# Patent

## Title
Modified surface having low adhesion properties to mitigate insect residue adhesion

## Abstract

A process to modify a surface to provide reduced adhesion surface properties to mitigate insect residue adhesion. The surface may include the surface of an article including an aircraft, an automobile, a marine vessel, all-terrain vehicle, wind turbine, helmet, etc. The process includes topographically and chemically modifying the surface by applying a coating comprising a particulate matter, or by applying a coating and also topographically modifying the surface by various methods, including but not limited to, lithographic patterning, laser ablation and chemical etching, physical vapor phase deposition, chemical vapor phase deposition, crystal growth, electrochemical deposition, spin casting, and film casting.

## Background
CROSS-REFERENCE TO RELATED PATENT APPLICATION(S)
This patent application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/657,322 filed on Jun. 8, 2012, and U.S. Provisional Patent Application No. 61/788,785, filed on Mar. 15, 2013, the contents of each of which are hereby incorporated by reference in their entirety.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of 51 U.S.C. §20135, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore.


BACKGROUND OF THE INVENTION
Laminar flow is the smooth, uninterrupted flow of air over a surface, such as the contour of wings, fuselage, or other parts of an aircraft in flight. Drag reduction through the maintenance of laminar flow over greater chord lengths during the cruise portion of an aircraft's flight can yield to improved fuel efficiency over long distances. However, surface imperfections, especially on the wing leading edge, can lead to transition from laminar to turbulent flow increasing drag and fuel burn.
Flight tests have shown that insect impacts on wing leading edge surfaces can leave residue with critical heights sufficient to disrupt laminar flow and decrease fuel efficiency. Since maintenance of laminar flow is most critical during cruise, insect residue adhesion mitigation is an operational necessity for fuel-efficient configurations.
Accordingly, there is a need to provide an improved method of mitigating insect residue adhesion to a surface that does not add significant weight to increase efficiency of the aircraft.
BRIEF SUMMARY OF THE INVENTION
The present invention includes a process to modify a surface to provide reduced adhesion surface properties to mitigate insect residue adhesion. The process includes providing at least one article having at least one surface, topographically modifying the surface, and chemically modifying the surface by coating said surface with a low surface energy coating. The low surface energy coating may include a polymer composition having a surface energy of less than about 50 mJ/m2, or alternatively less than about 40 mJ/m2. The surface may comprise a water contact angle of greater than about 80 degrees, or alternatively greater than about 110 degrees. The modified surface may also comprise a surface roughness of about 0.2 micron to about 50 microns after the surface is topographically and chemically modified. The surface may comprise a surface roughness of about 1 microns to about 10 microns. In one embodiment, the surface may be topographically modified by laser ablation with an ablation depth of about 0.5 μm to about 30 μm. The laser ablation depth may be about 1 μm to about 10 μm.
In another embodiment, the surface may be topographically and chemically modified by spray deposition of a polymer particulate composition comprising a nanocomposite material where the nanocomposite material comprises silica nanoparticles.
In yet another embodiment, the coating may include a copoly(imide fluorinated alkyl ether), fluorinated silanes, fluorinated aliphatic compounds, silicones, or fluorine-containing polymers. The coating may alternatively comprise a silane composition. The silane composition may be prepared by generating 1-2% weight aqueous ethanol solutions with glacial acetic acid to induce acid hydrolysis of the alkoxy functionality of said silane composition. Alternatively, the silane composition may comprise a mixture of Si—C6, Si—C12, and Si—C18; or Si—F17.
In yet another embodiment, chemical modification of the surface may also include chemical or physical vapor phase deposition, plasma deposition, submersion, spray coating, or spin casting.
These and other features, advantages, and articles of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
FIG. 1 is a side, cross-sectional view of a surface having a substantially smooth topography.
FIG. 2 is a side, cross-sectional view of a textured surface having an uneven topography.
FIG. 3 is a side, cross-sectional view of a textured surface having an uneven topography and a chemical coating thereon.
FIGS. 4A-4C are alternative cross-sectional drawings of the textured surface having a random uneven surface (FIG. 4A), a uniform symmetrically uneven surface (FIG. 4B), and a repeating uneven surface (FIG. 4C).
FIG. 5 is a side, cross-sectional view of a textured surface having an alternative rounded pattern of peaks and channels.
FIG. 6 is a side, cross-sectional view of a textured surface having the alternative pattern of peaks and channels as shown in FIG. 5 further modified with a chemical coating thereon.
FIG. 7 is a micrograph of a surface that has been modified to create uneveness.



DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, the present invention includes providing at least one article 10 having at least one surface 12 to be chemically and topographically modified to mitigate residue adhesion to the surface 12. The present invention can be used to reduce the adhesion of insect residue on various surfaces, including but not limited to, airplane, helicopter, airborne vehicles, automobiles, marine vessels, motorcycles, helmets, wind turbines, all-terrain vehicles, floors, building, exterior walls or windows, etc. The surface may comprise any surface which will be exposed to particles or objects that may become adhered to that surface, and may comprise various types of materials including, but not limited to, metal, inorganic materials, polymeric materials, composites, textiles, and combinations of the foregoing. Examples of metallic surfaces include aluminum, titanium and related alloys thereof, and examples of inorganic surfaces include glass and ceramic articles.
As discussed herein, the surface is described as a wing surface of an aircraft, which is one embodiment of the present invention. In this embodiment, the surface is substantially smooth and the topography of this unmodified surface facilitates the laminar flow of air across and/or around it.
One aspect of the present invention is chemical modification of the surface of the article. The chemical modification can occur through application of a chemical coating to the surface of the article. The chemical coating may comprise a low surface energy coating due to their minimization of interfacial interactions. In one embodiment, the surface energy of the coating comprises less than about 50 mJ/m2, or less than about 40 mJ/m2. The coating can be applied by various methods including, but not limited to, spray application, dip-coating, spin-coating, film casting, physical vapor phase deposition, chemical vapor phase deposition, crystal growth, electrochemical reaction, etc. For vapor phase deposition, the procedure may involve placing the surface to be functionalized into a sealed container along with a small amount of the coating composition (e.g. silianating agent). Evaporation of the coating composition enables the functionalization of the surface.
The chemical coating can persist on the surface either due to physical adsorption or chemical reaction with the article. For physical adsorption, the coating persists on the surface due to Van der Waals forces, electrostatic and magnetic interactions, mechanical interlocking, or any combination thereof. The chemical coating may be a fluorinated silanol and a precursor thereof, an aliphatic material, fluorinated aliphatic material, silicone, fluorine-containing polymer and copolymer, an epoxy, a urethane, and mixtures of two or more of the foregoing. Alternative chemical coatings that may be effective are also disclosed in U.S. patent application Ser. No. 13/286,715, filed on Nov. 1, 2011, which is incorporated by reference herein in its entirety.
For chemical reaction with the article, the species would consist of two components, a chemical moiety that tethers the chemical to the article through chemical reaction with functionalities on the article and a chemical moiety that generates a low surface energy coating. Chemical functionalities that could be used to tether the chemical species to the article include, but are not limited to: silanes, chlorosilanes, alkoxysilanes, primary amines, secondary amines, epoxides, alcohols, carboxylic acids, esters, among others. The number of chemical reactions between the article and the surface coating chemical can be any number equal to or greater than one. Similarly, the chemical species used to modify the surface may react with itself resulting in a multilayered coating. The chemical functionalities that result in a low surface energy include, but are not limited to: aliphatic functional groups including CH3, C2H5, any CnH2n+1 or any hydrocarbon chain with unsaturation arising from double or triple carbon-carbon bonds; fluorinated aliphatic groups consisting of any number of carbon atoms with hydrogen and fluorine bonds such as CH2F, CHF2, CF3, C2H2F3, C2F5, any CnF2n+1, or any combination of H and F such that the total number of H and F atoms is equal to 2n+1 with n equaling the number of carbon atoms, or any fluorinated hydrocarbon with unsaturation arising from double or triple carbon-carbon bonds.
The thickness of the chemical coating can vary from less than a molecular layer that is a surface coverage consisting of coated and uncoated regions with an average thickness of about 0.5 nm, to coatings that are about 80 microns thick. The preferential coating thickness for coatings that persist by physical adsorption would be from about 1 nm to about 80 microns, more preferential thicknesses would be from about 100 nm to about 50 microns. For chemical coatings that persist as a result of chemical reaction with the article, the preferential thickness would be from about 0.5 nm to about 1 micron with a more preferential thickness from about 0.5 nm to about 100 nm. Similarly, the coating thickness is preferred to be no greater than the separation distance between two topographical features representing the lowest frequency topographical pattern intentionally imparted on the surface. The chemical coating uniformity can be described as the continuity of the same surface chemical composition. The chemical coatings described herein can be either uniform or non-uniform. A uniform coating is described as a uniform chemical composition of a single or multiple species across the article. A non-uniform coating is described as a chemical coating of a single or multiple species that is not of uniform composition across the article. This could include regions with no coating at all with the uncoated regions ranging from about 0.01% to about 50% of the modified surface.
The present invention also includes topographical modification of the surface of the article. Chemical and topographical modification can alternatively occur in a single step or multiple steps.
For the chemical and topographical modification to occur in a single step, the coating may comprise particulate matter such that application of the coating would chemically and topographically modify the surface of the article. In one embodiment, the coating may comprise a polymer particulate composition having a nanocomposite material (e.g. silica particles).
The surface can also be topographically modified separately from the chemical modification. The topography can be modified to create unevenness by either additive or subtractive methods including, but not limited to: lithographic patterning, laser ablation and chemical etching, physical vapor deposition, chemical vapor deposition, vapor phase deposition, crystal growth, electrochemical deposition, spin casting, and film casting. The unevenness may be imparted and defined in terms of a defined or random pattern of unevenness on the surface. The topography may be uniformly symmetric or asymmetric across the surface of the article. FIGS. 4A-C demonstrate examples of uneven topographies (see surfaces 42, 44 and 46 of article 40) having a random (FIG. 4A), uniform symmetric (FIG. 4B), or uniform asymmetric (FIG. 4C) types of unevenness. The topography may be described by any manner of shapes including but not limited to: spheres, triangles, any polygon, pillars, recessed cavities, overhanging structures, etc.
Referring to FIG. 2, the pattern that is visible and that is created from the smooth surface defines peaks 22 and channels 24 in terms of the height 26 and width 28 respectively of the topographical variations. The channels are measured by width 28 as the distance between the tops of adjacent peaks 22 that are formed in the surface 20. As demonstrated in the profile view of FIG. 2, the peaks 22 and channels 24 may have the same or similar size relative to other peaks along the surface of the article. The peaks 22 may be different heights 26 (defined as the distance from the bottom of an adjacent channel to the top of a peak). The channels 24 between the peaks 22 may have the same or similar widths 28, or they may vary along the length of the channels that are formed along the surface. The peaks 22 and channels 24 may be relatively sharp in their shape, including perpendicular angles of the sides of the peaks and the floor of the channels. The shapes may also be rounded or curved or otherwise formed in the surface. Additionally, the surface topography could be comprised of any combination of rounded and sharp features.
FIGS. 5 and 6 illustrate a surface 50 having rounded, symmetric peaks 52 that are, as shown in FIG. 6, coated with a coating 64. Also, FIG. 7 is a micrograph that shows a surface textured using laser ablation as described herein.
The size of the peaks and channels on the modified surface of the article cannot be so great as to create significant turbulence in air that moves along the surface. Accordingly, the topographical features on the surface should vary between about 10 nanometers and about 80 microns in height, alternatively between about 0.1 microns and about 20 microns in height, or still further alternatively, between about 0.5 microns and about 10 microns in height. Similarly, the size of the channels is preferably in the range of about 10 nanometers to about four millimeters or alternatively about 10 microns to about 40 microns.
Additional topographical features may be present on the surface that are an order of magnitude smaller than the features described above generating a hierarchical or fractal surface topography. For example, a surface may consist of rectangular pillars with length, width, and height dimensions of about 10 microns. These pillars could have further topographical features on them consisting of rectangular pillars that have length, width, and height dimensions of about 500 nanometers. This would be considered a hierarchical structure.
An additional parameter to describe the surface topographical modifications encompassed in this invention is the surface roughness of the material. Although there are many different ways to calculate surface roughness, for this example, the average areal surface roughness values are used. These values can be determined by any microscopy or imaging technique that provides information in three dimensions such that the average of the peaks and valleys can be determined along both length and width axes. Surface roughness is calculated as the arithmetic mean of the absolute values for the vertical deviation of surface topographical features from the mean line. For the topographies described here, the surface roughness values should vary between about 0.2 micron and about 50 microns, between about 0.5 micron and 8 microns, between about 1 micron and about 10 microns, or yet further alternatively between about 1 micron and about 6 microns.
EXAMPLE
An aluminum alloy is used and topographically modified. The topographical modification of the aluminum alloy samples was realized using two techniques: spray deposition and laser ablation patterning following by chemical modification. For the spray deposition example, a solution was generated by combining nanometer sized silica particles with a hydrophobic coating (heptadecafluoro-1,1,2,2-tetrahydrodecyltriethoxysilane) in aqueous ethanol solution. For comparison, a similar solution was made with the only variation being the use of a hydrophilic coating (methoxy(polyethyieneoxy) propyltrimethoxysilane) instead of the hydrophobic coating. For the laser ablation patterned surfaces, patterning was performed with a PhotoMachining, Inc. laser ablation system equipped with a Coherent Avia® frequency-tripled Nd:YAG laser (355 nm, average power: 7 W). The laser beam diameter and scan speed were kept constant at 25 micrometers and 25.4 cm/s. A series of different laser parameters were evaluated with pulse energies ranging from 40 to 99 microjoules per pulse. The line spacing was also varied from 12 to 102 micrometers. Other examples and teachings regarding topographical modification using a laser are disclosed in U.S. patent application Ser. No. 12/894,279, filed Sep. 30, 2010, which is incorporated by reference herein in its entirety.
In this example, a chemical modification and coating was also performed. These surfaces were then chemically modified by exposure to (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane in a 19:1 ethanol:water solution with a minute amount of acetic acid. The solution pH was measured to be approximately five. A small volume of this solution was placed on the laser patterned surface and allowed to react for five minutes. The surface was then rinsed with copious amounts of ethanol.
The surface that is both topographically and chemically modified as described was measured and assessed by using water contact angle goniometry. The surfaces were characterized using water contact angle goniometry with a First Ten Angstroms FTA 1000B contact angle goniometer. Several of the modified surfaces exhibited water contact angles in excess of 170 degrees. These surfaces were tested for insect adhesion under dynamic conditions using a custom-built pneumatic insect delivery device that delivered fruit flies to the surface at an average speed of 138 mph with a standard deviation of 27 mph. High-speed photography was obtained during impact events using a Vision Research Phantom 12 camera at a speed of 50,000 frames per second. Digital images of the post-insect impacted samples were obtained using an Olympus C-740 UltraZoom Digital Camera.







TABLE 1







Surface characterization and fruit fly impact results of laser ablation


patterned Al alloy surfaces and coated surfaces.













Profilometry Results














Pulse
Line
Water Contact

Maximum
Areal



Energy,
Spacing,
Angle, °
Roughness,
Height,
Coverage,














Coating
μJ/pulse
μm
Ablated
Silanated
Ra, μm
μm
mm2

















Control
—
—
84a 
—
0.31
69
1.27


Control
—
—
—
110
—
66
0.71


& F17


LA-1
40
50.8
26 
170
1.854
60
0.26


LA-2
40
25.4
82 
162
2.227
66.1
1.33


LA-3
40
12.7
 5b
165
0.595
62
0.45


LA-4
65
101.6
5
162
2.549
89.7
0.31


LA-5
65
50.8
5
166
3.402
97
1.43


LA-6
65
25.4
5
167
4.166
100.7
0.77


LA-7
65
12.7
5
165
1.197
71.5
0.45


LA-8
99
101.6
5
166
2.154
66.4
0.40


LA-9
99
50.8
5
167
5.328
63.9
0.50


LA-10
99
25.4
5
171
5.11
66.6
1.31


LA-11
99
12.7
5
164
3.459
66.9
0.28





aControl surfaces were not laser ablation patterned.


bContact angles reported as 5° were not able to be accurately measured due to rapid wetting of the surface where contact angles on these surfaces approached 0°.






The “Areal Coverage” column in Table 1 was determined using the optical surface profilometer. The areal coverage represents the surface area that has insect residue remaining on it after the test described herein and the insect impact samples discussed. Both the area and the height of the insect residue may be relevant for the purpose of any subsequent aerodynamic testing.
The results above are for exemplary purposes only, and one of ordinary skill in the art would adjust the various parameters depending on the desired reduction in adhesion properties, which would vary depending on the type of surface, conditions under which adhesion should be mitigated, coating types and methods of topographical modification.
While some embodiments of the invention have been herein illustrated, shown and described, it is to be appreciated that various changes, rearrangements and modifications may be made therein, without departing from the scope of the invention as defined by the appended claims. It is intended that the specific embodiments and configurations are disclosed for practicing the invention, and should not be interpreted as limitations on the scope of the invention as defined by the appended claims and it is to be appreciated that various changes, rearrangements and modifications may be made therein, without departing from the scope of the invention as defined by the appended claims.

## Claims

1. A process of mitigating insect residue adhesion on an aerodynamic surface to maintain laminar flow, the process comprising:
a. providing at least one article having at least one aerodynamic surface that is impacted by insects in use;
b. topographically modifying said aerodynamic surface;
c. chemically modifying said aerodynamic surface by coating said aerodynamic surface with a low surface energy coating;
wherein the modified aerodynamic surface comprises a water contact angle of greater than about 80 degrees and a surface roughness of about 0.2 micron to about 50 microns after said aerodynamic surface is topographically and chemically modified, wherein the modified aerodynamic surface is configured to maintain laminar flow of air that moves across the modified aerodynamic surface and mitigate adhesion of insect residue without creating significant turbulence in air that moves along the modified aerodynamic surface in use.

2. The process of claim 1, wherein said modified aerodynamic surface comprises a surface roughness of about 1 micron to about 10 microns.

3. The process of claim 1, wherein said modified aerodynamic surface is topographically modified by laser ablation with an ablation depth of about 1 μm to about 30 μm.

4. The process of claim 1, wherein said aerodynamic surface is topographically modified by laser ablation with an ablation depth of about 1 μm to about 10 μm.

5. The process of claim 1, wherein said modified aerodynamic surface is topographically and chemically modified by spray deposition of a polymer particulate composition comprising a nanocomposite material.

6. The process of claim 5, wherein said nanocomposite material comprises silica nanoparticles.

7. The process of claim 1, wherein said coating comprises a polymer composition having a surface energy of less than about 50 mJ/m2.

8. The process of claim 1, wherein said coating comprises a polymer composition having a surface energy of less than about 40 mJ/m2.

9. The process of claim 1, wherein said coating is chosen from the group consisting of: copoly(imide fluorinated alkyl ether), fluorinated silanes, fluorinated aliphatic compounds, silicones, and fluorine-containing polymers.

10. The process of claim 1, wherein said coating comprises copoly(imide fluorinated alkyl ether).

11. The process of claim 1, wherein said coating comprises a silane composition.

12. The process of claim 11, wherein said silane composition is prepared by generating aqueous ethanol solutions with glacial acetic acid to induce acid hydrolysis of the alkoxy functionality of said silane composition.

13. The process of claim 11, wherein said silane composition comprises a mixture of Si—C6, Si—C12, and Si—C18.

14. The process of claim 11, wherein said silane composition comprises Si—F17.

15. The process of claim 1, wherein said water contact angle is greater than about 110 degrees.

16. The process of claim 1, wherein said coating is applied by a method chosen from the group consisting of: vapor phase deposition, plasma deposition, spin casting, submersion, and spray coating.

17. A process of mitigating insect residue adhesion on an aerodynamic surface to maintain laminar flow, the process comprising:
a. providing at least one article having at least one aerodynamic surface that is impacted by insects in use;
b. topographically modifying said aerodynamic surface;
c. chemically modifying said aerodynamic surface by coating said aerodynamic surface with a coating having a surface energy of less than about 50 mJ/m2;
wherein the modified aerodynamic surface comprises a water contact angle of greater than about 80 degrees and a surface roughness of about 1 micron to about 10 microns after said aerodynamic surface is topographically and chemically modified, wherein the modified aerodynamic surface is configured to maintain laminar flow of air that moves across the aerodynamic surface and mitigate adhesion of insect residue without creating significant turbulence in air that moves along the modified aerodynamic surface in use.

18. The process of claim 17, wherein said coating comprises a surface energy of less than about 40 mJ/m2.

19. The process of claim 18, wherein said modified aerodynamic surface is topographically and chemically modified by a coating comprising a polymer particulate composite having a nanocomposite material.