This research was supported by the projects: MAT2014-60615-R and MAT2017-82182-R funded by the&#160;State&#160;Research&#160;Agency (SRA) and&#160;European&#160;Regional&#160;Development Fund (ERDF).

Note: The protocol follows the scheme shown in Figure 2.
1. Sample Preparation
<ol>
	<li>Cutting and cleaning 
	<ol>
		<li>Using a metal shear, cut 250 mm x 250 mm x 0.5 mm sheets of aluminum into 25 mm x 45 mm x 0.5 mm pieces. 
		Note: Special care must be taken when using the metal shear, and special training may be necessary.</li>
		<li>Remove the protective film covering one side of the sample and wash this side using around 50 mL of cl...

<ol>
	<li>Fang, G., Amirfazli, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Understanding+the+anti-icing+behavior+of+superhydrophobic+surfaces.">Understanding the anti-icing behavior of superhydrophobic surfaces.</a> Surface Innovations. 2 (2), 94-102 (2014).</li><li>Wang, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Robust+superhydrophobic+coating+and+the+anti-icing+properties+of+its+lubricants-infused-composite+surface+under+condensing+condition.">Robust superhydrophobic coating and the anti-icing properties of its lubricants-infused-composite surface under condensing condition.</a> New Journal of Chemistry. 41 (4), 1846-1853 (2017).</li><li>Jung, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Are+superhydrophobic+surfaces+best+for+icephobicity?.">Are superhydrophobic surfaces best for icephobicity?.</a> Langmuir. 27 (6), 3059-3066 (2011).</li><li>Milionis, A., Loth, E., Bayer, I. 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S., Kumar, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superhydrophobic+coatings+for+aluminium+surfaces+synthesized+by+chemical+etching+process.">Superhydrophobic coatings for aluminium surfaces synthesized by chemical etching process.</a> International Journal of Smart and Nano Materials. 7 (4), 248-264 (2016).</li><li>Ruiz-Cabello, F. J. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Testing+the+performance+of+superhydrophobic+aluminum+surfaces.">Testing the performance of superhydrophobic aluminum surfaces.</a> Journal of Colloid and Interface Science. 508, 129-136 (2017).</li><li>Mahadik, S. 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We have nothing to disclose.

In this paper, we demonstrate strategies to produce water-repellent surfaces on aluminum substrates. In addition, we show methods to characterize their wetting properties, roughness, durability and anti-icing performance.
To prepare the SH surfaces, we used two strategies. The first strategy incorporated the proper roughness degree to achieve the intrinsic hierarchical structure of SH surfaces by acid etching. This process is particularly critical, which may require further work for other meta...

The ability of superhydrophobic (SH) surfaces to repel water is the reason that they are traditionally proposed as a solution to prevent icing1,2. However, there are concerns about the suitability of SH surfaces for anti-icing agents: 1) the high costs of production, 2) that superhydrophobicity does not always lead to ice-phobicity3, and 3) the questionable durability of the SH surfaces4. Superhydrophobic surfaces hold two properties related to their topography and chemical composition5: they are rough, with particular topographic features; and their surface energy is low (intrinsically hydrophobic).
The roughness on a hydrophobic surface serves to reduce the ratio between the real solid-liquid area and the apparent contact area. The water is not fully in contact with the solid due to the lotus effect6,7, when the drop rests or moves onto the surface asperities. In this scenario, the solid-liquid interface acts heterogeneously with two chemical domains: the solid surface itself and the tiny air-bubbles trapped between the solid and water8. The degree of water repellency is connected to the amount of trapped air because the air patches are smooth and its intrinsic contact angle is 180&#176;. Some studies report the incorporation of a hierarchical surface texture with micro and nano-asperities as the optimal strategy to provide better water-repellent properties (greater presence of air at the solid-liquid interface)9. For some metals, a low-cost strategy to create two-level roughness features is acid-etching10,11. This procedure is frequently used in industry. With certain acid concentrations and etching times, the metal surface reveals the proper hierarchical roughness. In general, the surface roughening is optimized by varying the acid concentration, etching time, or both12. The surface energy of metals is high and for this reason, the fabrication of water-repellent metal surfaces requires later hydrophobization.
Hydrophobization is generally achieved by hydrophobic film deposition using different methods: silanization10,13, dip-coating14, spin-coating15, spraying16 or plasma-deposition17. Silanization has been proposed18 as one of the most promising tool for improving the low durability of SH surfaces. Unlike other deposition techniques, the silanization process is based on a covalent bond between the Si-OH groups with the surface hydroxyl groups of the metal substrate10. A drawback of the silanization process is the need for previous activation of the metal substrate to create enough hydroxyl groups for a high degree of coverage and uniformity. Another strategy recently proposed to produce resistant superhydrophobic surfaces is the use of rare-earth coatings19,20. Ceria coatings have two properties that justify this use: they can be intrinsically hydrophobic21, and they are mechanically and chemically robust. In particular, one of the most important reasons why they are chosen as protective coatings is their corrosion-protection abilities20.
To produce long-lasting SH metal surfaces, two issues are considered: the surface texture must not be damaged, and the hydrophobic film/coating must be firmly anchored to the substrate. Surfaces are typically exposed to wear originated by lateral abrasion or particle impact4. If the asperities are damaged, the water-repellency may be substantially reduced. Under extreme environments, the hydrophobic coating may be partially removed from the surface or may be chemically degraded by UV exposure, humidity or corrosion. The design of durable SH surfaces coatings is an important challenge for coating and surface engineering.
For metals, one of the most demanding requirements is that the anti-icing ability is based on three interconnected aspects22 as illustrated in Figure 1: subcooled water repellency, freezing delay, and low ice-adhesion. Outdoor icing occurs when subcooled water, typically rain drops, comes into contact with a solid surface and is rapidly frozen by heterogeneous nucleation23. The formed ice (rime) is firmly attached to the surface. Thus, the first step to avoid icing is to reduce the solid-water contact time. If the surface is superhydrophobic, rain drops may be expelled from the surface before freezing. In addition, it has been proven that, under humid conditions, surfaces with a high contact angle delay freezing more efficiently than those ones with a low contact angle24. For these two reasons, SH surfaces are the most appropriate surfaces to mitigate icing. However, the lifetime of superhydrophobic surfaces may be a key point since icing conditions are typically aggressive25. Some studies have concluded that SH surfaces are not the best choice for decreasing ice adhesion26. Once the ice forms on the surface, it stays firmly attached due to surface asperities. The roughness increases the ice-surface contact area and the asperities act as interlocking agents26. The use of durable SH surfaces is recommended to avoid icing if there are no traces of ice already present on the surface.
In this work, we present several protocols to produce durable SH surfaces on metal substrates. We use aluminum (Al) as the substrate because it is widely used in industry, and the incorporation of anti-icing properties is particularly relevant for certain applications (ski resorts facilities, aeronautics, etc.). We prepare three types of surfaces: a textured Al surface coated with a fluoropolymer coating, a textured Al surface silanized with a fluorosilane, and a ceria-stearic acid bilayer on an Al substrate. Similar techniques17,27,28,29 provide 100-300 nm film thicknesses or even monolayer films. For each surface, we measured their wetting properties and conducted wear tests. Finally, we analyzed their anti-icing performance by using three tests aimed to probe independently the three properties shown in Figure 1.
Our protocol is based on the scheme shown in Figure 2. Once the SH Al surfaces are prepared, their wetting properties and topography are analyzed to determine their repellency properties and roughness features. The wetting properties are analyzed by bouncing drop experiments, which is a technique connected to the water tensile adhesion. Since the observation of drop bounces is required, this technique is only suitable for superhydrophobic surfaces13. For each surface treatment, we prepared at least four samples to conduct the anti-icing tests and another four samples to perform the durability tests. The damage caused after each durability test was analyzed by measuring the loss of wetting properties and roughness features. Similar durability tests to the proposed ones in this work were recently used for other metal surfaces27,30.
Concerning the anti-icing tests, the aim of this study is to determine whether the use of the produced SH Al surfaces are convenient as anti-icing agents. Hence, we analyzed, for comparison, the performance of two control samples: a) an untreated Al sample (smooth hydrophilic sample) and b) a hydrophobized but not textured sample (smooth hydrophobic sample). For the same purpose, the use of a textured but not hydrophobized surface might be of interest. Unfortunately, this surface is extremely wettable and anti-icing tests cannot be carried out for them.

<table>
	<tbody>
		<tr>
			<td>Hydrochloric acid, 37%</td>
			<td>SICAL, S.A.</td>
			<td>AC07411000</td>
			<td>used for acid etching</td>
		</tr>
		<tr>
			<td>1H,1H,2H,2H-Perfluorodecyltriethoxysilane, 97%</td>
			<td>Sigma-Aldrich</td>
			<td>658758</td>
			<td>used for silanization with FAS-17</td>
		</tr>
		<tr>
			<td>Dupont AF1600</td>
			<td>Dupont</td>
			<td>D10389631</td>
			<td>used for fluropolymer deposition</td>
		</tr>
		<tr>
			<td>FC-72</td>
			<td>3M, Fluorinet</td>
			<td>1100-2-93</td>
			<td>used for fluropolymer deposition (flurocarbon solvent)</td>
		</tr>
		<tr>
			<td>Cerium(III) chloride heptahydrate, 99.9%</td>
			<td>Sigma-Aldrich</td>
			<td>228931</td>
			<td>used for Ceria coating deposition</td>
		</tr>
		<tr>
			<td>Hydrogen peroxide solution, 30%</td>
			<td>Sigma-Aldrich</td>
			<td>H1009</td>
			<td>used for Ceria coating deposition</td>
		</tr>
		<tr>
			<td>Stearic acid, &#8805;98.5%</td>
			<td>Sigma-Aldrich</td>
			<td>S4751</td>
			<td>used for Ceria coating deposition</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>SICAL, S.A.</td>
			<td>16271</td>
			<td>used throughout</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>SICAL, S.A.</td>
			<td>1090</td>
			<td>used throughout</td>
		</tr>
		<tr>
			<td>Aluminum sheets 0.5mm</td>
			<td>MODULOR (Germany)</td>
			<td>125993</td>
			<td>substrates used throught</td>
		</tr>
		<tr>
			<td>Micro-90 concentrated cleaning solution</td>
			<td>Sigma-Aldrich</td>
			<td>Z281506</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultra pure Milli-Q water</td>
			<td>Millipore</td>
			<td>discontinued</td>
			<td>used throughout</td>
		</tr>
		<tr>
			<td>Plasma Etcher/Asher/Cleaner EMITECH K1050X</td>
			<td>Aname</td>
			<td>K1500XDEV-001</td>
			<td>used throughout</td>
		</tr>
		<tr>
			<td>PCC software</td>
			<td>AMETEK</td>
			<td>discontinued</td>
			<td>sofware controlling the high speed camera Phantom MIRO 4</td>
		</tr>
		<tr>
			<td>High Speed Camera Phantom Miro 4</td>
			<td>AMETEK</td>
			<td>discontinued</td>
			<td>used for bouncing drop experiments</td>
		</tr>
		<tr>
			<td>Open Loop PL&#181; 2.32</td>
			<td>UPC-CD6 &#38; Sensofar Tech S.L.</td>
			<td>version 2.32</td>
			<td>Sofware controlling PL&#181; Confocal Imaging Profiler</td>
		</tr>
		<tr>
			<td>Pl&#181;-Confocal Imaging Profiler 2300</td>
			<td>Sensofar Tech S.L.</td>
			<td>discontinued</td>
			<td>used for roughness measurements</td>
		</tr>
		<tr>
			<td>TABER 5750 LINEAL ABRASER</td>
			<td>TABER</td>
			<td>5750</td>
			<td>used for lateral abrasion tests</td>
		</tr>
		<tr>
			<td>Abbrasive sand: ASTM 20-30 SAND C778</td>
			<td>U.S. SILICA COMPANY (USA)</td>
			<td>1-800-635-7263</td>
			<td>used for abrasive partcile impact tests</td>
		</tr>
		<tr>
			<td>Ozone cleaner: PSDP-UV4T, Digital UV Ozone System</td>
			<td>Novascam</td>
			<td>discontinued</td>
			<td>UV-ozone degradation test</td>
		</tr>
		<tr>
			<td>Peristalitic Pump GILSON 312, France</td>
			<td>GILSON (France)</td>
			<td>discontinued</td>
			<td>used for water dripping test</td>
		</tr>
		<tr>
			<td>Nylon thread</td>
			<td>Dracon fishing line, Izorline internacional, inc. (USA)</td>
			<td>discontinued</td>
			<td>used for ice adhesion tests</td>
		</tr>
		<tr>
			<td>Digital force gauge (ZTA-200N, ZTA Series</td>
			<td>IMADA (USA)</td>
			<td>370199</td>
			<td>used for ice adhesion tests</td>
		</tr>
		<tr>
			<td>Motorized test stand I, MH2-500N-FA</td>
			<td>IMADA (USA)</td>
			<td>366942</td>
			<td>used for ice adhesion tests</td>
		</tr>
		<tr>
			<td>Force Recorder Professional</td>
			<td>IMADA (USA)</td>
			<td>version 1.0.2</td>
			<td>software provided by IMADA to register the force</td>
		</tr>
		<tr>
			<td>HYGROCLIP XD - STANDARD PROBE</td>
			<td>Rotronic</td>
			<td>discontinued</td>
			<td>Temperature and humidity probe</td>
		</tr>
		<tr>
			<td>HW3 Lite software</td>
			<td>Rotronic</td>
			<td>version 2.1.2</td>
			<td>Sofware controlling the HYGROCLIP Probe</td>
		</tr>
	</tbody>
</table>

fabrication of superhydrophobic metal surfaces for anti-icing applications

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in industry. The wettability of the produced surface was analyzed by bouncing drop experiments and the topography was analyzed by confocal microscopy. In addition, we show various methodologies to measure its durability and anti-icing properties. Superhydrophobic surfaces hold a special texture that must be preserved to keep their water-repellency. To fabricate durable surfaces, we followed two strategies to incorporate a resistant texture. The first strategy is a direct incorporation of roughness to the metal substrate by acid etching. After this surface texturization, the surface energy was decreased by silanization or fluoropolymer deposition. The second strategy is the growth of a ceria layer (after surface texturization) that should enhance the surface hardness and corrosion resistance. The surface energy was decreased with a stearic acid film.
The durability of the superhydrophobic surfaces was examined by a particle impact test, mechanical wear by lateral abrasion, and UV-ozone resistance. The anti-icing properties were explored by studying the ability to repeal subcooled water, freezing delay, and ice adhesion.

The wetting and roughness properties of the SH surfaces that were used in this study are shown in Figure 5. The average number of bounces measured for each sample is displayed in Figure 5a and the average roughness is shown in Figure 5b. There is no correlation between the roughness and wetting properties. The number of bounces measured for the polytetrafluoroethylene coated sample agrees with the Ce...

Watch this Scientific Journal Video about Fabrication of Superhydrophobic Metal Surfaces for Anti-Icing Applications at JoVE.com

Fabrication of Superhydrophobic Metal Surfaces for Anti-Icing Applications

We illustrate several methodologies to produce superhydrophobic metal surfaces and to explore their durability and anti-icing properties.

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in ...

This method can help answer key questions in the coating fields about the convenience and suitability of using superhydrophobic surfaces as anti-icing agents in real applications. The main advantage of these techniques is that analysis with a surface both prevents icing and is durable enough to resist the wetting conditions when icing occurs. To begin the sample preparation, in a fume hood, place a 100 milliliter beaker containing 80 milliliters of four molar hydrochloric acid in a 500 milliliter beaker.Fill another 500 milliliter beaker with ultrapure water. Immerse an aluminum sample in the acid, using polytetrafluoroethylene tweezers and allow it to soak for eight minutes. Then, quench the etched sample in ultrapure water.Thoroughly rinse the sample before proceeding. Next, dry the sample under a steam of filtered compressed air until no water is visible on the surface. Lastly, dry the sample in an oven at 120 degrees Celsius for 10 minutes.To hydrophobize the sample by vapor-phase silanization, first plasma-treat the sample with air plasma at 100 watts for 10 minutes. Then, immediately place the sample in a plastic Petri dish and incline the sample slightly with a 20 to 200 microliter pipette tip. Apply two 50 microliter drops of the silane about one centimeter from the sample.Cover most of the dish with the lid. Promptly place the dish in a vacuum dessicator. Cover about 80%of the dish with the lid and evacuate it to begin silanization.Silanize the sample overnight or for at least eight hours. To hydrophobize a sample by fluoropolymer deposition, first spray the sample from 10 centimeters away with a one to 20 by volume solution of an amorphous fluoropolymer in a fluorocarbon solvent. Allow the coating to dry in air at room temperature for 10 minutes.Apply a second coat and dry the sample at 110 degrees Celsius for 10 minutes to evaporate the solvent and crosslink the fluoropolymer coating. Then, check that the surface is superhydrophobic. To hydrophobize an etched sample by ceria-stearic acid deposition, first clean and dry the sample.Then, immerse the sample in 50 milliliters of an aqueous solution containing two grams of cerium trichloride heptahydrate and three milliliters of 30%by weight hydrogen peroxide. Keep the immersed sample in an oven at 40 degrees Celsius for one hour. Then, rinse the sample in distilled water, dry it at 100 degrees Celsius for 10 minutes and allow it to cool to room temperature.Then, soak the sample in a 30 millimolar solution of stearic acid in ethanol for 15 minutes. Afterwards, rinse the sample in ethanol, dry it at 100 degrees Celsius for 10 minutes and let it cool. Place a sample on the bouncing drop experiment platform under a fixed insulin needle.Set the high-speed camera acquisition rate to 4200 images per second and the exposure time to 235 microseconds. Then, fill a one milliliter syringe with ultrapure water and connect it to the needle. Confirm that the needle and sample are in focus and then begin recording.Stop recording a few seconds after a drop falls on the sample. Trim the video to start from the moment of the drop being released from the needle and end at the static drop being in full contact with the sample with no further bouncing observed. Create the drop profiles with image analysis software and quantify the number of bounces by watching the video sequence.To perform a lateral abrasion test, first set a linear abrader with a mild to moderate tip to 20 cycles per minute with a stroke length of 38.1 millimeters. Configure the abrader for the minimum applied pressure. Mount the sample on the abrader and perform an abrasion cycle.Then, gently clean and dry the sample. Evaluate the wetting properties with a tilting plate experiment. Repeat the abrasion and evaluate the sample again after two, three and five total cycles.To perform an abrasive particle impact test, first fix a sample on a surface with a 45 degree incline. Mount a glass funnel 24 to 26 centimeters above the sample. Pour 30 milliliters of ASTM 2030 abrasive test sand into the funnel and wait for the sand to flow over the sample.Evaluate the wetting properties with a tilting plate experiment. Perform this test up to three times per sample. To perform a UV-ozone degradation test, treat the sample in a UV-ozone cleaner for 10 minutes at room temperature.Afterwards, rinse the sample in ultrapure water, dry it with compressed air and evaluate the wetting properties. Repeat this test up to three times. To begin the freezing delay test, place a large sample on an isolated platform In the center of a freezing chamber.Ensure that the chamber humidifier is filled with water. Then, use a pipette to gently deposit an array of 25 drops of water on the sample, if it is superhydrophobic. For other samples, deposit an array of 70 drops.Close the chamber and start the cooling system. Once the chamber reaches about five degrees Celsius, start the humidifier. The drops will begin to nucleate below zero degrees Celsius.Once nucleation begins, record the number of frozen drops at intervals of 0.5 degrees Celsius until all drops have frozen. Repeat the test until about 200 drops have been tested on the sample. To begin the tensile ice-adhesion test, Tie 0.25 millimeter nylon thread to a digital force gauge mounted on a motorized test stand above the freezing chamber.Then, cut a 28 millimeter long piece of PTFE pipe with a 10 millimeter inner diameter. Drill two one millimeter-wide holes across from each other, five millimeters from one end of the cylinder. Put a narrow bar through these holes.Then, set the cylinder upright on a sample and fill it with 1.2 milliliters of distilled water. Carefully place the sample and cylinder on a platform in the freezing chamber, keeping the cylinder pressed against the sample. Close the freezing chamber, turn on the cooling system and wait one hour for the water to freeze.Then, firmly fix the sample to the platform using a striker plate with a hole slightly wider than the cylinder. Connect the sample to the force gauge by feeding the nylon thread through the bar at the top of the cylinder. Close the chamber and wait for 10 minutes.Then, when recording the force, vertically displace the gauge from the sample at 10 millimeters per minute until the ice separates from the sample. For the shear adhesion test, freeze 1.2 milliliters of water against a sample in a PTFE cylinder in the same way. Clamp the sample vertically in the freezing chamber and fit a metal ring attached to the force gauge via nylon thread over the cylinder.Close the chamber and wait 10 minutes. Then, vertically displace the gauge at 10 millimeters per minute until the ice shears. No correlation was observed between roughness and wetting properties of the superhydrophobic surfaces.For example, while the PTFE sample had a similar roughness value to the fluoro alkoxysilane sample, its average number of bounces was closer to that of the ceria-stearic acid sample. All three sample types exhibited poor mechanical resistance, losing their water-repelling properties after two cycles. The fluoro alkoxysilane and the ceria-stearic acid samples were degraded by UV-Ozone exposure, but the PTFE sample was not.At high humidity, the superhydrophobic surfaces delayed freezing longer than the smooth hydrophobic surfaces did. However, at low relative humidity, the superhydrophobic surfaces delayed freezing less efficiently than smooth hydrophobic surfaces did. Consistent with previous findings, the superhydrophobic surfaces showed greater shear and tensile ice-adhesion than coated and uncoated smooth surfaces.This was attributed to the roughness of the superhydrophobic surfaces. A hierarchical texture is required to produce a metal superhydrophobic surface. Acid etching is a very successful strategy, but the conditions depend on the metal.Advance screening is required to insure an optimal degree of roughness. The etching reactions on the hydrophobization molecules might be the most critical steps for successfully producing superhydrophobic surface on aluminum. Following this procedure, other materials like lubricant-impregnated slippery surfaces can be fabricated to answer additional questions about which surfaces avoid icing more efficiently.By combining the analysis of two important aspects of anti-icing agents'suitability, this technique paves the way for researchers in the coatings field to explore certain surfaces and coatings as anti-icing agents that are resistant to air in environmental conditions. Don't forget that working on acid is extremely dangerous. should be taken like training and wear different devices for protection, like goggles, gloves and so.

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8.3K Views.  University of Granada. This method can help answer key questions in the coating fields about the convenience and suitability of using superhydrophobic surfaces as anti-icing agents in real applications. The main advantage of these techniques is that analysis with a surface both prevents icing and is durable enough to resist the wetting conditions when icing occurs. To begin the sample preparation, in a fume hood, place a 100 milliliter beaker containing 80 milliliters of four molar hydrochloric acid in a 500 millil...