micro-scale roughened surface. “Micro-scale,” as used herein, should be understood to describe an article or feature generally measured in microns such as, for example, 1 micron to 100 microns. “Submicron” or “nanoscale,” as used herein, should be understood to describe an article or feature generally measured in nanometers such as, for example, 1 nanometer to 500 nanometers.

[0042] As illustrated in FIG.3, the substrate 100 includes microstructure 105, as discussed herein, but includes nanostructures 109 engraved into the microstructure 105. The nanostructures 109 can be nano-cavities.

[0043] Microstructure 105 can take the form of a microwire, a microrod, a microtube, a microsphere, or a microdroplet. Nanostructure 109 can take the form of a nanowire, a nanorod, a nanocavity, a nanotube, a nanosphere, or a nanodroplet. Individual microstructures can independently have a major dimension in a range of from about 1 µm to about 1000 µm, about 250 µm to about 750 µm, less than, equal to, or greater than about 1 µm, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, or about 1000 µm. Individual nanostructures 109 can independently have a major dimension in a range of from about 1 nm to about 100 nm, about 10 nm to about 70 nm, less than, equal to, or greater than about 1 nm, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or about 100 nm.

[0044] Although not shown the gas is typically delivered from a pump. In some examples, the pump can be an independent apparatus; in some other examples, the pump may be tied to an engine, generator or other power source. In some examples, e.g., where the marine surface is a portion of a vessel, the vessel may have a compressed air source or other air source with a bleed line for air to be supplied to porous surface 102. Although air (e.g., atmospheric air) is described as the gas, other gases can be used. For example, emission gas can beused as the gas source. The gas can be delivered at a constant rate, a controlled variable rate, or a random rate.

[0045] Marine surface 100 can be one of many different surfaces or part of many different marine surfaces. For example, marine surface 100 can be a vessel hull, a dock, a wharf, a buoy, a marine motor, or a propeller. Examples

[0046] Various aspects of the present invention can be better understood by reference to the following Examples which are offered by way of illustration. The present invention is not limited to the Examples given herein. EXAMPLE 1 Experimental Methods

[0047] A super-hydrophobic surface on porous material was created by a procedure illustrated in Figure 4(a). First, a porous disk with a diameter of 25.4 mm made of 316L stainless-steel (McMaster Carr, #9446T34, thickness 1.59 mm, porosity of 20 to 25%) was cleaned and dried. A Scanning Electron Microscopy (SEM) image of the porous disk was shown in Figure 4(b). The pore size measured from the SEM image ranged from 10 to 50 µm. Then, a commercial super-hydrophobic coating was sprayed (UltraEver Dry) on one side of the porous plate. The application of UltraEver Dry coating involved two steps: a bottom coat for generating surface roughness, and a top coat for altering the surface hydrophobic chemistry. SEM images of the porous plate after applying the super-hydrophobic coating were shown in Figures 4(c-d). Clearly, after the coating, the surface consisted of both micro and nano-scale surface roughness.

[0048] A number of micro-pores were not entirely covered by the coating materials, which provided pathways for the gas to pass through the surface. As shown in Figure 4(e), the water contact angle of the coated porous disk was 162°, confirming that the surface was super-hydrophobic. As shown in Figure 4(f), when immersed in water, the entire disk was covered by a thin air layer (the fraction of surface area covered was close to 1).

[0049] To characterize how well air could pass through the fabricated sample, we experimentally measured the flow rate of air (Q) as a function ofpressure difference (∆p) on two sides of the sample. During the measurement, both sides of the porous material were exposed to air. The pressure difference was measured by a differential pressure transmitter (Omega Engineering, #PX3005-160WDWBI, range 80 kPa, precision 0.075%). The flow rate was measured by counting the time required to displace a specific amount of water (300 ml) by the air at the exit of porous material. Figure 5 shows the measured Q as a function of ∆p for the porous material before and after applying the superhydrophobic coating. As expected, for both cases, Q followed a nearly linear relationship with ∆p. According to the Darcy’s law, the flow rate passing through a porous medium can be expressed as: Q ∆ =kA p, (1) µ L where k is the permeability, A is the surface area, µ is the dynamic viscosity of the fluid, and L is the thickness of the porous material. By fitting the two curves in Figure 5, an air permeability of k=5.5×10−13m2and k=2.7×10−14m2were found for the porous material before and after applying the coating, respectively. As expected, the permeability reduced due to the addition of coating on the porous material.

[0050] To study the plastron restoration by gas injection, experiments were performed in an acrylic tank as shown in Figure 6. The fabricated porous SHS was installed at the bottom of a tank, with the coated side facing to the water and the uncoated side connecting to an air compressor and a vacuum pump. The height of water in the tank was fixed at 0.15 m. The water surface was exposed to atmosphere. The air compressor aimed to replenish the plastron by injecting air upward through the porous material, and the vacuum pump removed the trapped air on SHS via suction. In this work, we mainly studied the impacts of two parameters on the plastron restoration: (i) the pressure difference across the porous SHS denoted as ∆p (a positive ∆p indicated that air is forced upward through the SHS), and (ii) the time duration of gas injection denoted as ∆t. To control ∆p, we used a high-precision air regulator (McMaster Carr, #1888K1, range 25 psi) to vary the pressure at the bottom of the SHS. The magnitude of ∆p was measured by a high precision pressure gauge (Omega Engineering, #DPG108-030G, range 30 psi, precision 0.25%). To control ∆t, weused a valve (McMaster Carr, #3976T1) whose opening duration can be programmed between 1 to 10 s. To visualize the status of gas layer on SHS, a CMOS camera (FLIR, #GS3-U3-41C6M-C, 2048 by 2048 pixels) and a LED light located on two sides of the tank were used. The light illuminated the surface at an angle such that a total internal reflection occurred at the air-water interface. To capture the dynamic process of plastron depletion or restoration, images at 82 frames per second (fps) were recorded.

[0051] Before each plastron restoration test, all the gas was removed on SHS by opening the vacuum pump for a short duration of about 1 s. Figure 7 shows a typical plastron depletion process. Clearly, the gas layer initially attached to the SHS was quickly removed, causing the SHS to reach a fully wetted state. After achieving the fully wetted state, the plastron restoration was tested by gas injection with different magnitudes of ∆p and ∆t. Results and Discussion

[0052] The plastron was tested to determine if it could be restored after injecting gas for a short period of time. Experiments were performed for ∆p varying from 10 to 81 kPa and ∆t from 1 to 10 s. For each case, an image was captured of the SHS shortly after the gas injection stopped and the surface status was stable. The results are shown in Figure 8(a). For small ∆p and ∆t, the plastron was only restored over a small portion of the entire surface. Instead of forming a uniform layer, isolated gas bubbles appeared on the surface. Increasing either ∆p or ∆t resulted in a larger surface area being recovered with the gas layer or gas bubbles. When ∆p or ∆t was sufficiently large, the plastron on the entire surface was fully restored. In some cases, a single air bubble presented close to the center of the SHS, due to the residual air left on the surface after the gas injection. Results for ∆p>40 kPa were similar to ∆p=40 kPa and were therefore not included in Figure 8(a).

[0053] To better quantify the degree of plastron restoration, the percentage of surface area covered by gas was measured and defined as φgbased on the images shown in Figure 8(a). Figure 8(b) plots φg as a function of ∆t for different values of ∆p. In all cases, an increase of φg with increasing ∆t was observed. This trend was most obvious for cases where ∆p was small. For example, as increasing ∆t from 1 to 10 s, φgincreased from 30% to 50% for thelowest ∆p=12 kPa, and from 50% to 90% at ∆p=19 kPa. At ∆p=54 kPa, φgwas very close to 90% at the smallest ∆t=1 s and increased to 100% when ∆t exceeded 8 s. A criticalφgreached to a threshold of 90%. Figure 8(c) shows ∆tcr as a function of ∆p. As expected, increasing ∆p led to a smaller ∆tcr. Moreover, comparing the experimental data to the scaling ∆tcr~1 / ∆p indicated that ∆tcrreduced at a rate faster than 1 / ∆p.

[0054] Next, to the dynamic process of how the plastron was restored, plastron restoration process at different levels of ∆p while keeping ∆t=10 s (the longest duration of gas injection) was studied. Figure 9 shows a plastron restoration process by gas injection at ∆p=33 kPa and ∆t=10 s. Clearly, a short period after the gas injection has stopped (t=13 s), the plastron on the entire SHS was restored. The plastron restoration process can be generally separated into three phases, as illustrated in Figure 10. During Phase I (0<t<183 ms in Figure 9), a number of micro-bubbles merged and randomly distributed on the SHS. It should be noted that we defined t=0 as the time right before the presence of these tiny bubbles. The shape of these micro-bubbles was similar to these formed on an underwater orifice. These bubbles grew due to the gas flow. The growth rate varied at different positions, probably due to spatial variation of the porosity of the SHS. These bubbles could grow to a size of an order of O(1 mm) before merging with neighboring ones. A detachment of these small bubbles was not observed probably because the surface tension force was larger than the buoyant force and momentum force.

[0055] During Phase II (183 ms<t<10 s in Figure 9), small bubbles grew, merged into large bubbles, and detached from the surface. We found two types of bubble merging: (i) merging of two bubbles that were directly contact with each other (e.g., t=244 ms in Figure 9), and (ii) merging of two non-contact bubbles that were separated at a certain distance (e.g., t=366 ms in Figure 9). In both cases, air flowed from the smaller bubbles to the larger ones, which had a smaller pressure inside the bubble according to the Young-Laplace equation. The latter case occurred due to the existence of a thin air layer on the surface, which bridged the two bubbles and promoted mass transfer between them. The consequence of bubble merging was the formation of a bubble with a larger base, which increased the surface tension force and promoted the furtherbubble growth. The contact angle at the three-phase contact line was larger than 150° due to the hydrophobic coating. The merged bubbles kept growing until they formed a neck and detached from the surface when the buoyant and momentum forces overcame the surface tension force. Interestingly, immediately after the bubble detachment, an air layer was left at the same position on the surface. This was due to the super-hydrophobic coating, which caused the air trapped Cassie-Baxter state to be thermodynamically favorable compared to the wetted Wenzel state. As gas continued to be injected through the surface, at the position where the air layer was left, new air bubbles of a similar size to the detached ones repeatedly formed, grew, and detached.

[0056] During the early stage of Phase II (183<t<1037 ms in Figure 9), multiple air bubbles located at different positions along the SHS grew and detached. As time progressed, the bubbles further merged to bigger ones. At the late stage of Phase II (t>1098 ms), there was no further bubble merging. Only a single large bubble located at the center of surface repeatedly formed, grew, and detached. During Phase III after the gas injection has stopped (t>10 s), an uneven air layer covering the entire SHS was left on the surface and slowly stabilized to a thin, uniform layer under the influence of surface tension.

[0057] Figure 11 shows the plastron restoration process at ∆p=19 kPa and ∆t=10 s. Due to the relatively low ∆p, a small portion (≈10%) of the SHS remained wetted following a short period after the gas injection. The gas pressure was not strong enough to displace all the liquid within the micro-pores on the surface. Yet, the overall plastron restoration process was similar to the one shown in Figure 9. The key difference was at the late stage of Phase II. At low ∆p, multiple small bubbles separated at certain distances, repeatedly formed, grew, and detached, without further merging to a single larger one. It was found that the surface areas between these small bubbles remained wetted.

[0058] Figure 12 shows the plastron restoration process at ∆p=68 kPa and ∆t=10 s. As expected, with a large ∆p, the plastron over the entire SHS was restored shortly after the gas injection. The plastron restoration process followed similar steps as the schematic shown in Figure 10. The merging of two non- contact bubbles (seen in Figure 9) was also observed at t=98 ms and t=268 ms in Figure 12. Compared to the low-pressure case shown in Figure 9, the plastron restored at a much faster rate, including an earlier formation of micro-bubblesfrom the micro-pore on the SHS, a faster growth rate of gas bubbles, and the earlier merging of small bubbles to a single large bubble. Interestingly, it was also found that the size of the bubbles detached from the SHS was larger compared to the one at the lower ∆p.

[0059] To systemically investigate the impact of ∆p on the plastron restoration process, the time-variation of the diameter of the detached bubbles Dbwas measured. Figure 13(a) shows the time-variations of Db for five different values of ∆p in the range of 19 to 81 kPa. For large ∆p, the detached bubbles were not perfectly spherical in shape, Db was approximated by the horizonal dimension of the bubbles. For all cases, as increasing time, Dbgradually increased due to the merging of small bubbles at the early stage of plastron restoration before reaching to a stable value (Dbstable) when no more bubble merging occurred.

[0060] Figure 13(b) shows the impact of ∆p on the diameter of the bubble that firstly detached from SHS (Db1st) as well as Dbstable. Both Db1stand Dbstableincreased monotonically when ∆p increased. Two possible reasons were identified for this trend. First, as ∆p increased, the bubble base diameter (Dbbase) increased, as can be seen by images shown in Figures 9, 11 and 12. A larger Dbbaseled to an increase of the surface tension force, allowing the bubble to grow larger before the detachment. Second, the larger values of Db1stand Dbstablecould also be caused by the higher gas flow rates (Q) as increasing ∆p. It is well known that for a bubble detaching from an orifice and at the dynamic region (when Q exceeds a critical flow rate Qc), the detached bubble size increases as increasing Q. According to a model proposed by Oguz and Prosperett , Qc=π(16σ5R5 / 3ρ5g2)1 / 6, where σ was the surface tension, R=Dbbase / 2 was the bubble base radius, ρ was the density of liquid, and g was the gravitational acceleration. Based on this model, for current maximum Dbbase=20 mm, Qc=885 ml / min. As will be shown later, at the highest ∆p=81 kPa, Q was found to be approximately 3500 ml / min which was larger than the value of Qc.

[0061] For a comparison, the size of the bubble that detached from an unbounded SHS at the quasi-static region, DbSta,Ub=9.1 mm was also plotted in Figure 13(b). At small ∆p, Db1stwas smaller than DbSta,Ubsince the SHS was not yet fully covered by air layer and the unbounded condition was not establishedyet. At large ∆p, both Db1stand Dbstableexceeded DbSta,Ubprobably because the current flow rate belonged to the dynamic region as previously explained.

[0062] Figure 13(c) shows the impact of ∆p on the time when the first bubble detached from the surface (tb1st), and the time when Db reached to a stable value (tbstable). Here, tb1stand tbstablecould be approximated as the minimum duration of gas injection required for the plastron to be partially and fully recovered, respectively. In particular, injecting gas for a duration longer than tbstablewill not significantly improve φg. As shown in Figure 13(c), as increasing ∆p from 19 to 81 kPa, both tb1stand tbstablequickly reduced and reached to stable values of about 0.15 s and 0.32 s, respectively. For a comparison, the in-situ gas generation techniques required more than 20 s for the plastron to be restored. The reason that tb1stand tbstablereduced as increasing ∆p included the faster merging of small bubbles and the larger gas flow rate. However, the reason that tb1stand tbstabledid not further reduce at ∆p>68 kPa was unclear, and required

[0063] To better understand how gas passed through the porous SHS during the plastron restoration process, we calculated the gas flow rate Q based on the detached bubbles using the following equation: n 3 Q (tpi ) =Db n n −1, (2) where superscriptdetached from the surface, and tn=0=0. Noted that this method might under- or over-estimate the value of Q for low ∆p since multiple bubbles grew and detached from the surface, and only the volume of a single bubble was account at a time. The magnitude of Q was an important parameter because it determined whether the bubble formation process was at the quasi-static regime or at the dynamic regime. Figure 14(a) shows the time-variations of Q corresponding to the five cases shown in Figure 13(a). The magnitude of Q fell within the range of 10 to 3500 ml / min with the upper limit belonging to the dynamic regime as explained earlier. As expected, in agreement with the Darcy’s law, Q increased as increasing ∆p. Moreover, it was found that for all cases, Q initially increased with time and then became stable.

[0064] To explain the observed trends for Q, we defined and calculated a dimensionless flow rate Q’ as:Q'=QD2. (3) k ∆ pb / µ L For the when both sides of. the time-variations of Q’. Interestingly, after the normalization, all of the profiles collapsed nicely, having a value slightly above 1. This suggests that the flow of air through the underwater porous SHS followed a modified Darcy’s law, given as: Q ≈kD2 b ∆p, (4) µ L where A in the original Darcy’s law expressed in Equ. (1) was replaced by Db2. Indeed, when one side of the porous SHS was submerged underwater, the area which allowed gas to pass through scales with Db2, which itself depended on ∆p and t. According to the proposed Equ. (4), the reason Q increased with time was due to the increase of Db2. It is also worth noting that the permeability for the porous SHS with one side exposed to water was nearly same to the permeability for the SHS with both sides were exposed to air. Future studies are needed to understand the underlying mechanism of this phenomenon.

[0065] An experiment for gas injection through an un-coated, hydrophilic porous disk was also performed. The result was shown in Figure 15. The process of bubble formation, growth, and detachment was very different from the coated porous disk or the SHS. Unlike the coated one, each time after a bubble detached from the uncoated surface, no air layer was left behind on the surface. The size of detached bubble was about 2 to 3 mm, much smaller compared to these detached from the SHS. A merging of two non-contact bubbles separated at a short distance, as seen for the coated porous disk, was not observed here.

[0066] The terms and expressions that have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the aspects of the present invention. Thus, it should be understood that although the present invention has been specifically disclosed by specific aspects and optional features, modification and variation ofthe concepts herein disclosed may be resorted to by those of ordinary skill in the art, and that such modifications and variations are considered to be within the scope of aspects of the present invention. EXAMPLE 2 Bubble formation on super-hydrophobic surface

[0067] Bubble formation on super-hydrophobic surface (SHS) was studied. The gas was injected to the SHS through a single orifice on the SHS. A SHS with a radius RSHSwas submerged in water with a density of ^L=997 kg / m3. An air bubble with a density of ^G=1.2 kg / m3formed on the SHS. The air was supplied to the bubble at a constant flow rate Q by using a syringe pump. The bubble shape was recorded by a high-speed camera with a maximum frame rate of 1000 frames per second. The static contact angle (SCA) of the SHS was 152°. To fabricate the SHS, a surface roughness was created by sandblasting the entire aluminum surface using an abrasive medium of grit size 60 (particle mesh size 35 to 100). Then, the rough surface was cleaned in the ultrasonic bath and coated by hydrophobic nano-particles (Glaco Mirror Coat Zero, by SOFT99 Corp). Only the center region with a radius of RSHS was coated.

[0068] Before a bubble formed, a uniform air layer presented on the submerged SHS. When a bubble formed, the bubble bases either pinned on the rim of SHS for small RSHS or moved on the SHS without reaching to the SHS rim for sufficiently large RSHS. The experimental parameters are listed in Table 1. The value of RSHSchanged from 4.2 mm to 19.0 mm. The value of Q changed from 1 to 150 ml / min. Table 1 also provided the range of Q / Qcr, where Qcrwas the critical flow rate for a transition from the quasi-static regime to the dynamic regime. Qcr was calculated as: Qcr=^(16 / 3g2)1 / 6(^Rbmax / ^L)5 / 6, where ^=72 mN / m was the surface tension and g=9.78 m2 / s was the gravitational acceleration. RSHS (mm) Rbmax(mm) Q (ml / min) Q / Qcr Vd (ml) 4.2 4.2 1 to 150 0.0023 to 0.35 0.17 to 0.29 6.3 6.3 1 to 150 0.0016 to 0.25 0.30 to 0.43 19.0 7.2 to 7.9 1 to 150 0.0071 to 0.21 0.34 to 0.58 Table 1. Main experimental parameters and results in current study.

[0069] First, the effect of RSHSon bubble formation was examined. The gas flow rate was fixed at Q=1 ml / min. t=0 was defined as the time of the detachment of previous bubble, T0was the bubbling period. For small RSHS=4.2 and 6.3 mm, as bubble grows, the contact line quickly pinned at the rim of the SHS after an initial expansion. As a result, the bubble base radius Rb reached to a maximum value of Rbmax=RSHS. For large RSHS=19.0 mm, the contact line expanded and never reached to the edge of SHS. The maximum bubble base radius was Rbmax=7.5 mm. The bubble detached volume Vdand Vd / VT, respectively, as a function of Rbmax. Here, VT=2^Rbmax^ / ^Lg was the Tate volume derived based on the balance between surface tension force and buoyancy force. Regardless the types of surfaces and the values of ^0, the Tate volume provided a good approximation for the bubble detached volume. For SHS with small RSHS, Rbmax=RSHS. For SHS with large RSHS, Rbmaxwas independent of RSHSand might increase as increasing ^0.

[0070] Second, the effect of Q on bubble formation was studied. For different flow rates, the bubble shape was similar, and the time duration of necking was nearly same Tn~20 ms. These results indicate that the bubble shape and the bubble necking were not affected by the momentum of the injected gas. Nevertheless, as increasing Q, T0 reduced and thereby Tn / T0 (the percentage of necking duration to the bubbling period) increased from <1% to ~20%. Moreover, Vdincreased by 1.4 to 1.7 times as increasing Q from 1 to 150 ml / min. For a comparison, results for bubble formation on a hydrophobic surface and a hydrophilic surface were also plotted. Regardless of the surface type, the profiles generally followed the same trends. For sufficiently low flow rates (Q<<Qcr), Vd / VTwas close to 1, suggesting that the bubble was governed by a balance between surface tension and hydrostatic pressure. However, as increasing Q close to Qcr, Vd / VTincreased and was larger than 1. Effect of gas permeability of porous material on the gas restoration process

[0071] Gas permeability of the porous material on gas restoration process on SHS was examined. To fabricate the porous SHS, a commercial superhydrophobic coating (UltraEver Dry) was sprayed on one side of the porous stainless-steel plates. The application of UltraEver Dry coating involvedtwo steps: a bottom coat for generating surface roughness, and a top coat for altering the surface hydrophobic chemistry.

[0072] Gas permeability of the porous SHS was examined before and after the coating. The gas permeability was characterized by measuring the flow rate of air (Q) as a function of pressure difference (∆p) on two sides of the sample. Both sides of the porous material were exposed to air. The pressure difference was measured by a differential pressure transmitter (Omega Engineering, #PX3005-160WDWBI, range 80 kPa, precision 0.075%). The flow rate was measured by counting the time required to displace a specific amount of water by the air at the exit of porous material. The measurement results are provided in Table 2. Clearly, the porous SHS with a larger pore size had a larger gas permeability. permeability permeability after Sample # pore size before coating coating (22m ) (m ) 1 2 µm 5.5 × 10-131.5 × 10-142 20 µm 2.2 × 10-121.2× 10-12Table 2. Experimentally measured permeability of the porous samples before and after applying the super-hydrophobic coatings.

[0073] To study the effect of gas permeability on the gas restoration, the status of gas layer on SHS was measured by injecting gas at different pressure and with different time duration of gas injection. The measurements were started from a state where all the gas on the SHS was removed (e.g., started from a fully wetted SHS). For both SHSs with different gas permeabilities, a uniform air layer can be re-formed on the surface when the pressure was high. Moreover, for SHS with larger pore sizes and larger gas permeabilities, it required less pressure for the restoration of air plastron and thus more economically feasible. EXAMPLE 3 Experimental setup

[0074] The experimental setup for studying the bubble formation on a SHS includes a transparent tank made of acrylic that is filled with water. The tank has an inner dimension of 100 mm by 100 mm. The height of the water is maintained at 70 mm. These dimensions ensure that the walls and the upper free surface have a negligible impact on the bubble formation. At the bottom of the tank, a 50 mm by 50 mm aluminum surface is installed. The center area with a radius of RSHS of this aluminum surface is made to be superhydrophobic. Outside this region, the surface is hydrophilic. The equilibrium water contact angles (^0) on the super-hydrophobic and hydrophilic regions measured by placing a small water drop on the surface are 152±2° and 32±2°, respectively. The sliding angle of a water droplet on the SHS is 5±2°. The procedure is briefly described below. First, surface roughness is fabricated on the entire aluminum surface by sandblasting (abrasive medium aluminum oxide, grit size 60, particle mesh size 35 to 100). The surface is clean in ultrasonic bath before and after applying the sandblasting. Then, super-hydrophobicity is achieved by spray-coating the rough surface with hydrophobic nano-particles (Glaco Mirror Coat Zero, by SOFT99 Corp). Only the center aera with the radius of RSHS is coated.

[0075] A 0.5 mm diameter orifice is fabricated at the center of the SHS to allow the gas to be injected into the bubble. The gas flow is generated and controlled by a syringe pump (Model #NE-1010 SyringeONE, by New Era Pump System Inc). A long needle with a length of 152 mm and an inner diameter of 0.61 mm is used to guide the gas to the orifice. The purpose of this long needle is to achieve a large pressure drop so that the pressure variation in the bubble does not cause a notable change of the pressure in the syringe. As will be shown later, during the bubble growing process, a constant gas flow rate is achieved in current study.

[0076] A high-speed camera (PCO.dimax S4, pixel size 11 µm, 2016×2016 pixels) is used to record the bubble formation and detachment. A collimated light (Thorlabs, model #QTH10, power 50 mW) with a diffuser is used to illuminate the bubble. To record the bubble growing process, a frame rate of from 50 frames per second (fps) is used. To record the necking process which occurs at a very short timescale, a frame rate of 1000 fps is used. The spatial resolution of the imaging system is 34 µm / pixel. The data is recorded after a series of bubbles have formed and detached from the surface. The light isturned on for a short duration of time and has negligible influence on the temperature of the water.

[0077] The key experimental parameters are listed in Table 3. Three values of RSHS=4.2, 6.3, and 19.0 mm are used in current study. The reason of choosing these values is to cover the two modes A and B of bubble formation, the two modes are observed in current study. In Mode A for small RSHS=4.2 and 6.3 mm, as bubble grows, the contact line quickly pins at the rim of the SHS after an initial expansion. As a result, the bubble base radius Rbreaches to a maximum value of Rbmax=RSHS. In Mode B for large RSHS=19.0 mm, the contact line expands and never reach to the edge of SHS. The maximum bubble base radius is Rbmax=7.5 mm, which has a fair agreement with the numerical simulations. The values of Rbmaxfor the three cases are provided in Table 3. As will be shown later, the two cases for Mode A (RSHS=4.2 and 6.3 mm) are sufficient to understand the bubble formation in Mode A since the bubble geometrical parameters and the forces acting on the bubble follow similar trends regardless of RSHS. Moreover, the one case for Mode B (RSHS=19.0 mm) is sufficient, since in Mode B the bubble has the same maximum based radius (Rbmax=7.5 mm) and same geometrical parameters regardless of RSHS.

[0078] Also listed in Table 3 is the gas flow rate Q=1.2 ml / min for all three cases. The value of Q is calculated by fitting the curve of bubble volumes at the constant flow region. The bubbling frequency is less than 0.2 Hz, minimizing the impact of previously formed bubble on the reference one. Moreover, the small value of Q ensures a quasi-static condition. The critical flow rate for a transition to dynamic region can be estimated as: Qcr=^(16 / 3g2)1 / 6(^Rbmax / ^L)5 / 6, where ^=72 mN / m is the surface tension of water, ^L=997 kg / m3is the density of water, and g=9.78 m2 / s is the gravitational acceleration. As shown in Table 3, Q / Qcr is much smaller than 1. Table 3 also lists the dimensionless Rbmax*=Rbmax / l^=(Bo)1 / 2, where l^=(^ / ^Lg)0.5=2.7 mm is the capillary length and Bo is the Boud number. RSHS RbmaxmaQ Modex / l (mm)(mm)Rb^(ml / min)Q / QcrA 4.2 4.2 1.5 1.2 0.0023A 6.3 6.3 2.3 1.2 0.0016 B 19.0 7.5 2.8 1.2 0.0014 Table 3. Summary of experimental parameters in current study. Data analysis and force calculation

[0079] To characterize the shape of the bubble as it grows, we calculate several parameters, including the volume (V), base radius (Rb), height (H), radius at the apex (Ra), and contact angle at the three-phase contact line (^). V is calculated by accumulating the cross-section area at each height level from the bottom to the top of the bubble. Rais obtained by fitting the bubble apex with a circle of radius Ra. ^ is found by linearly fitting the bubble shape near the three- phase contact line. The velocity of bubble in the vertical direction is also estimated as Ub=dH / dt, where t is the time. To understand the forces acting on the bubble, six calculated forces, as summarized in Table 4 are produced. The six forces include two lifting forces (pressure force FP and gas momentum force FGM) and four restraining forces (surface tension force FS, buoyancy force FB, drag force FD, and liquid inertia force FLI). The pressure force is mainly caused by the surface tension at the bubble apex, which leads to a higher pressure in the bubble than that in the liquid (2^ / Ra is the Laplace pressure at the bubble apex). The gas momentum force is due to the momentum of the gas flowing through the orifice. The surface tension force applies at the three-phase contact line of the bubble base. The buoyancy force is due to the hydrostatic pressure applied on the bubble surface. As will be shown later, FB for bubble presented on SHS is mostly in the downward direction, which is in opposite to bubble formation at a nozzle. The liquid inertia force accounts for the momentum of surrounding liquid due to the acceleration of the bubble as it grows. The drag force is approximated as the force applied on a bubble of radius Rb moving at a constant velocity Ub in the liquid. The drag coefficient CD takes the form of CD=24 / Reb(1+0.15Reb0.687)

[0066] , where Reb=^LUbRb / ^L is the Reynolds number and ^L=1.0×10−3N s / m2is the dynamic viscosity of the water. This expression is valid for the current range of Reb<1000.Forces Expressions Direction Laplace pressure force FP =(2^ / Ra+^GgH)^Rb2UpwardTable 4. Expressions and directions of six forces acting on the bubble as it grows on SHS. Here, ^G=1.3 kg / m3is the density of air, negative sign indicates the force is the downward direction. The uncertainties of measurement parameters are mainly caused by the errors in determining the boundary of the bubble during the image processing. To estimate the measurement uncertainties, we select different intensity thresholds to get binarized images of the bubble and bubble boundaries. We found that the uncertainty of H is about the size of a pixel (34 µm), the uncertainty of Rbis 0.1 mm, the uncertainty of Ra reduces as increasing bubble volume and varies in the range of 0.5 mm to 0.05 mm, the uncertainty of V is 0.004 ml, and the uncertainty of ^ is about 5°. Based on the uncertainties of geometrical parameters, the uncertainties of forces are also calculated and fall below 0.1 mN. Results and Discussion Bubble volume

[0080] For the smallest RSHS=4.2 mm, V increases linearly with t, indicating a constant gas flow rate. Very interestingly, for the two large RSHS, there is a “waiting time” for a bubble with a detected volume to appear. The waiting time is more obvious for larger RSHS, with a magnitude increasing from ~1 s to ~2 s as increasing RSHSfrom 6.3 mm to 19.0 mm. After the waiting time, a bubble with finite volume is detected and V increases linearly with t. The slope of V for the three cases is nearly same, indicating a nearly same gas flow rate.

[0081] The observed waiting time is not due to the measurement uncertainty. Given the uncertainty of bubble volume is 0.004 ml and the flow rate is Q=1.2 ml / min, the uncertainty of the waiting time is 0.2 s. To understand the reason of waiting time for SHS with large RSHS (cases with RSHS=6.3 mm and19.0 mm), we show the time-variations of FP(i.e., the primary lifting force acting on the bubble). Clearly, for the two large RSHS, during the waiting time, FP is close to 0, which can be explained by the nearly flat bubble interface. During the waiting time, the syringe pump continuously supplies gas, causing the pressure of gas in the orifice to increase. When the gas pressure in the orifice is sufficient to overcome the hydrostatic pressure above air-water interface, the interface deforms, FP becomes finite, leading to the subsequent growth of the bubble. The waiting time reduces as increasing Q, probably due to the reduced build-up time for the gas to overcome the hydrostatic pressure. Waiting time was also observed for the bubble formation at a micro-orifice due to a different mechanism. For micro-orifice, the waiting time is because when a micro-size bubble forms at the micro-orifice, the pressure inside the bubble is larger than the pressure in the gas reservoir. Bubble geometrical parameters

[0082] The evolutions of H, Rb and Ra for different RSHS generally follow the same trends as described below. H increases linearly with V and t until experiencing a rapid jump due to the necking; Rb experiences an initial increase due to the expansion of contact line, then a nearly constant region, and finally a sudden reduction due to contraction of contact line during the necking; Ra reduces continuously as increasing V and experiences a slight increase during the necking. Furthermore, for the two large RSHS cases and during the beginning of bubble formation period V / l^3<3 (i.e., before the contact line reaching to the edge of the SHS), the profiles showing as a function V for different RSHS overlap, indicating that the bubble shape is only a function of V and independent of RSHS.

[0083] Although the profiles of H, Rb and Ra for different RSHS share similar trends, there are notable differences. Comparing the results at a same V, as increasing RSHS, the bubble has a smaller H, a larger Rb and a larger Ra (i.e., the bubble is wider in the horizonal direction). For the two small RSHS=4.2 and 6.3 mm cases, Rb quickly increases to a maximum value of Rbmax=RSHS. However, for the largest RSHS=19.0 mm case, Rbvery slowly reaches to the maximum value of Rbmax=7.5 mm, which is smaller than RSHS. As explained early, the two different trends correspond to two different bubble formation modes. For small RSHS and Mode A, the bubble base reaches and pins at the rimof SHS, so that Rbmax=RSHS. While for large RSHSand Mode B, the bubble base is not able to reach to the edge of the SHS and the contact line continuously moves. The value of Rbmaxin Mode B depends on the equilibrium contact angle (^0) of the surface. Contact angle

[0084] For all three cases, ^ is close to 180° since the interface is nearly flat. However, as increasing time, the trends of ^ are different among the three cases. For the small RSHS=4.2 mm and 6.3 mm (i.e., Mode A), the evolutions of ^ can be generally separated into three stages: At stage I (0<t<0.65T0), as increasing time, ^ reduces, indicating the deformation of three-phase contact line as bubble grows. This reduction is more obvious for smaller RSHS. At stage II (0.65<t<0.9T0), interestingly, ^ maintains as a nearly constant value (denoted as ^min), which suggests that the contact line does not deform further while the bubble continuously growing. The value of ^minis smaller for smaller RSHS. For sufficiently small RSHS<3 mm results showed that ^mincan be less than 90°. Finally, at stage III just before necking (t>0.9T0), ^ experiences an either significant or mild increase, depending on the value of RSHS. During the necking period (t>0.999T0), ^ has a nearly constant value of ^=^0, regardless of RSHS. This result indicates that during the necking period and as the contact line retracts, the contact angle mainly depends on the material properties of the substrate.

[0085] For the largest RSHS=19.0 mm (Mode B), as increasing time, ^ continuously reduces until reaching to a value close to ^0 during the necking period. A period of constant value as bubble grows (the stage II in Mode A) is not observed in Mode B. It was observed that ^ has a nearly constant value of ^=^0, during the necking process, regardless of RSHS. Bubble necking

[0086] To better understand the pinch-off the gas bubble from the SHS, the evolutions of minimal neck radius are discussed. Bubble shapes for the other two cases: RSHS=4.2 and 19.0 mm, follow similar trends and are not shown. The time to pinch-off is defined as ^=T0–t. The evolution of bubble shape is generally consistent to the results discussed in previous sections: during thenecking, H increases rapidly, Rbreduces, and ^ remains nearly a constant. A neck seems to occur at ^~20 ms.

[0087] For all three cases (except for the largest RSHS at ^<3 ms), Rneck reduces as increasing time and follows a power-law relation Rneck~^0.54. The power-law exponent is in good agreement these reported in the literature for the pinch-off of a gas bubble from a nozzle submerged in water. This agreement indicates that the pinch-off of a gas bubble from SHS is similar to that from a nozzle, which can be predicted by the Rayleigh-Plesset equation and ignoring the effects of gas and liquid momentums.

[0088] However, for the largest RSHSat ^<3 ms, as increasing time, Rneckreduces at a rate faster than the prediction by the power-law relations. This deviation might be attributed to the reduction of Rbwhich is different from the constant Rb during the necking for a bubble at a nozzle. Since the reduction of Rb is most significantly for the largest RSHS, the deviation is only seen for this case. Another possible reason for this deviation might be the uncertainty of time. The time of ^=0 is defined as the time (or frame) when the bubble just detached from the surface. Due to the finite 1000 frames-per-second data requisition rate in this work, the uncertainty of ^ is 1 ms. To examine whether a similar deviation exists for the other two cases at ^<1 ms or whether the deviation is due to measurement error, a higher frame rate (e.g., 100,000 frames rate per second) is required.

[0089] The results for different RSHSare better collapsed compared to these before normalization. Moreover, the profiles for SHS largely overlap with the one for nozzle, except for the largest RSHSat ^<3 ms. The overlapping of profiles on SHS and nozzle, again, indicates that the bubble pinch-off is universal. The time-duration of pinch-off is governed by Rbmaxand increases as increasing Rbmax. Forces acting on the bubble

[0090] Six forces act on the bubble, as defined in Table 4. The magnitudes of six forces are calculated based on the bubble geomatical parameters and the expressions listed in Table 4. Our goal is to understand how different forces are in balance and contribute to the quasi-static growth of the bubble. Results are only shown for the bubble growing period, since in necking period the bubble is not in equilibrium. We find that the magnitudes of FGM, FLI,and FDare nearly zero, which is expected due to the small gas flow rate in current study. The momentums of gas and liquid are negligible. Thus, the main forces acting on the bubble are FP, FB.

[0091] There are similarities among the three different RSHS. For all cases, as increasing V, the magnitude of FP increases primarily due to a smaller Raand the raised gas pressure within the bubble. The magnitude of FBalso increases because of the larger H and the higher hydrostatic pressure acting on the bubble. The trend of FSdepends on RSHS: for small RSHSin Mode A, the magnitude of FS initially increases and then reaches to a stable value; while for large RSHSin Mode B, the magnitude of FSincreases continuously. The summation of these three forces, FP+FB+FS, is close to zero. This result confirms that the bubble is governed by the balance among one lifting forces (pressure force) and two retaining forces (buoyancy force and surface tension force). The balance of forces observed on SHS is very different from that at a nozzle, where the main lifting and retaining forces are FB and FS respectively.

[0092] There are notable differences among the three different RSHS. As increasing RSHS, the maximum value of |FP| increases from ~3 to ~6 mN, the maximum value of |FB| increases from ~2 to ~5 mN. The reason for these trends is because the maximum values of |FP| and |FB| are proportional to Rbmax, which increases as increasing RSHS. However, even though the maximum value of |FS| is also proportional to Rbmax, it does not increase as increasing RSHS. Instead, it remains as a nearly constant of 1.6 mN. The trend is induced by the combination effects of larger Rbmaxand larger contact angle as increasing RSHS.

[0093] Typically, for a bubble growing from a nozzle, a detachment occurs when the buoyancy force overcomes the surface tension force. However, this is not the case here: the main lifting force (FP) does not exceed the retaining forces during the bubble growing process. For bubble growing on a SHS, there is a maximum volume at which the bubble can maintain a stable shape. A detachment must occur when the bubble volume exceeds this maximum stable volume. We suspect that the necking process as well as the detachment are driven by the surface tension, which minimizes the surface area of the bubble. Due to the surface tension, the bubble quickly shrinks in the horizontal direction and changes into a spherical shape. Future studies are required to understand the force balance during the necking process.Bubble detached volume

[0094] Bubble detached volume (Vd) measured from the experiments are compared to the theoretical prediction by Tate volume VT=2^Rbmax^ / ^Lg. Here, the Tate volume is derived based on the balance between surface tension force and buoyancy force, and the assumptions that the detached bubble is nearly spherical so that FB=^LgVT and has a contact angle of 90° so that the FS=2^Rbmax^. At the quasi-static region, the Tate volume has been shown to well predict Vd for a bubble detaching from a nozzle and from an orifice on hydrophilic and hydrophobic surfaces (^0<120°). However, it is unclear whether the Tate volume applies for a bubble detaching from the superhydrophobic surface (^0>150°). Regardless the types of surfaces and the values of ^0, the Tate volume provides a good approximation for the bubble detached volume. Although the assumptions of spherical bubble shape and 90° contact angle used for deriving the Tate volume are not validate for SHS, the applicability of Tate volume is not affected.

[0095] The results indicate that regardless of the surface type, the bubble detached volume is linearly proportional to Rbmax. Surfaces with different wetting properties produce different Rbmax. For hydrophobic surface, bubble formation followed Mode B, and Rbmaxincreased as increasing ^0. Clearly, for SHS with small RSHS where bubble formation follows Mode A, Rbmax=RSHS. While for SHS with large RSHSwhere bubble formation follows Mode B, Rbmaxis independent of RSHS and might increase as increasing ^0. Future studies are required to determine the relationship between Rbmaxdepends on ^0. A bubble with smaller detached volume can be expected for SHS with smaller RSHSand smaller ^0. Conclusions

[0096] In summary bubble formation was experimentally measured on three SHS with a radius RSHSranging from 4.2 mm to 19.0 mm. We studied the evolutions of bubble volume, bubble height, bubble base radius, bubble radius at the apex, contact angle, as well as the minimal neck radius. We also calculated six forces acting on the bubble during the growing process. The main conclusions are listed below:• Two bubbling modes A and B, where observed, depending on RSHS. In Mode A for small RSHS, the contact line quickly pins at the rim of SHS after an initial expansion. In Mode B for large R , the contacSHS t line expands as the bubble grows. • For large RSHS, a waiting time, was found for a finite volume to be detected. The possible reason is that at the beginning of bubble formation, the radius at the apex is large, causing a nearly zero pressure force (which is the primary lifting force acting on the bubble). • The contact angle follows different trends in the two bubbling modes: in Mode A, ^ initially reduced, then maintained as a constant, and finally increased; in Mode B, ^ continuously reduced. In both modes and during the necking, the contact line retracts, and ^ is close to the equilibrium contact angle. • For all RSHS, the pinch-off of bubble on SHS at the early stage (^>1 ms) follows a power-law relation Rneck~^0.54, which agrees well with the pinch- off of bubble on a nozzle.• At quasi-static region, the main forces acting on the bubble are one lifting force (pressure force) and two retaining forces (surface tension force and buoyancy force). As increasing RSHS, the maximum pressure force and maximum buoyancy force increases, while the maximum surface tension force remains nearly constant. • Similar to hydrophilic and hydrophobic surfaces, Tate volume can be used to predict the detached bubble volume on SHS, which is a function of the maximum bubble base radius. A bubble with smaller detached volume can be expected for SHS with smaller RSHS and smaller ^0. EXAMPLE 4 Methods

[0097] The experiments were performed in a transparent acrylic tank filled with water. The tank had an inner dimension of 100 mm by 100 mm, and the height of the water was 70 mm. These dimensions were sufficiently larger than the size of the bubble, ensuring a negligible influence on the bubble formation. A super-hydrophobic surface (SHS) with a radius of 4.2 mm<RSHS<19.0 mm was installed at the bottom of the tank. It was created on a 50 mm by 50 mm aluminum surface. To fabricate the SHS, wefirst created surface roughness by sandblasting the entire aluminum surface using an abrasive medium of grit size 60 (particle mesh size 35 to 100). Then, the rough surface is cleaned in the ultrasonic bath and coated by hydrophobic nano-particles (Glaco Mirror Coat Zero, by SOFT99 Corp). Only the center region with a radius of RSHS was coated. The static contact angle on the super-hydrophobic and hydrophilic regions was 152±2° and 32±2°, respectively. The sliding angle of a water droplet on the SHS is 5±2°.

[0098] To promote the bubble formation on SHS, a 0.5 mm diameter orifice was fabricated at the center of the SHS. A syringe pump (Model #NE-1010 SyringeONE, by New Era Pump System Inc) was used to supply gas through the orifice. A long needle with a length of 152 mm and an inner diameter of 0.61 mm was installed below the orifice to ensure that the pressure variation in the bubble does not cause a notable change of the pressure in the syringe.

[0099] To record the bubble formation, a high-speed camera (PCO.dimax S4, pixel size 11 µm, 2016×2016 pixels) and a collimated light (Thorlabs, model #QTH10, power 50 mW) were used. The maximum frame rate used in this study was 1000 frames per second, which provided a sufficient temporal resolution to capture the bubble growing and necking processes. The spatial resolution of the imaging system was 34 µm / pixel. The data was recorded after a series of bubbles had formed and detached from the surface. The light was only turned on for a short duration of time and didn’t change the water temperature.

[0100] An image processing procedure was used to measure the bubble geometrical parameters, including the volume (V), base radius (Rb), height (H), radius at the apex (Ra), and contact angle at the three-phase contact line (^). The image processing procedure included the following steps. First, the wall on the raw images was removed by subtracting an image containing only the wall. Then, an intensity threshold was applied to segment the area of the bubble from the background. Final, V was calculated by accumulating the cross-section area at each height level from the bottom to the top of the bubble. Ra was obtained by fitting the bubble apex with a circle of radius Ra. ^ was found by linearly fitting the bubble shape near the three-phase contact line. The velocity and acceleration of the bubble in the vertical direction were calculated as Ub=dyb / dt and ab=d2yb / dt2, where t was the time and yb was the position of center-of-mass in the vertical direction.

[0101] A SHS with a radius RSHS is submerged in water with a density of ^L=997 kg / m3. An air bubble with a density of ^G=1.2 kg / m3forms on the SHS. The air is supplied to the bubble at a constant flow rate by using a syringe pump. The bubble shape is recorded by a high-speed camera with a maximum frame rate of 1000 framesper second. The static contact angle (SCA) of the SHS is 152°. Before a bubble forms, a uniform air layer presents on the submerged SHS. When a bubble forms, the bubble bases either pins on the rim of SHS for small RSHS or moves on the SHS without reaching to the SHS rim for sufficiently large RSHS.

[0102] Table 5 lists the main experimental parameters. Three different RSHS=4.2, 6.3 and 19.0 mm and five different gas flow rates Q=1, 5, 20, 50 and 150 ml / min are considered in this study. For RSHS=4.2 and 6.3 mm, the bubble base is fixed at the rim of SHS so the maximum bubble base radius Rbmax=RSHS. For RSHS=19.0 mm, the bubble base does not reach to SHS boundary so that Rbmax<RSHS, and Rbmaxvaries slightly at different flow rates. The effect of Q on bubble formation is similar for different R . In the followinSHS g, results obtained for RSHS=6.3 mm are

[0103] Table 5 provides the range of Q / Qcr, where Qcris the critical flow rate for a transition from the quasi-static regime to the dynamic regime. Qcris calculated as: Qcr=^(16 / 3g2)1 / 6(^Rbmax / ^L)5 / 6, where ^=72 mN / m is the surface tension and g=9.78 m2 / s is the gravitational acceleration. In current study, Q / Qcrdoes not exceed 0.35. Similar, the Weber number We=^LQ2 / ^^Rbmax3(ratio of gas momentum to surface tension) does not exceed 0.12. Due to the small values of Q and We, we will show later that the dynamic forces due to the momentums of injected gas and surrounding liquid are negligible, and the gas flow rate has a minor impact on the bubble shape. Vd increases as increasing Q. RSHS(mm) Rbmax(mm) Q (ml / min) Q / QcrVd(ml) 4.2 4.2 1 to 150 0.0023 to 0.35 0.17 to 0.29 6.3 6.3 1 to 150 0.0016 to 0.25 0.30 to 0.43 19.0 7.2 to 7.9 1 to 150 0.0071 to 0.21 0.34 to 0.58 Table 5. Main experimental parameters and results in current study. Conclusions

[0104] In summary, we studied the effect of gas flow rate on the bubble formation on SHS, including bubble detached volume, bubbling period, bubble geometrical parameters, necking process, as well as the forces acting on the bubble. The gas flow rate varied in the range of 1<Q<150 ml / min and 0.001<Q / Qcr<0.35. The main conclusions are listed below:• Bubble detached volume: Although current flow rates were in the quasi-static regime (Q<Qcr), an increased Vd as increasing Q was observed. The increased Vd was mainly due to the increased bubble volume during the necking process. After proper normalization, the relationship between Vdand Q for hydrophilic, hydrophobic and super-hydrophobic surfaces generally followed similar trends. • Bubble shape and necking: The flow rate had a minor impact on the bubble shape and the time duration of necking due to the small momentum of injected gas. • Forces acting on bubble: For current flow rates, bubble was driven by a balance between one lifting force (pressure force) and two retaining forces (surface tension force and buoyancy force). The dynamics forces caused by the momentums of the injected gas and surrounding liquid were negligible.

[0105] Overall, the results enhanced the understanding of bubble formation on SHS. Considering the wide applications of SHS such as drag reduction, anti-bacteria, anti-icing and anti-corrosion, our results may have significant impacts. For example, our results may lead to the development of new techniques for extending the longevity of gas and thereby SHS functions for submerged SHS. Our results could also inspire new methods to control bubble size that involve micro / nano-engineered surfaces. Exemplary Aspects.

[0106] The following exemplary aspects are provided, the numbering of which is not to be construed as designating levels of importance:

[0107] Aspect 1 provides a marine surface comprising: a porous substrate a superhydrophobic coating applied to at least a portion of the porous substrate; a gas source adapted to deliver gas through the porous substrate and to contact the superhydrophobic coating.

[0108] Aspect 2 provides the marine surface of Aspect 1, wherein the porous substrate comprises a metal, a plastic material, a ceramic, a glass, or a combination thereof.

[0109] Aspect 3 provides the marine surface of Aspect 2, wherein the plastic material comprises a thermoplastic polymer, a thermoset polymer, or a mixture thereof.

[0110] Aspect 4 provides the marine surface of any of Aspects 2 or 3, wherein the plastic material comprises a polyamide, a polycarbonate, apolyolefin, a polyester, a polyurethane, an epoxy, a copolymer thereof, or a mixture thereof.

[0111] Aspect 5 provides the marine surface of any of Aspects 2-4, wherein the plastic material comprises polytetrafluoroethylene, a styrene- butadiene copolymer, ethylene tetrafluoroethylene, a polyvinyl chloride, polyether urethane, a phenyl formaldehyde polymer, or a mixture thereof.

[0112] Aspect 6 provides the marine surface of any of Aspects 2-5, wherein the ceramic comprises yttria (Y2O3), magnesia (MgO), aluminum oxide (Al2O3), a magnesium aluminum oxide (MgAl2O4), a carbide, an oxycarbide, a nitride, an oxynitride, a boride, an oxyboride, a sulfide, a selenide, a sulfo- selenide, silica, zirconia, silicon-carbide, silicon-nitride, aluminum nitride, or a mixture thereof.

[0113] Aspect 7 provides the marine surface of any of Aspects 2-6, wherein the glass comprises soda lime silicate glass, alkali aluminosilicate glass, alkali containing borosilicate glass, alkali aluminophosphosilicate glass, alkali aluminoborosilicate glass, or a mixture thereof.

[0114] Aspect 8 provides the marine surface of Aspect 2, wherein the metal comprises steel.

[0115] Aspect 9 provides the marine surface of any of Aspects 1-8, wherein the porous substrate comprises a plurality of through pores.

[0116] Aspect 10 provides the marine surface of any of Aspects 1-9, wherein the porous substrate comprises a plurality of micropores, nanopores, or a combination thereof.

[0117] Aspect 11 provides the marine surface of any of Aspects 1-10, wherein the superhydrophobic coating is disposed over about 10% to about 100% total surface area of the porous substrate.

[0118] Aspect 12 provides the marine surface of any of Aspects 1-11, wherein the superhydrophobic coating is disposed over about 10% to about 50% total surface area of the porous substrate.

[0119] Aspect 13 provides the marine surface of any of Aspects 1-12, wherein the superhydrophobic coating comprises a superhydrophobic material comprising a metal, a polymeric material, a ceramic, a glass, or a combination thereof.

[0120] Aspect 14 provides the marine surface of Aspect 13, wherein the superhydrophobic coating comprises tungsten disulfide, hexamethyldisiloxane, tetramethyldisiloxane, fluorosilane, a glass, a perfluoropolyether, manganese oxide polystyrene, zinc oxide polystyrene, precipitated calcium carbonate, or a mixture thereof.

[0121] Aspect 15 provides the marine surface of any of Aspects 13 and 14, wherein the superhydrophobic coating comprises a microstructure, a nanostructure, or a combination thereof.

[0122] Aspect 16 provides the marine surface of any of Aspects 13-15, wherein the microstructure comprises a plurality of structural features independently having a major dimension in a range of from about 1 µm to about 1000 µm.

[0123] Aspect 17 provides the marine surface of any of Aspects 13-16, wherein the microstructure comprises a plurality of structural features independently having a major dimension in a range of from about 250 µm to about 750 µm.

[0124] Aspect 18 provides the marine surface of any of Aspects 13-17, wherein the nanostructure comprises a plurality of structural features independently having a major dimension in a range of from about 1 nm to about 100 nm.

[0125] Aspect 19 provides the marine surface of any of Aspects 13-18, wherein the nanostructure comprises a plurality of structural features independently having a major dimension in a range of from about 10 nm to about 70 nm.

[0126] Aspect 20 provides the marine surface of any of Aspects 13-19, wherein the superhydrophobic coating comprises a plurality of structural microfeatures and a plurality of structural nanofeatures.

[0127] Aspect 21 provides the marine surface of any of Aspects 13-20, wherein the plurality of nanostructures extend from the plurality of microstructures and are a substrate to the plurality of nanostructures.

[0128] Aspect 22 provides the marine surface of any of Aspects 13-21, wherein the microstructure comprises a microwire, a microrod, a microtube, a microsphere, or a microdroplet.

[0129] Aspect 23 provides the marine surface of any of Aspects 13-22, wherein the nanostructure comprises a nanowire, a nanorod, a nanotube, a nanosphere, or a nanodroplet.

[0130] Aspect 24 provides the marine surface of any of Aspects 1-23, wherein a thickness of the superhydrophobic coating is substantially constant across the article.

[0131] Aspect 25 provides the marine surface of any of Aspects 1-24, wherein the thickness of the superhydrophobic coating is variable across the article.

[0132] Aspect 26 provides the marine surface of any of Aspects 1-25, wherein a contact angle of the superhydrophobic coating is at least 120 degrees, as determined using ASTM D7334-08.

[0133] Aspect 27 provides the marine surface of any of Aspects 1-26, wherein a contact angle of the superhydrophobic coating is at least 150 degrees, as determined using ASTM D7334-08, as determined using ASTM D7334-08.

[0134] Aspect 28 provides the marine surface of any of Aspects 1-27, wherein a contact angle of the superhydrophobic coating is in a range of from about 120 degrees to about 180 degrees, as determined using ASTM D7334-08.

[0135] Aspect 29 provides the marine surface of any of Aspects 1-28, wherein a contact angle of the superhydrophobic coating is in a range of from about 140 degrees to about 160 degrees, as determined using ASTM D7334-08.

[0136] Aspect 30 provides the marine surface of any of Aspects 1-29, wherein the gas source comprises a pump to deliver the gas.

[0137] Aspect 31 provides the marine surface of Aspect 30, wherein the gas comprises air.

[0138] Aspect 32 provides the marine surface of any of Aspects 1-31, wherein the marine surface comprises a vessel hull, a dock, a wharf, a buoy, a marine motor, or a propeller.

[0139] Aspect 33 provides a marine vessel comprising the marine surface of any of Aspects 1-32.

[0140] Aspect 34 provides a method of operating the marine surface of any of Aspects 1-33, the method comprising delivering a gas through the porous substrate.

[0141] Aspect 35 provides the method of Aspect 34, wherein the gas is continuously delivered through the porous surface.

[0142] Aspect 36 provides the method of Aspect 35, wherein the gas is delivered through the porous surface in discrete increments.

Claims

CLAIMS What is claimed is:

1. A marine surface comprising: a porous substrate a superhydrophobic coating applied to at least a portion of the porous substrate; a gas source adapted to deliver gas through the porous substrate and to contact the superhydrophobic coating.

2. The marine surface of claim 1, wherein the porous substrate comprises a metal, a plastic material, a ceramic, a glass, or a combination thereof.

3. The marine surface of claim 2, wherein the plastic material comprises a thermoplastic polymer, a thermoset polymer, or a mixture thereof.

4. The marine surface of claim 2, wherein the plastic material comprises a polyamide, a polycarbonate, a polyolefin, a polyester, a polyurethane, an epoxy, a copolymer thereof, or a mixture thereof.

5. The marine surface of claim 2, wherein the plastic material comprises polytetrafluoroethylene, a styrene-butadiene copolymer, ethylene tetrafluoroethylene, a polyvinyl chloride, polyether urethane, a phenyl formaldehyde polymer, or a mixture thereof.

6. The marine surface of claim 1, wherein the ceramic comprises yttria (Y2O3), magnesia (MgO), aluminum oxide (Al2O3), a magnesium aluminum oxide (MgAl2O4), a carbide, an oxycarbide, a nitride, an oxynitride, a boride, an oxyboride, a sulfide, a selenide, a sulfo-selenide, silica, zirconia, silicon-carbide, silicon-nitride, aluminum nitride, or a mixture thereof.

7. The marine surface of claim 2, wherein the glass comprises soda lime silicate glass, alkali aluminosilicate glass, alkali containing borosilicate glass,alkali aluminophosphosilicate glass, alkali aluminoborosilicate glass, or a mixture thereof.

8. The marine surface of claim 2, wherein the metal comprises steel.

9. The marine surface of claim 1, wherein the porous substrate comprises a plurality of through pores.

10. The marine surface of claim 1, wherein the porous substrate comprises a plurality of micropores, nanopores, or a combination thereof.

11. The marine surface of claim 1, wherein the superhydrophobic coating is disposed over about 10% to about 100% total surface area of the porous substrate.

12. The marine surface of claim 1, wherein the superhydrophobic coating is disposed over about 10% to about 50% total surface area of the porous substrate.

13. The marine surface of claim 1, wherein the superhydrophobic coating comprises a superhydrophobic material comprising a metal, a polymeric material, a ceramic, a glass, or a combination thereof.

14. The marine surface of claim 13, wherein the superhydrophobic coating comprises tungsten disulfide, hexamethyldisiloxane, tetramethyldisiloxane, fluorosilane, a glass, a perfluoropolyether, manganese oxide polystyrene, zinc oxide polystyrene, precipitated calcium carbonate, or a mixture thereof.

15. The marine surface of claim 13, wherein the superhydrophobic coating comprises a microstructure, a nanostructure, or a combination thereof.

16. The marine surface of claim 13, wherein the microstructure comprises a plurality of structural features independently having a major dimension in a range of from about 1 µm to about 1000 µm.

17. The marine surface of claim 13, wherein the microstructure comprises a plurality of structural features independently having a major dimension in a range of from about 250 µm to about 750 µm.

18. The marine surface of claim 13, wherein the nanostructure comprises a plurality of structural features independently having a major dimension in a range of from about 1 nm to about 100 nm.

19. The marine surface of claim 13, wherein the nanostructure comprises a plurality of structural features independently having a major dimension in a range of from about 10 nm to about 70 nm.

20. The marine surface of claim 13, wherein the superhydrophobic coating comprises a plurality of structural microfeatures and a plurality of structural nanofeatures.

21. The marine surface of claim 13, wherein the plurality of nanostructures extend from the plurality of microstructures and are a substrate to the plurality of nanostructures.

22. The marine surface of claim 13, wherein the microstructure comprises a microwire, a microrod, a microtube, a microsphere, or a microdroplet.

23. The marine surface of claim 13, wherein the nanostructure comprises a nanowire, a nanorod, a nanotube, a nanosphere, or a nanodroplet.

24. The marine surface of claim 1, wherein a thickness of the superhydrophobic coating is substantially constant across the article.

25. The marine surface of claim 1, wherein the thickness of the superhydrophobic coating is variable across the article.

26. The marine surface of claim 1, wherein a contact angle of the superhydrophobic coating is at least 120 degrees, as determined using ASTM D7334-08.

27. The marine surface of claim 1, wherein a contact angle of the superhydrophobic coating is at least 150 degrees, as determined using ASTM D7334-08, as determined using ASTM D7334-08.

28. The marine surface of claim 1, wherein a contact angle of the superhydrophobic coating is in a range of from about 120 degrees to about 180 degrees, as determined using ASTM D7334-08.

29. The marine surface of claim 1, wherein a contact angle of the superhydrophobic coating is in a range of from about 140 degrees to about 160 degrees, as determined using ASTM D7334-08.

30. The marine surface of claim 1, wherein the gas source comprises a pump to deliver the gas.

31. The marine surface of claim 30, wherein the gas comprises air.

32. The marine surface of claim 1, wherein the marine surface comprises a vessel hull, a dock, a wharf, a buoy, a marine motor, or a propeller.

33. A marine vessel comprising the marine surface of claim 1.

34. A method of operating the marine surface of claim 1, the method comprising delivering a gas through the porous substrate.

35. The method of claim 34, wherein the gas is continuously delivered through the porous surface.

36. The method of claim 35, wherein the gas is delivered through the porous surface in discrete increments.

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