Atmospheric Ice Adhesion on Water-Repellent Coatings: Wetting and

Nov 13, 2015 - Atmospheric Ice Adhesion on Water-Repellent Coatings: Wetting and Surface Topology Effects. Yong Han Yeong, Athanasios Milionis, and Er...
8 downloads 17 Views 6MB Size
Download PDF
Article pubs.acs.org/Langmuir

Atmospheric Ice Adhesion on Water-Repellent Coatings: Wetting and Surface Topology Effects Yong Han Yeong,* Athanasios Milionis, and Eric Loth* Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, United States

Jack Sokhey Rolls-Royce North America, Indianapolis, Indiana 46241, United States

Alexis Lambourne Rolls-Royce, Plc. Derby DE24 8EJ, U.K. S Supporting Information *

ABSTRACT: Recent studies have shown the potential of waterrepellent surfaces such as superhydrophobic surfaces in delaying ice accretion and reducing ice adhesion. However, conflicting trends in superhydrophobic ice adhesion strength were reported by previous studies. Hence, this investigation was performed to study the ice adhesion strength of hydrophobic and superhydrophobic coatings under realistic atmospheric icing conditions, i.e., supercooled spray of 20 μm mean volume diameter (MVD) droplets in a freezing (−20 °C), thermally homogeneous environment. The ice was released in a tensile direction by underside air pressure in a Mode-1 ice fracture condition. Results showed a strong effect of water repellency (increased contact and receding angles) on ice adhesion strength for hydrophobic surfaces. However, the extreme water repellency of nanocomposite superhydrophobic surfaces did not provide further adhesion strength reductions. Rather, ice adhesion strength for superhydrophobic surfaces depended primarily on the surface topology spatial parameter of autocorrelation length (Sal), whereby surface features in close proximities associated with a higher capillary pressure were better able to resist droplet penetration. Effects from other surface height parameters (e.g., arithmetic mean roughness, kurtosis, and skewness) were secondary.

1. INTRODUCTION Atmospheric icing on infrastructure and vehicles such as on power lines, wind turbines, and aircraft creates undesirable effects. The formation of ice occurs when supercooled water droplets, traveling through air in the form of an icing cloud, impinge on exposed freezing surfaces. This results in stall conditions or engine operating anomalies on an aircraft, both of which could lead to catastrophic consequences.1 In addition, the event of severe icing on wind turbines could adversely affect its operation and was estimated to cause a reduction in annual power production by up to 50%.2 Current ice mitigation methods mostly rely on active strategies such as electrothermal systems to melt and remove ice from a surface. Although these systems can successfully protect the surfaces from ice accretion, they require a constant source of energy which increases the operating cost of the system and, in some cases, reduces its operating efficiency.3 Passive anti-icing techniques such as the application of permanent or sacrificial coatings on surfaces to retard the © XXXX American Chemical Society

formation of ice are concepts that have been extensively studied since the 1950s.4−6 These strategies focus on tuning the surface energy of the coating to increase its hydrophobicity or applying a layer of lubricant/wax to the surface.7,8 Recently, superhydrophobic coatings have shown potential in ice mitigation due to its extreme water repellency. In a study performed by Mischenko et al., a supercooled water drop was shown to rebound from a cooled superhydrophobic surface before it could nucleate.9 Therefore, superhydrophobic surfaces have been touted by many to be a more attractive anti-icing technology as compared to current active anti-icing techniques. However, realistic atmospheric icing studies that were recently performed showed that while ice formation was initially prevented on superhydrophobic surfaces, accretion eventually occurred after an extended exposure to supercooled dropReceived: July 23, 2015 Revised: November 5, 2015

A

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir lets.10,11 For example, in a study by Jung et al. where supercooled droplets were allowed to impinge a superhydrophobic surface, droplet freezing was found to be delayed for an average of 100 s, after which ice accretion would occur.12 Several investigations that studied superhydrophobic ice accretion under different icing conditions also reported similar results,13−20 the exception being a study conducted by Antonini et al. where a complete prevention of ice accretion was observed when heating was employed on the superhydrophobic surface.21 The comprehensive agreement of this observation from these studies proved that a superhydrophobic surface cannot inherently thwart ice accretion. Therefore, on the basis of these findings, researchers have altered their studies to focus on investigating the effectiveness of superhydrophobic surfaces in reducing ice adhesion strength. This is of great relevance in applications such as on aircraft and wind turbines as intrinsic operating forces such as centrifugal and/or aerodynamic forces can be significant enough to detach the accreted ice from the superhydrophobic surface. This is based on the hypothesis of reduced liquid−surface contact area when in a Cassie−Baxter wetting state for a lower ice adhesion strength.22 Various researchers investigated this hypothesis with conflicting results. In general, they observed a decrease in ice adhesion but a few reports disagreed with this assessment.23−31 For example, Chen et al.32 reported a significant increase in ice adhesion strength on superhydrophobic surfaces fabricated from photolithography techniques as compared to control surfaces. Although Bharathidasan et al.33 did find that the ice adhesion strength of superhydrophobic nanocomposite coatings were lower than of aluminum and polyurethane surfaces, they were still significantly higher than hydrophobic materials. In addition, Sussoff et al.34 reported conflicting trends in superhydrophobic ice adhesion strength of a composite coating with other measurements published in the literature. They discovered that while certain superhydrophobic surfaces had lower adhesion strength, others did not. As such, researchers have conducted various studies to investigate the factors that control ice adhesion strength on superhydrohobic surfaces. A few parameters were identified which include surface wettability properties such as static contact angle (CA), contact angle hysteresis (CAH), and receding contact angle (RCA). In addition, the surface texture characteristic of surface roughness was considered as well. These parameters have been previously linked to ice adhesion strength on control surfaces and hydrophobic materials, with studies showing a decrease in ice adhesion strength on high-CA surfaces due to its low surface energy.5,35 Rougher surfaces were also shown to increase the bond between ice and these surfaces.36,37 Further studies were conducted to investigate the role of wettability properties on superhydrophobic ice adhesion strength, most of which resulted in inconsistent findings. While some studies reported a relationship between ice adhesion strength and CA,23−25,31 others did not.4,20,32,34 Recently, researchers focused on dynamic angles, one of the studies being Kulinich et al.,20,26−28 who reported a favorable correlation between superhydrophobic ice adhesion and CAH. However, Meuler et al.38 conducted a detailed study on over 20 materials and showed that RCA had a stronger effect on ice adhesion as compared to CAH, but this dependency has not been proven on superhydrophobic coatings. These inconsistencies and disagreements show the lack of understanding of the mechanisms that govern ice adhesion strength on super-

hydrophobic materials as well as its general effectiveness in reducing the ice-coating bond. It should also be noted that majority of these previous studies were conducted based on “static” ice accretion processes that involved the freezing of either a water droplet or a body of water on top of a superhydrophobic surface. For example, Ge et al.24 positioned a static droplet on a superhydrophobic surface to be frozen. Chen et al.32 placed water-filled cuvettes on the tested surfaces and on a cooling stage for freezing. Yang et al.,18 Bharathidasan et al.,33 and Susoff et al.34 utilized a zero degree cone test where a cylinder coated with superhydrophobic materials was submerged into a water bath and frozen. These ice accretion methods do not resemble the atmospheric ice accretion process of supercooled drop impact. More importantly, “static” ice accretion methods subject a hydrostatic force on the superhydrophobic surface and can cause the air layer within the surface asperities to gradually diminish.39,40 This results in inconsistencies in the ice adhesion measurements. Therefore, the objective of this study was to comprehensively assess the effects of wettability parameters and a variety of surface texture characteristics on hydrophobic and superhydrophobic ice adhesion in atmospheric icing conditions, so that the properties that govern ice adhesion on these surfaces could be elucidated. We performed the above study by exposing substrate disks to the impact of an icing cloud consisting of 20 μm mean volume diameter (MVD) supercooled droplets in a freezing (−20 °C), thermally homogeneous environment conditions which are typical of atmospheric icing. The impact speed of the droplets on the substrate was estimated to be 5 m/ s. The accreted ice was detached from the surface by gradually increasing underside air pressure for a Mode-1 ice fracture. This fracture air pressure was then converted into an ice fracture energy for further analysis.

2. EXPERIMENTAL METHODS A. Tested Coatings. The coatings that were tested include commercially available products and coatings developed in-house at the Fluids Research Innovation Lab (FRIL) at the University of Virginia. These coatings had a wide range of surface wettabilities and texture characteristics and could be separated into three categories: (1) control surfaces, (2) hydrophobic coatings, and (3) superhydrophobic coatings. The three categories with their respective coatings and their fabrication techniques and procedures are described in the following paragraphs. The first category of surfaces consisted of three control surfaces of different materials and finish quality: (i) as-received Titanium 6−4 (Ti 6−4), (ii) shot-peened Ti 6−4 (Ti 6−4 SP), and (iii) as-received aluminum 6061. Ti 6−4 is a titanium alloy containing 6% aluminum and 4% vanadium and is a material that is commonly used in aircraft engines. In addition to this surface, a shot-peened Ti 6−4 surface was tested as well. Shot peening is a procedure where a Ti 6−4 surface is roughened by the impact of glass beads and is a procedure commonly conducted to improve their fatigue strength.41 All of the above control surfaces were fabricated to be in the shape of a 30 mm diameter disk. With the exception of two coatings (RR1 and RR2 described in the next paragraph), the rest of the coatings tested in this study were applied on aluminum control substrate disks. The second category of surfaces consisted of hydrophobic coatings. Five coatings were selected under this category. These include two proprietary coatings (named RR1 and RR2) and were applied on Ti 6−4 shot-peened disks. The rest of the coatings were commercial coatings applied on aluminum disks and consisted of Nusil R-2180, DuPont Teflon, and Aculon coatings. Nusil R-2180 is a silicone-based, semitranslucent coating targeted at aerospace applications for a high ice-release performance. A primer (Nusil SP-270) was first wiped on B

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Table 1. Summary of Coating Formulations, Application Method, Surface Wettability, and Texture Characteristics for All Tested Hydrophobic and Superhydrophobic Coatingsa coating

type

application method

RR1 RR2 Nusil R-2180 Teflon Aculon UVA SH-1 UVA SH-3 UVA SH-8 UVA SH-a UVA SH-c UVA SH-d UVA ABS Hydrobead 1 Hydrobead 2 Teflon SH

H H H H H SH SH SH SH SH SH SH SH SH SH

proprietary proprietary spray brushed dip coat spray spray spray spray spray spray spray spray spray spray

coating solids proprietary proprietary proprietary Dupont 852G-201 proprietary polyurethane, PMC, SiO2 particles polyurethane,waterborne PMC, SiO2 polyurethane,waterborne PMC, SiO2 polyurethane, PMC, SiO2 particles polyurethane,waterborne PMC, SiO2 polyurethane,waterborne PMC, SiO2 ABS, Aerosil R812 proprietary proprietary DuPont 852G-201

particles particles particles particles

mean CA (deg)

mean RCA (deg)

mean Sa (μm)

mean Sal (μm)

128.6 108.7 116.7 126.8 111.0 156.5 159.8 159.0 162.0 161.5 154.4 162.8 166.1 162.1 160.4

101.3 91.0 84.6 65.1 99.4 150.9 144.5 151.5 147.5 150.9 139.1 160.0 158.2 162.0 154.2

1.3 0.9 1.0 3.9 1.1 1.5 2.8 7.7 1.1 3.5 1.2 2.4 1.2 1.9 8.3

159.3 127.1 128.2 78.1 154.8 56.7 38.7 69.2 72.3 63.8 50.8 40.0 42.4 24.5 37.1

a

Nomenclature: H = hydrophobic, SH = superhydrophobic, PMC = perfluoroalkyl methacryclic copolymer, and ABS = acrylonitrile−butadiene− styrene. roughness on the fluorocarbon polymer matrix to result in a superhydrophobic surface. Detailed fabrication procedures of the SH coating series and information about its surface and chemical topography characteristics can be found in a previous work by Davis et al.44 A novel nanocomposite coating fabricated with acrylonitrile− butadiene−styrene (ABS), a thermoplastic polymer commonly used in 3-D printing, was also tested. The ABS (ABSplus-430, Stratasys) is hydrophobic by nature (CA of approximately 85) and consists of a styrene/acrylonitrile continuous phase partially grafted to a dispersed butadiene (rubber phase) which enhances the mechanical properties of the material.45 It was first dissolved in acetone and spray coated on the substrate with an airbrush (Passche, USA) to create a binder with microscale roughness, before a dispersion of hydrophobic fumed silica nanoparticles (HFS) (Aerosil R812, Evonik Industries) in acetone was applied on top of the ABS polymer coat to introduce secondary roughness. The HFS consist of spherical nanoparticles which are fused together forming secondary particles and then agglomerate into tertiary particles. This product is chemically identified by the manufacturer as Silanamine, 1,1,1-trimethyl-N-(trimethylsilyl), hydrolysis products with silica. After spraying, the coating was heat treated at 240 °C for 30 min to promote the bonding between the polymer and the particles to result in a nanocomposite surface. Further details on the coating fabrication method and surface topography can be found in Milionis et al.45 Another superhydrophobic coating that was tested consisted of the Teflon polymer. The preparation procedure and materials used in this coating were similar to the hydrophobic coating that was previously described. However, for the superhydrophobic coating, the top coat solution (DuPont 852G-201) was spray-coated on the primer layer. The spray process introduced surface texture on the coating which transitioned the surface from a Wenzel to a Cassie− Baxter wetting state to result in high water repellency. Finally, the commercial superhydrophobic coating Hydrobead was prepared by spray coating as-received two-part solutions on the substrates. In addition, a Hydrobead enhancer was sprayed on top of the original coating to improve its water-repellency for anti-ice applications. By varying the spray distances, two Hydrobead coatings with different surface topology characteristics were fabricated and were named Hydrobead 1 and 2, respectively. Since Hydrobead is a commercial product, its chemical composition is proprietary. The wettability and surface topology characteristics for the 18 coatings and surfaces described above were characterized using a goniometer (Model 290, Ramé Hart) and with a laser confocal scanning microscope. The wettability parameter of CA was measured using a 10 μL water drop while a 20 μL water drop was used for the

an aluminum disk and left to dry in room temperature for 1 h before two-part R-2180 solutions were mixed together with xylene and sprayed on top of the primer with an internal mix, siphon-fed airbrush (Passche, USA). The fabrication process was completed by curing the coating on a hot plate set to incrementally increasing temperatures (room temperature for 30 min, 75 °C for 45 min, and 150 °C for 135 min). A mixture of acid primer solutions (DuPont 850-7799 and DuPont 850G-314) was applied on roughened aluminum disks with a high volume low pressure (HVLP) spray gun for the fabrication of the Teflon coating. This primer was first cured at 260 °C for 10 min, followed by a temperature of 400 °C for another 10 min. The Teflon topcoat solution (DuPont 852G-201) was then applied on top of the primer coat with a paint brush and cured at 385 °C for 30 min. Finally, for the Aculon coating, the aluminum control disk was dip-coated with as-received Aculon solution from the manufacturer for 8 h and dried in room temperature. It should be noted that Aculon should be considered as a sacrificial surface finish due the fragile nature of the coating. The third category of surfaces consisted of superhydrophobic coatings developed at the University of Virginia (UVA SH coating series and ABS coating) as well as a commercially available product (Hydrobead). All the superhydrophobic coatings were fabricated on aluminum disk substrates by the spray deposition technique. This technique has gained prominence as it is simple, cost-effective, and also suitable for large-scale applications. The UVA SH coating series were nanocomposite coatings with hierarchical surface texture composed of aerospace grade polyurethane (Imron AF3500, DuPont) for superior coating durability, waterborne perfluoroalkyl methacryclic copolymer (PMC) (Capstone ST-110, DuPont), and silicon dioxide nanoparticles (Sigma-Aldrich). These components were mixed together to form a slurry and applied on substrates based on spray-casting techniques described by Yeong et al.42 By slightly modifying the weight percentages of the chemical formulations and spray distances, three different versions of the coating could be produced (SH-a, -c, and -d). These superhydrophobic coatings had similar CA and RCA values but consisted of different surface topology characteristics and were therefore differentiated accordingly. Energy dispersive X-ray spectroscopy (EDS) measurements revealed the presence of fluorocarbons on the SH coating series. Fluorocarbons are known for their intrinsic lowsurface energy properties and have been widely used by researchers to create superhydrophobic surfaces.43 In addition, titanium dioxide (TiO2) and silicon dioxide (SiO2) nanoparticles were detected. The TiO2 particles originated from the as-received polyurethane and gave the coating a white hue while the SiO2 particles were dispersed as part of the coating formulation. These nanoparticles provided a hierarchical C

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 1. Schematic (a) and picture (b) of the ice adhesion experiment. measurement of dynamic angles of CAH and RCA. The use of a 20 μL water drop for dynamic angle measurements was due to the wide wettability range of the test surfaces, i.e., hydrophilic (control samples), hydrophobic, and superhydrophobic coatings. Since the tilt method was utilized to measure the dynamic angles, it was discovered that 10 μL drops would release themselves on superhydrophobic coatings only. By increasing the volume of the drop to 20 μL, the drop could be released from all test coatings, therefore providing a more consistent measurement of the dynamic angles. These wettability measurements were conducted on three different spots on the coating to provide an averaged wettability value. A laser confocal microscope (Model LSM 510 Meta, Zeiss) with a metrology software (Mountains Map, Digital Surf) was used to image the coating surface texture and to convert them into 3-D topology images with image dimensions of 800 × 800 μm. This software also provided surface texture information such as arithmetic mean roughness (Sa), skewness (Ssk), kurtosis (Sku), and autocorrelation length (Sal). As with the wettability measurements, the imaging was also performed at three different spots on the coating so that the variability of the surface feature characteristics could be determined. A summary of the chemical formulations, application methods, surface wettability (CA and RCA), and topology parameters (Sa and Sal) for all hydrophobic and superhydrophobic coatings are listed in Table 1. B. Experimental Setup. The ice adhesion experiment was based on a test designed by Andrews et al. to be a plane-strain, mode-1, ice fracture toughness test.46−48 First, a polytetrafluoroethylene (PTFE) disk was secured on the hole of a disk substrate as a “defect”. After ice was accreted on the substrate and PTFE disk, an underside air pressure was continuously applied through the hole of the disk substrate and on the defect to initiate the process of ice fracture.47 Upon release of the ice from the surface, the critical pressure which was required to fracture the ice was recorded and referred to as f racture pressure. This fracture pressure could be further converted to an energy release rate as the f racture energy.47 A schematic of the experiment based on the principles described above is shown in Figure 1. A spray consisting of 20 μm MVD water droplets was generated by an air-atomizing nozzle (model Mod-1) which was fabricated by the Icing Branch at the NASA Glenn Research Center. It was shielded with heat tapes to prevent water from freezing in the nozzle. This nozzle setup was placed in a walk-in cold chamber and above the substrate disk. Preliminary tests were performed to optimize the distance between the Mod-1 spray nozzle and the substrate disk so that the water droplets would adequately supercool to form a consistent and correct type of ice (rime ice) on the substrate. This distance was determined to be 78 cm. The impact speed of the droplets on the substrate disks when positioned at this height was

estimated to be 5 m/s based on computational fluid dynamics simulations performed by Davis et al.49 Deionized water was separately cooled to 5 °C and delivered to the spray nozzle via a water pump (PO101X, Berns Corp., USA) installed outside of the cold chamber. Similarly, an air compressor (1.5 hp, Craftsman) outside the cold chamber supplied the necessary operating air pressure to the spray nozzle. Both the water pump and air compressor were connected to the spray nozzle via hoses which were thermally wrapped to prevent internal ice formation, a condition which could potentially obstruct the passage for air and water flow. Deionized water was used as water impurities could result in a large discrepancy in ice adhesion strength.50 The coatings were all applied to 30 mm diameter substrate disks and adhered to an aluminum fixture, called a boss, using hot melt adhesive (dry film adhesive #224, Lenderink Technologies, USA). The combined test piece (combination of the substrate disk and boss piece) was then attached to a vertical pipe and placed in the cold chamber and under the Mod-1 nozzle. A 50 μm polytetrafluoroethylene (PTFE) “defect” disk was used to cover the substrate disk access hole and was secured in place by vacuum pressure. This “defect disk” acted as a pre-existing defect/flaw for the growth of Mode-1 cracks along the ice-coating interface when supplied with air pressure for ice removal. This substrate disk setup is shown in Figure 2. The experiment was initiated by first setting the walk-in cold chamber to reach a temperature of −20 °C. Once the target temperature was reached, a 3 min long pretest spray was initiated at a water pressure of 450 kPa (65 psi) and at an air pressure of 35 kPa (5 psi). This 3 min spray was conducted to allow the water temperature

Figure 2. Setup and positioning of the aluminum disk substrate with boss piece for ice accretion: (a) components of the setup; (b) 10 mm ice accretion on the disk substrate. Direction of force provided by pressurized air to release ice from the surface is as indicated. D

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

⎛ 100 − % area ice released ⎞ ⎜ × 2τ ⎟ ⎝ ⎠ 100 ⎛ % area ice released ⎞ +⎜ × ω⎟ ⎝ ⎠ (5) 100

in the system to reach steady-state conditions. As shown in Figure 1a, a shield was utilized to prevent the prespray from coming in contact with the substrate disk. At the end of this 3 min procedure, the air pressure was increased to an operating air pressure of 138 kPa (20 psi). After an additional minute, the shield was retracted so that ice could accrete on the coating and “defect” disk. After ice was accreted to a thickness of 10 mm, air was manually supplied at an increasing rate of approximately 14 kPa/s (2 psi/s) from a pressurized air tank placed outside of the walk-in freezer to the underside of the substrate disk until the point where ice was detached from the test surface. This ice fracture pressure was recorded with a high frequency pressure transducer (recording rate of 1 kHz). A picture of a 10 mm ice accretion on a substrate disk is shown in Figure 2b. This test procedure of ice accretion and fracture was repeated three times for each coating so that an average value of ice fracture energy could be obtained. C. Fracture Energy Calculation. The detachment of ice from the coatings could lead to two different kinds of ice fractures: adhesive or cohesive ice fracture. Adhesive fracture refers to the ice-surface interfacial failure while cohesive fracture denotes an internal structural failure of the ice. Two ice fracture conditions were present in this experiment: a complete adhesive fracture mode (no remnant ice on test surface) and mixed-mode fracture (a combination of cohesive and adhesive failures resulting in partial attachment of ice on the surface). The difference in these ice fracture modes is shown in Figure 3. The cohesive (2τ) and adhesive (ω) ice fracture energies are defined in eqs 1 and 2, respectively.46

2τ =

ω=

Pc 2c Ef1

(1)

Pc 2c Ef2

(2)

fracture energy (J/m 2) =

3. RESULTS AND DISCUSSION The effect of surface wettability of the coatings on ice fracture energy was investigated and is shown in Figure 4. Two

Figure 3. Two different ice fracture conditions on the substrate disk from the top-down view: (a) mixed-mode fracture (cohesive and adhesive); (b) full adhesive fracture.

Figure 4. Effect of surface wettability on ice fracture energy of control, hydrophobic, and superhydrophobic coatings. The averaged arithmetic mean roughness (Sa) of the coatings were color-coded in the legend: (a) effect of CA; (b) effect of RCA.

The constants f1 and f 2 are defined as −1 3 ⎛c⎞ 4 ⎤ 1 ⎧ 3 ⎡⎜⎛ c ⎟⎞ 1⎫ ⎜ ⎟ ⎨ ⎬ ⎢ ⎥ + + f1 = ⎝h ⎠1 − υ ⎦ π ⎭ 1 − υ2 ⎩ 32 ⎣⎝ h ⎠ ⎪

⎪

⎪

⎪

3 ⎛c⎞ 4 ⎤ 1 ⎧ 3 ⎡⎜⎛ c ⎟⎞ 2⎫ ⎨ ⎢ ⎥+ ⎬ +⎜ ⎟ 2 ⎝ ⎠ ⎝ ⎠ h 1 − υ⎦ π⎭ 1 − υ ⎩ 32 ⎣ h

wettability parameters were studied. These were the static wettability parameter of CA and the dynamic wettability parameter of RCA. In addition, a third parameter describing the surface topology amplitude (Sa) was included in this analysis, defined as 1 Sa = | z(x , y )| d x d y (6) A A where z refers to a function of surface heights relative to the mean plane and A is the imaged area of the coating by the confocal microscope. This parameter provides a general quantification of the degree of coating roughness. The Sa of the coatings were classified into six ranges (from less than 1 μm to over 6 μm) and color-coded to each data point in Figures 4a and b according to their roughness classifications.

(3)

−1

f2 =

⎪

⎪

⎪

⎪

∫∫

(4)

where υ is the ice Poisson’s ratio (0.35), c is the “defect” radius, h is the ice accretion height, E is the Young’s modulus of ice (8.5 GN/m2 at temperatures above −20 °C),47 and Pc is the critical air pressure required for ice fracture. Image analysis was performed on the topdown images shown in Figure 3 to determine the area of adhesive fracture. The fracture energy can be then determined in eq 5. In general, coatings with favorably low fracture energies will have majority of the ice removed from the sample surface. 47

E

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

and minimal contact area between the surfaces and liquid, both of which contributes to low ice adhesion strength. Contrary to results from previous reports,20,26,28 no relationship was observed between CAH and ice fracture energy. This was true for all the coatings (control, hydrophobic, and superhydrophobic alike). A figure depicting the absence of this relationship can be found in the Supporting Information. These results (correlation with RCA and lack of with CAH) were consistent with the recent ice adhesion measurements of hydrophobic materials by Meuler et al.38 and most recently by Momen et al.53 on superhydrophobic surfaces. The lack of correlations between ice adhesion strength and superhydrophobic wettability in this study validated the observation of scattered superhydrophobic ice adhesion measurements that were acquired by Sussoff et al.34 and indicated that surface topology effects were more prominent in affecting the bond between ice and the superhydophobic coating than compared to wettability parameters. The presence of surface roughness and texture on a low surface energy material is an important criterion for the Cassie−Baxter wetting state. However, surface roughness has been perceived as detrimental to low ice adhesion as it could create additional surface area for the ice to interlock with. Results obtained from this experiment do not support this hypothesis. For example, as shown in Figure 4b, the superhydrophobic surfaces of Teflon SH and Hydrobead 2 were measured to be of lower ice adhesion strength than SH-d, even though both of these surfaces had Sa values that were larger. This indicated that the Sa parameter was not the primary influence of ice adhesion strength for the superhydrophobic surfaces. The reason is Sa only provides a general quantification of the surface texture and is not sensitive to the detailed changes in the features, e.g., shape of the feature peaks, spacing between features, etc.54 Hence, a further investigation was conducted to examine the relationships between additional superhydrophobic surface topology characteristics and ice adhesion. The autocorrelation length (Sal) is a surface topology parameter that quantifies the spatial relationship between randomly structured surface features. This parameter is quantified by employing the autocorrelation function (G) which is defined as

As indicated in Figures 4a and 4b, the relationship between the coating wettability and ice fracture energy can be separated into two sets: (a) the control and hydrophobic surfaces and (b) the superhydrophobic surfaces. These sets displayed significant differences in surface wettability−ice adhesion correlations. One could observe in Figure 4a that an increasing CA resulted in a reduction in ice facture energy of the control and hydrophobic coatings. This was not the case with the superhydrophobic coatings. Although all of the superhydrophobic coatings had CA values of approximately 160°, significant fluctuation in the ice fracture energies was observed. In fact, the ice fracture energy of the SH-c superhydrophobic coating was measured to be higher than the control surface of Ti 6−4 SP. This was also observed for the dynamic wettability parameter of RCA which is shown in Figure 4b. A strong relationship between RCA and ice fracture energy for control and hydrophobic surface was noted with decreased ice adhesion strength for increased surface RCA’s. However, as shown in Figure 4b, this relationship was not observed for superhydrophobic surfaces. In addition, it should be noted that ice adhesion of the surfaces were generally not affected by Sa. The coatings on control and hydrophobic surfaces were applied on two different substrate materials (aluminum and Ti 6−4) which are of dissimilar thermal conductivities. For example, the thermal conductivity of aluminum at 20 °C is 167 W/(m K) while the thermal conductivity for Ti 6−4 is 6.6 W/(m K) at the same temperature.51,52 This could affect the ice adhesion strength of the coatings due to varying heat transfer rates between the interfaces of supercooled droplets and different substrates. Therefore, coating results shown in Figure 4b were separated into their respective substrate materials by color-coding and plotted in Figure 5. Results showed no

G(τx , τy) =

1 Sq

∬s z(x , y)z(x + τx , y + τy) dx dy

(7)

where τx and τy are the lateral displacements in the x and y directions and Sq is the root-mean-squared (RMS) surface roughness shown in eq 8.

Figure 5. Effect of receding contact angle on ice fracture energy of control and hydrophobic coatings separated by substrate disk material which are color-coded.

Sq =

significant difference in ice adhesion trends between coatings applied on aluminum or Ti 6−4; i.e., decreasing RCA on both substrate materials led to a lower ice adhesion strength, a relationship which was previously observed in Figure 4b. The strong influence of surface wettability on ice adhesion for the set of control/hydrophobic surfaces was expected. The CA describes the coating surface energy while the RCA quantifies the mobility of a drop on the surface when at tilt. Concurrently high CA and RCA values indicate a low attraction

1 A

∫ ∫A z 2(x , y) dx dy

(8)

The autocorrelation function (G) multiplies a surface with another that is separated by τx and τy distances to provide a measure of texture similarity from the original location. If the product of the surfaces is of unity, the textures at the new location is similar to the textures at the original location, vice versa. One may then define an autocorrelation length (Sal) as the minimum lateral distance such that G decays to a value of 0.2. This is shown in eq 9. F

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Sal = min

τx , τy ∈ R

decrease in ice adhesion strength was accompanied by a reduction in averaged Sal of the surfaces (with the exception of an outlier point of SH-a coating). Ice fracture energy data on superhydrophobic coatings (named SH-1, -3, and -8) acquired by Davis et al.44 with a similar ice adhesion measurement technique were also superimposed in Figure 6 and compared with current results. The comparison showed that the trend from previous results also compared favorably with current measurements even though previous measurements were acquired using a “thick” defect (1 mm thick). The correlation between ice adhesion strength and Sal is due to the impingement of supercooled droplets (estimated to be traveling at 5 m/s in the experiment) on the superhydrophobic surface. The impingement imparts a Bernoulli pressure, which is a strong function of droplet liquid velocity, on the surface textures. This droplet pressure could potentially overcome the capillary pressure of the surface textures to infiltrate the air gaps between the textures and fill the asperities. This causes a change in surface wetting state from Cassie to Wenzel. Therefore, the presence of high capillary pressure is desired to resist the infiltration of the droplets. Present results suggest that surface feature spacing controls capillary pressure, a finding which was validated by Extrand, Fortin et al. and Varanasi et al., who demonstrated a significant increase in resistive pressure when surface features were positioned in closer proximities.55−57 In addition, surfaces with features spaced further apart (high Sal) also provide an easier mean for small droplets to physically “fit” itself into the space between the surface features. This explains the strong ice adhesion strength for superhydrophobic surfaces with larger Sal values (features spaced further apart) in Figure 6 as they were unable to withstand droplet penetration. This feature-spacing effect could also be observed with 3D topology and scanning electron microscope (SEM) images of selected superhydrophobic coatings of Hydrobead 2, SH-d, and SH-a shown in Figure 7. The figure shows that the spatial distances and order of the features become further apart and less prominent, respectively, with increasing coating Sal (ranging from 24.5 to 72.3 μm). In addition to autocorrelation length, the relationship between higher order surface topology amplitude parameters

τx 2 + τy 2

where R = {(τx , τy): G(τx , τy) ≤ 0.2}

(9)

The description of Sal in eq 9 provides a statistical quantification of the random texture spacing on a surface. Surface textures that are positioned in close proximity will yield low Sal values whereas textures that are not will result in large Sal values. Figure 6 shows the effect of Sal on the ice fracture energy of the superhydrophobic coatings. Information on the ranges of

Figure 6. Effect of surface autocorrelation length (Sal) on ice fracture energy of superhydrophobic coatings. The averaged arithmetic mean roughness (Sa) of the coatings was color-coded in the legend. Data previously acquired by Davis et al.44 on coatings SH-1, -3, and -8 were superimposed with the current data. Note that the previous data were acquired with the same ice fracture technique used in the current study but with a thicker “defect” (1 mm).

superhydrophobic coating roughness was also provided in the figure by color code. It could be observed that the averaged Sal correlated favorably with the ice fracture energy and that a

Figure 7. 3D topology images of superhydrophobic coatings with their corresponding SEM pictures at different Sal’s (26 to 76 μm). G

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir such as the surface feature skewness (Ssk) and kurtosis (Sku) on superhydrophobic ice adhesion strength were studied. These parameters were previously reported by Kulinich et al.28 to affect the strength of ice adhesion on a superhydrophobic coating. Surface skewness (Ssk) is a nondimensional parameter that describes the distribution symmetry of feature heights. It is mathematically defined in eq 10.54 Sssk =

1 ⎡ 1 ⎢ Sq 3 ⎣ A

⎤

∫ ∫A z3(x , y) dx dy ⎥⎦

(10)

One could use surface skewness measurements to obtain information on the surface feature peaks and valleys, i.e., surfaces which consists of predominantly peaks will yield positive Ssk values while surfaces consisting of primarily valleys will yield negative Ssk values. The Sku parameter is obtained if eq 10 is extended to include the fourth central moment of the feature height (z) distribution.54 This is shown in eq 11. S ku =

1 ⎡ 1 ⎢ Sq 4 ⎣ A

⎤

∫ ∫A z4(x , y) dx dy ⎥⎦

(11)

Surfaces with Sku of greater than 3 will contain inordinately sharp peaks/deep valleys while surfaces with gently varying features will result in Sku values of less than 3. The effect of Ssk and Sku on superhydrophobic ice adhesion strength was studied in Figures 8a,b whereby averaged Ssk and Sku of the coatings were color-coded accordingly on data points for a fracture energy vs Sal graph previously shown in Figure 6. Fracture energy data obtained by Davis et al.44 were also included. It could be shown that coatings with the highest adhesion strength (SH-c and SH-8) exhibited a surface topology composed of predominant valleys (Ssk < 0) which were inordinately deep (Sku > 5). However, no further correlations between Ssk and Sku and superhydrophobic ice fracture energies were observed. This is hypothesized to be due to insufficient range in the Ssk and Sku values. For example, the difference between the maximum and minimum Ssk and Sku values for coatings tested in the current experiment was 0.85 and 1.9, respectively. In comparison, the difference between the maximum and minimum Ssk and Sku values for Kulinich and Farzaneh’s coatings, who observed decreased ice adhesion strength with increased Ssk and Sku, were 6.29 and 17.87.28 This proved that Ssk and Sku values of current coatings did not vary enough to significantly alter the ice adhesion strength. The results from Figures 6 and 8 show that Sal is the primary parameter that controls the strength of superhydrophobic ice adhesion while effects of surface feature heights (Sa, Ssk, and Sku) are secondary. Consider the case of supercooled icing on a superhydrophobic surface with low Sal. Since the surface features of this surface are positioned within close proximities, supercooled droplets are unable to infiltrate the asperities, thus ensuring that the coating remains in Cassie wetting state when ice nucleation occurs. In this scenario, the characteristics of the secondary surface features are not significant as ice nucleates only on the tips of the features. This was true for coatings Teflon SH and Hydrobead 2 in Figure 6. Although the disparity of Sa for these coatings was large (8.35 μm for Teflon SH and 1.87 μm for Hydrobead 2), the ice adhesion strength for both of these coatings was low. The effects of secondary surface features only become more prominent if supercooled droplets penetrate and nucleate within the surface asperities. For example, coatings SH-1, SH-a, and SH-c in Figure 8 were all

Figure 8. Effect of (a) surface skewness and (b) kurtosis on ice fracture energy of superhydrophobic coatings. These two parameters were color-coded on data points on a graph of fracture energy− autocorrelation length graph previously shown in Figure 6. Data previously acquired by Davis et al.44 on coatings SH-1, -3, and -8 were also superimposed with the current data.

likely to be in a Wenzel wetting state during ice accretion due to its high Sal values. However, the ice adhesion strength of SHc coating was measured to be higher than SH-1 and SH-a due to its rougher surface (larger Sa) with predominant and inordinate valleys (larger S ku and negative S sk ). The combination of these secondary characteristics provided a larger surface area for ice to interlock with and as a consequence, resulted in a high ice adhesion strength. SEM and 3D topology images of the SH-c coating depicting the characteristics of these secondary surface features are shown in Figure 9. It should be noted the superhydrophobic ice adhesion is also dependent on coating chemical composition. This could explain the imperfect correlation between ice adhesion strength and Sal previously shown in Figure 6. Further studies would be required to fully understand the effect of coating chemistry on ice adhesion strength. The effect of additional surface topology characteristics such as the Hurst exponent (h*), root-meansquare gradient (Sdq), and maximum height−height correlation length (ξ) on superhydrophobic ice adhesion was also investigated. These parameters were recently reported by Bottiglione et al. to be influential in controlling the superhydrophobicity of a surface.58 With the exception of ξ, which H

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 9. SEM and 3D surface topology pictures of the SH-c superhydrophobic coating. The large spacing of surface features (high Sal) combined with the presence of inordinately deep valleys (as shown in both SEM and surface topology scan) resulted in the penetration of supercooled droplets into the surface asperities with high ice adhesion strength.

■

was found to be of similar value to Sal, in general, no relationships were observed between the parameters of h*, Sdq, and ice fracture energy. Additional details regarding this investigation could be found in the Supporting Information.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02725. Effect of contact angle hysteresis on coating ice adhesion; effect of additional surface topology characteristics on superhydrophobic ice adhesion; Figures 1−4 (PDF)

4. CONCLUSIONS The majority of previous superhydrophobic icing studies have utilized “static” ice accretion methods which involved submersion of superhydrophobic surfaces in water that could lead to ice accretion in the partial wetted/fully wetted states. The present study investigated the ice adhesion on waterrepellent coatings of different wettability levels (hydrophobic and superhydrophobic) and surface topology under atmospheric icing conditions, i.e., supercooled icing cloud of 20 μm MVD droplets in a −20 °C, thermally homogeneous environment. The coatings that were tested include commercial hydrophobic and superhydrophobic coatings as well as inhouse-developed nanocomposite superhydrophobic surfaces. An underside air pressure was supplied to detach the accreted ice from the surface for a Mode-1 fracture and further converted into ice fracture energy. Results showed that ice adhesion strength of hydrophobic and superhydrophobic surfaces correlated differently with surface wettability. While increasing CA and RCA resulted in a reduction of hydrophobic surface ice adhesion, no correlation could be established between ice fracture energy of hydrophobic surfaces and CAH. In addition, ice adhesion on superhydrophobic surfaces was not influenced by any of the wettability parameters (CA, RCA, and CAH). Instead, its ice adhesion strength depended primarily on the surface topology spatial parameter of Sala parameter that quantifies the wavelengths of the surface features. In particular, adhesion strength was found to decrease as Sal was reduced. This trend was attributed to the surface features being spaced at close proximities, which increased the capillary pressure between the surface asperities needed for droplet penetration associated with a Cassie state. By preventing such penetration and subsequent ice-locking during freezing, the ice adhesion strength was reduced. The effects of surface roughness parameters such as Sa, Ssk, and Sku were found to be secondary and would only affect superhydrophobic ice adhesion strength if the supercooled droplets have penetrated the surface asperities.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.H.Y.). *E-mail: [email protected] (E.L.). Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Kinnersley, S.; Roelen, A. The contribution of design to accidents. Saf. Sci. 2007, 45, 31−60. (2) Parent, O.; Ilinca, A. Anti-icing and de-icing techniques for wind turbines: Critical review. Cold Reg. Sci. Technol. 2011, 65, 88−96. (3) Shires, G.; Munns, G. The Icing of Compressor Blades, and Their Protection by Surface Heating; Aeronautical Research Council Technical Report: 1955; Vol. Reports and Memoranda No. 3041. (4) Bascom, W.; Cottington, R.; Singleterry, C. Ice adhesion to hydrophilic and hydrophobic surfaces. J. Adhes. 1969, 1, 246−263. (5) Raraty, L.; Tabor, D. The adhesion and strength properties of ice. Proc. R. Soc. London, Ser. A 1958, 245, 184−201. (6) Jellinek, H. H. G. Adhesive properties of ice. J. Colloid Sci. 1959, 14, 268−280. (7) Subramanyam, S. B.; Rykaczewski, K.; Varanasi, K. K. Ice adhesion on lubricant-impregnated textured surfaces. Langmuir 2013, 29, 13414−13418. (8) Itagaki, K. The Implications of Surface Energy in Ice Adhesion. J. Adhes. 1983, 16, 41−48. (9) Mishchenko, L.; Hatton, B.; Bahadur, V.; Taylor, J. A.; Krupenkin, T.; Aizenberg, J. Design of ice-free nanostructured surfaces based on repulsion of impacting water droplets. ACS Nano 2010, 4, 7699−7707. (10) Cao, L.; Jones, A. K.; Sikka, V. K.; Wu, J.; Gao, D. Anti-icing superhydrophobic coatings. Langmuir 2009, 25, 12444−12448. (11) Farhadi, S.; Farzaneh, M.; Kulinich, S. A. Anti-icing performance of superhydrophobic surfaces. Appl. Surf. Sci. 2011, 257, 6264−6269. (12) Jung, S.; Dorrestijn, M.; Raps, D.; Das, A.; Megaridis, C. M.; Poulikakos, D. Are superhydrophobic surfaces best for icephobicity? Langmuir 2011, 27, 3059−3066.

I

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (13) Wang, F.; Li, C.; Lv, Y.; Lv, F.; Du, Y. Ice accretion on superhydrophobic aluminum surfaces under low-temperature conditions. Cold Reg. Sci. Technol. 2010, 62, 29−33. (14) Tourkine, P.; Le Merrer, M.; Quéré, D. Delayed freezing on water repellent materials. Langmuir 2009, 25, 7214−7216. (15) Meuler, A. J.; McKinley, G. H.; Cohen, R. E. Exploiting topographical texture to impart icephobicity. ACS Nano 2010, 4, 7048−7052. (16) Sarshar, M. A.; Swarctz, C.; Hunter, S.; Simpson, J.; Choi, C. Effects of contact angle hysteresis on ice adhesion and growth on superhydrophobic surfaces under dynamic flow conditions. Colloid Polym. Sci. 2013, 291, 427−435. (17) He, M.; Wang, J. J.; Li, H. L.; Song, Y. L. Super-hydrophobic surfaces to condensed micro-droplets at temperatures below the freezing point retard ice/frost formation. Soft Matter 2011, 7, 3993− 4000. (18) Yang, S. Q.; Xia, Q. A.; Zhu, L.; Xue, J. A.; Wang, Q. J.; Chen, Q. M. Research on the icephobic properties of fluoropolymer-based materials. Appl. Surf. Sci. 2011, 257, 4956−4962. (19) Jung, S.; Tiwari, M. K.; Doan, N. V.; Poulikakos, D. Mechanism of supercooled droplet freezing on surfaces. Nat. Commun. 2012, 3, 615. (20) Kulinich, S. A.; Farhadi, S.; Nose, K.; Du, X. W. Superhydrophobic surfaces: are they really ice-repellent? Langmuir 2011, 27, 25−29. (21) Antonini, C.; Innocenti, M.; Horn, T.; Marengo, M.; Amirfazli, A. Understanding the effect of superhydrophobic coatings on energy reduction in anti-icing systems. Cold Reg. Sci. Technol. 2011, 67, 58− 67. (22) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (23) Jafari, R.; Menini, R.; Farzaneh, M. Superhydrophobic and icephobic surfaces prepared by RF-sputtered polytetrafluoroethylene coatings. Appl. Surf. Sci. 2010, 257, 1540−1543. (24) Ge, L.; Ding, G.; Wang, H.; Yao, J.; Cheng, P.; Wang, Y. Antiicing property of superhydrophobic octadecyltrichlorosilane film and its ice adhesion strength. J. Nanomater. 2013, 2013, 1. (25) Dodiuk, H.; Kenig, S.; Dotan, A. Do Self-cleaning Surfaces Repel Ice? J. Adhes. Sci. Technol. 2012, 26, 701−714. (26) Kulinich, S. A.; Farzaneh, M. On ice-releasing properties of rough hydrophobic coatings. Cold Reg. Sci. Technol. 2011, 65, 60−64. (27) Kulinich, S. A.; Farzaneh, M. Ice adhesion on superhydrophobic surfaces. Appl. Surf. Sci. 2009, 255, 8153−8157. (28) Kulinich, S. A.; Farzaneh, M. How wetting hysteresis influences ice adhesion strength on superhydrophobic surfaces. Langmuir 2009, 25, 8854−8856. (29) Wang, F.; Lv, F.; Liu, Y.; Li, C.; Lv, Y. Ice adhesion on different microstructure superhydrophobic aluminum surfaces. J. Adhes. Sci. Technol. 2013, 27, 58−67. (30) Wang, Y. Y.; Xue, J.; Wang, Q. J.; Chen, Q. M.; Ding, J. F. Verification of icephobic/anti-icing properties of a superhydrophobic surface. ACS Appl. Mater. Interfaces 2013, 5, 3370−3381. (31) Dotan, A.; Dodiuk, H.; Laforte, C.; Kenig, S. The relationship between water wetting and ice adhesion. J. Adhes. Sci. Technol. 2009, 23, 1907−1915. (32) Chen, J.; Liu, J.; He, M.; Li, K.; Cui, D. P.; Zhang, Q. L.; Zeng, X. P.; Zhang, Y. F.; Wang, J. J.; Song, Y. L. Superhydrophobic surfaces cannot reduce ice adhesion. Appl. Phys. Lett. 2012, 101, 111603. (33) Bharathidasan, T.; Kumar, S. V.; Bobji, M.; Chakradhar, R.; Basu, B. J. Effect of wettability and surface roughness on ice-adhesion strength of hydrophilic, hydrophobic and superhydrophobic surfaces. Appl. Surf. Sci. 2014, 314, 241−250. (34) Susoff, M.; Siegmann, K.; Pfaffenroth, C.; Hirayama, M. Evaluation of icephobic coatingsScreening of different coatings and influence of roughness. Appl. Surf. Sci. 2013, 282, 870−879. (35) Ahlborn, G.; Poehlmann, H. Development of a hydrophobic substance to mitigate pavement ice adhesion. Environmental Protection Technology Series, Environmental Protection Technology Series, 1976; EPA-600/2-76-242.

(36) Zou, M.; Beckford, S.; Wei, R.; Ellis, C.; Hatton, G.; Miller, M. Effects of surface roughness and energy on ice adhesion strength. Appl. Surf. Sci. 2011, 257, 3786−3792. (37) Laforte, C.; Laforte, J.-L.; Carrière, J.-C. How a solid coating can reduce the adhesion of ice on a structure; Proceedings of the International Workshop on Atmospheric Icing of Structures (IWAIS); 2002; Vol. 9. (38) Meuler, A. J.; Smith, J. D.; Varanasi, K. K.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Relationships between water wettability and ice adhesion. ACS Appl. Mater. Interfaces 2010, 2, 3100−3110. (39) Lee, C.; Kim, C. Underwater restoration and retention of gases on superhydrophobic surfaces for drag reduction. Phys. Rev. Lett. 2011, 106, 014502. (40) Boinovich, L.; Emelyanenko, A. M.; Pashinin, A. S. Analysis of long-term durability of superhydrophobic properties under continuous contact with water. ACS Appl. Mater. Interfaces 2010, 2, 1754−1758. (41) Namjoshi, S.; Jain, V.; Mall, S. Effects of shot-peening on fretting-fatigue behavior of Ti-6Al-4V. J. Eng. Mater. Technol. 2002, 124, 222−228. (42) Yeong, Y. H.; Davis, A.; Steele, A.; Loth, E.; Bayer, I. S. Spray deposition effects on superhydrophobicity and durability of nanocoatings. Surf. Innovations 2014, 2, 70. (43) Ma, M.; Hill, R. M. Superhydrophobic surfaces. Curr. Opin. Colloid Interface Sci. 2006, 11, 193−202. (44) Davis, A.; Yeong, Y. H.; Steele, A.; Bayer, I. S.; Loth, E. Superhydrophobic Nanocomposite Surface Topography and Ice Adhesion. ACS Appl. Mater. Interfaces 2014, 6 (12), 9272. (45) Milionis, A.; Yeong, Y.; Loth, E.; Bayer, I. S. Mechanically Durable Superhydrophobic Nanocomposites from Acrylonitrile Butadiene Styrene (ABS); Technical Proceedings of the TechConnect World Nanotechnology Conference and Expo: Washington, DC, 2014. (46) Andrews, E.; Stevenson, A. Fracture energy of epoxy resin under plane strain conditions. J. Mater. Sci. 1978, 13, 1680−1688. (47) Andrews, E.; Lockington, N. The cohesive and adhesive strength of ice. J. Mater. Sci. 1983, 18, 1455−1465. (48) Andrews, E.; Majid, H.; Lockington, N. Adhesion of ice to a flexible substrate. J. Mater. Sci. 1984, 19, 73−81. (49) Davis, A.; Yeong, Y. H.; Steele, A.; Loth, E.; Bayer, I. S. Spray impact resistance of a superhydrophobic nanocomposite coating. AIChE J. 2014, 60, 3025−3032. (50) Alansatan, S.; Papadakis, M. SAE Technical Paper, 1999, 199901-1584. (51) Aluminum Association Aluminum standards and data; Aluminum Association, 2000. (52) Welsch, G.; Boyer, R.; Collings, E. Materials Properties Handbook: Titanium Alloys; ASM International, 1993. (53) Momen, G.; Jafari, R.; Farzaneh, M. Ice repellency behaviour of superhydrophobic surfaces: Effects of atmospheric icing conditions and surface roughness. Appl. Surf. Sci. 2015, 349, 211. (54) Gadelmawla, E.; Koura, M.; Maksoud, T.; Elewa, I.; Soliman, H. Roughness parameters. J. Mater. Process. Technol. 2002, 123, 133−145. (55) Extrand, C. Designing for optimum liquid repellency. Langmuir 2006, 22, 1711−1714. (56) Fortin, G.; Perron, J. Ice adhesion models to predict shear stress at shedding. J. Adhes. Sci. Technol. 2012, 26, 523−553. (57) Varanasi, K.; Deng, T.; Hsu, M.; Bhate, N. Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact; Technical Proceedings of the TechConnect World Nanotechnology Conference and Expo: Houston, TX, 2009. (58) Bottiglione, F.; Carbone, G. Role of statistical properties of randomly rough surfaces in controlling superhydrophobicity. Langmuir 2013, 29, 599−609.

J

DOI: 10.1021/acs.langmuir.5b02725 Langmuir XXXX, XXX, XXX−XXX