Anti-icing Potential of Superhydrophobic Ti6Al4V Surfaces: Ice

Sep 14, 2015 - *Telephone: +86-25-52112911. ... the solid–liquid interface nucleation rate, owing to the extremely low actual solid–liquid contact...
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Anti-icing Potential of Superhydrophobic Ti6Al4V Surfaces: Ice Nucleation and Growth Yizhou Shen, Jie Tao,* Haijun Tao, Shanlong Chen, Lei Pan, and Tao Wang College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu 210016, People’s Republic of China ABSTRACT: On the basis of the icing-delay performance and ice adhesion strength, the anti-icing potential of the superhydrophobic surface has been well-investigated in the past few years. The present work mainly emphasized the investigations of ice nucleation and growth to fully explore the anti-icing potential of the superhydrophobic surface. We took the various surfaces ranging from hydrophilic to superhydrophobic as the research objects and, combining the classical nucleation theory, discussed the ice nucleation behaviors of the water droplets on these sample surfaces under the condition of supercooling. Meanwhile, the macroscopical growth processes of ice on these surfaces were analyzed on the basis of the growth mechanism of the ice nucleus. It was found that the superhydrophobic surface could greatly reduce the solid−liquid interface nucleation rate, owing to the extremely low actual solid−liquid contact area caused by the composite micro−nanoscale hierarchical structures trapping air pockets, leading to the bulk nucleation dominating the entire ice nucleation at the lower temperatures. Furthermore, ice on the superhydrophobic surface possessed a lower macroscopical growth velocity as a result of the less ice nucleation rate and the insulating action of the trapped air pockets. texture and surface free energy.19,20 Smooth substrates with the appropriate surface chemical components (self-assembled monolayers of fluoroalkylsilanes and polytetrafluoroethylene) have been known to display an intrinsic hydrophobicity with the contact angle only reaching 120°. Therefore, the surface texture is crucial to superhydrophobicity, and the ideal surface microstructure is widely considered to be a class of hierarchical textures containing the microscale (dozens of micrometers) and nanoscale (hundreds of nanometers) structures.21,22 Under the action of the hierarchical textures, a large amount of air pockets can be trapped between the water droplet and the surface (i.e., Cassie wetting model), resulting in the water droplet almost being in a suspending state. On the basis of this wetting regime of water droplets on the superhydrophobic surfaces, workers variously characterize, analyze, and improve the anti-icing properties of superhydrophobic surfaces, because they believe that the trapped air pockets can effectively form a thermal barrier to hinder the heat transfer during icing and have the capacity to reduce the ice adhesion strength, leading to the ice on the superhydrophobic surfaces easily slipping off.23,24 Jiang and his coworkers designed a micro−nanostructured superhydrophobic surface using poly(vinylidene difluoride) (PVDF) polymer in combination with ZnO materials via heat-pattern-transfer and crystal-growth techniques and demonstrated the superhydrophobic surface displaying an excellent icing-delay property

1. INTRODUCTION Icing on exposed surfaces has exhibited a rising issue to our everyday lives through disturbing aircraft flying, downing power lines, freezing roads, obstructing telecommunication, affecting household refrigerators, etc., resulting in an enormous economic loss in every year and life safety impact.1−7 Over the past few decades, many efforts have been made to hinder ice formation and accretion and develop the two main antiicing/deicing strategies, i.e. active and passive anti-icing/deicing techniques.8,9 Currently, the active anti-icing/deicing approaches, such as scattering ice-melting agent, mechanical vibration, and thermally melting ice, have been widely applied in the aircraft field, power transmission, and daily life based on the purposes of melting or breaking ice.10 However, these methods are considered to be obsolescent as a result of their unacceptable design complexity, weight, and vast energy cost. Oppositely, with the advance of nano and bionic techniques, anti-icing superhydrophobic coatings (as a passive anti-icing technique) have been proposed and investigated for the ideal application conditions without any other energy consumption.11−13 To replace the traditional active strategy well, the superhydrophobic coatings and their anti-icing/deicing performance are widely investigated and also have become one of the most significant research issues in recent years.14,15 Bio-inspired superhydrophobic surfaces have aroused intense attention of researchers owing to their remarkable waterrepellency and potential application values.16−18 Generally, the superhydrophobicity refers to the higher contact angle (greater than 150°) and lower contact angle hysteresis (less than 10°), and it can only be achieved through a cooperation of surface © 2015 American Chemical Society

Received: August 9, 2015 Revised: September 14, 2015 Published: September 14, 2015 10799

DOI: 10.1021/acs.langmuir.5b02946 Langmuir 2015, 31, 10799−10806

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Langmuir (icing-delay time up to 7360 s at −10 °C).25 Also, reports from other authors showed that ice adhesion strength on the micro− nanoscale hierarchical structured superhydrophobic surfaces was significantly lowered, precisely because of water droplets freezing in the Cassie wetting state and forming a “Cassie ice” state, i.e., the air pockets trapped underneath the water droplets being maintained after freezing.26−29 In our opinions, the antiicing property and application potential of superhydrophobic surfaces have been well-researched around the two aspects of icing-delay performance and ice adhesion strength. However, we still cannot thoroughly grasp what the connection between superhydrophobicity and anti-icing is and how the surface micro−nanoscale structures can affect the anti-icing properties via the trapped air pockets, to date. As widely known, freezing is also a nucleation and growth process with the change of thermal energy. However, few studies focus on the investigation of ice nucleation itself on the superhydrophobic surface, which is a pressing need to provide an underlying understanding in the development of passive anti-icing coatings.30,31 We believe that ascertaining the action mechanism of the trapped air pockets on the ice nucleation and growth has a greater significance than those studies on measuring the icing-delay performance and ice adhesion strength. Herein, the present work mainly emphasized the investigations of ice nucleation and growth based on our previously reported superhydrophobic surfaces for further exploring their anti-icing potential. We took the various hydrophobic surfaces as the research objects and theoretically and experimentally investigated the ice nucleation mechanism and growth process. Also, the role played by the trapped air pockets underneath the water droplet during the ice nucleation process was discussed in detail, expecting to produce an echo to those measured icedelay performances.

apparent contact angle (APCA) and the contact angle hysteresis (CAH) were measured via a contact angle analyzer (Kruss DSA100, Germany). The average value of three measurements was determined. 2.3. Calculation of the Actual Water−Solid Contact Area. With regard to the calculation of the actual water−solid contact area, the nominal contact area between the water droplet and solid surface with a radius r can be calculated by34

s = πr 2

(1)

⎡ ⎤1/3 3V r=⎢ ⎥ sin θ* ⎣ π(2 − 3 cos θ* + cos3 θ*) ⎦

(2)

where V is the volume of the reference water droplet and θ* is the APCA of the water droplet on the sample surface. If the water droplet on a rough hydrophobic surface follows the Wenzel wetting model

cos θ* = r* cos θ

(3)

where r* is the roughness factor, i.e., the ratio of the actual contact area and the nominal contact area. θ is the Young contact angle of the water droplet on the smooth surface. In this case, the actual contact area can be easily calculated. When the wetting state is consistent with the Cassie wetting model, the actual contact area can be achieved via the equation cos θ* = r*f cos θ + f − 1

(4)

where f is the fraction of the projected area of the solid surface that is wet by the liquid and r* is the roughness factor of the wetting area.35 However, in this wetting case, the contact interface mainly consists of the solid−liquid and solid−air interfaces. Also, the fraction ( f) of the projected area of the solid surface that is wet by the liquid is very small (approximately 10%); therefore, the wetting interface can be considered to follow Young’s equation, and r* is considered to be approximately equal to 1. Lastly, the actual water−solid contact area can be easily calculated. 2.4. Measurement of the Icing-Delay Time before Ice Nucleation. The icing-delay time before ice nucleation was measured via a self-made experimental apparatus, as shown in Figure 1. It mainly

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Ti6Al4V titanium alloy [composition (wt %): ≤0.3% Fe, ≤0.1% C, ≤0.05% N, ≤0.015% H, ≤0.2% O, 5.5− 6.8% Al, and 3.5−4.5% V, with the rest being Ti] was used as the substrate material. The Ti6Al4V substrate was first cut into a square sample (10 × 10 × 1 mm) and sequentially cleaned by acetone, alcohol, and distilled water. Subsequently, on the basis of the design of this work, we prepared five classes of sample surfaces on the substrate, labeled as Surf 1, smooth substrate surface without any further processing; Surf 2, smooth substrate surface fluorated by heptadecafluorodecyl trimethoxysilane (FAS-17, purchased from Tokyo Chemical Industry Co., Ltd., Japan); Surf 3, microscale (constructed by sandblasting) structured surface with the fluoridation of FAS-17; Surf 4, nanoscale (the planted nanowires via a hydrothermal technique) structured surface with the fluoridation of FAS-17; and Surf 5, composite micro−nanoscale hierarchical structured (constructed by a cooperation of sandblasting and the hydrothermal technique) surface with the fluoridation of FAS-17. The detailed operational procedures are described in our previous paper.32 2.2. Characterizations. Field emission scanning electron microscopy (FE-SEM; Hitachi S4800, Japan) was used to observe the morphologies of these surfaces. The surface roughness was measured via a roughmeter (Mitutoyo SJ-210, Japan). With regard to the characterization of the hydrophobicity, a 4 μL water droplet was used as the reference droplet to avoid gravity deformation. According the reported result by Dorrer and Rühe,33 the reference droplet should meet the demand that the diameter of the water droplet should be less than the capillary length of 2.7 mm (for water). Furthermore, to ensure that the 4 μL water droplet successfully dripped on the sample surfaces, we chose the ultrafine syringe needle with the inner diameter of only 0.03 mm, which was also hydrophobized with FAS-17. The

Figure 1. Schematic representation of the self-made experimental apparatus for testing the icing-delay time before ice nucleation. includes the three units of the infrared (IR) thermodetector, microinjector, and cooling plate. All of these units are enclosed in a transparent plastic chamber, and the inner humidity is controlled below 5% by adding enough desiccants to avoid frosting. An external cooling system is used to independently control the temperature of the cooling plate. Before measuring, the cooling plate would reduce the temperature of the measured sample over it to a set value. Subsequently, the microinjector squeezed out a small water droplet (room temperature) on the sample surface. The temperature of the water droplet was recorded in real time by the IR thermodetector and transmitted to a connected computer. Thus, we could achieve the icing-delay time before ice nucleation because of the temperature of 10800

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Langmuir water droplet exhibiting a sudden fluctuation during ice nucleation. Meanwhile, we used a high-speed camera (Photron Mini 100) to film the growth process of ice on the sample surface.

the actual water−solid contact area being up to 7.136 mm2. After fluorination with FAS-17, the resultant surface (Surf 2) is hydrophobic with the APCA of 116°, which is attributed to the self-assembled monolayer reducing the surface free energy and compelling the water droplet to bulge to achieve the lowest interfacial energy with the smallest contact interface.36−38 With the introduction of microscale structures, the hydrophobicity (Surf 3) is further enhanced and the APCA reaches 135°. This result is expected because the microscale (∼30 μm) structures geometrically lead to the increase of APCA, which follows the Wenzel wetting model.39,40 However, such a wetting model also brings some disadvantages with the CAH increasing from 34° to 40° as a result of the completely impregnated interface.41 On the contrary, with the addition of nanowires (Surf 4) or the composite micro−nanoscale structures (Surf 5), the resultant surfaces both reach criterion of superhydrophobicity with the APCA being up to 153° and 161° and CAH reducing to 7° and 2°, respectively, which both adapt to the Cassie wetting model.42−45 The difference of the superhydrophobicity between Surf 4 and Surf 5 is mainly due to the composite micro−nanoscale structures trapping more air pockets underneath the water droplet than single-scale nanowire structures, which can be further evidenced by the actual water−solid contact area. 3.2. Icing-Delay Performance of the Superhydrophobic Surface. The anti-icing potential of the superhydrophobic surface will be discussed around the two aspects of ice nucleation and growth. For the rational logicality of data, the icing-delay time before ice nucleation on these above surfaces at −10, −20, and −30 °C is also detected and shown in Figure 2. The transient temperature curves well reflect the heat change during the ice nucleation process (a raised place), consequently achieving the icing-delay time.46 When the sample surfaces remain at −10 °C, the detected temperature−time curves show some obvious difference between each other. The water droplet on the Surf 1 cools rapidly, and ice nucleation has been triggered before the droplets cool to −10 °C. On the Surf 2 and Surf 3, although the ice nucleation is slightly delayed with respect to the Surf 1, it is still initiated during the cooling stage. Markedly, there are different phenomena observed on the Surf 4 and Surf 5, with the water droplets spending a prolonged period of time (approximately 620 and 750 s for Surf 4 and Surf 5, respectively) to trigger the ice nucleation. This result is mainly attributed to the textures on the superhydrophobic surfaces being resistant to contact with ice and water with the reduced water−solid contact area caused by the trapped air pockets.47 The small water−solid contact area also produces a very low solid−liquid nucleation rate during the frozen stage, which is further evidenced in the Ice Nucleation Discussion section. Also, the trapped air pockets hinder the heat transfer and lead to a certain extent of delay during the cooling stage to −10 °C.48 In comparison to Sur 4 (single-scale nanowire structures), Surf 5 shows a greater icing-delay performance as a

3. RESULTS AND DISCUSSION 3.1. Morphologies and Non-wettability. On the basis of the principle of fabricating a superhydrophobic surface, five classes of sample surfaces with various wettabilities are manufactured and shown in Table 1. We can find that the Table 1. Morphologies and Roughness of the Sample Surfaces and the Static Contact Optical Photographs of the Water Droplet on the Surfaces

selected surfaces exhibit a greater dimension span from ∼30 μm (Surf 3) to hundreds of nanometers (Surf 4) and evenly distribute on the substrate surface. The composite micro− nanoscale structured surface (Surf 5) retains the microscale structure morphology well and simultaneously introduces the nanoscale structure features, resulting in the roughness (1.352 μm) being located between those of the microscale (2.287 μm) and nanoscale (0.271 μm) structured surfaces. In combination of the fluorination of FAS-17, these surfaces display different hydrophobicities, causing the water droplet on the surfaces to show a hemispheric (Surf 2 and Surf 3) or perfect spherical (Surf 4 and Surf 5) shape. Table 2 illustrates the measured results of the hydrophobicity of these surfaces. Corresponding to the above analysis, the APCAs and CAHs of the water droplets on the surfaces also appear with a similar change. As a smooth substrate without fluorination, Surf 1 shows some extent of hydrophilicity with the APCA of 56° and the CAH of as high as 61°, resulting in

Table 2. Measured Results of the Hydrophobicity of These Surfaces sample ID Surf Surf Surf Surf Surf

1 2 3 4 5

description

APCA (deg)

CAH (deg)

wetting state

actual water−solid contact area (mm2)

smooth substrate without fluorination smooth substrate with fluorination micropattern structures with fluorination nanowire structures with fluorination micropattern−nanowire structures with fluorination

56 116 135 153 161

61 34 40 7 2

Wenzel Wenzel Wenzel Cassie Cassie

7.136 2.687 2.232 0.510 0.025

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Figure 2. Icing-delay time before ice nucleation on the sample surfaces at (a) −10 °C, (b) −20 °C, and (c) −30 °C.

result of the composite micro−nanoscale hierarchical structures trapping more air pockets underneath the water droplet and further reducing the actual water−solid contact area, resulting in the textures being more resistant to contact with ice and water. According to the above analysis, the actual water−solid contact area between the water droplet and the composite micro−nanoscale structured surface (Surf 5) is only 0.025 mm2, greatly reducing the solid−liquid nucleation rate during the ice nucleation stage. Thus, the superhydrophobic surface displays a robust icingdelay performance with the delay time reaching approximately 750 s at −10 °C. Although the icing-delay performance reduces with the decrease of the sample surface temperature, in comparison to the smooth substrate sample (Surf 1), superhydrophobic surfaces (Surf 4 and Surf 5) still require a long time to trigger the ice nucleation at −20 and −30 °C. The similar results on the icing-delay performance of the superhydrophobic surface have been widely reported previously and mainly attributed to the wetting regime with the composite contact interface of liquid/gas and liquid/solid.49,50 3.3. Ice Nucleation Discussion. To further explore the anti-icing performance of the superhydrophobic surface, the ice nucleation behaviors of the water droplets on these sample surfaces were investigated via employing the classical nucleation theory. In this work, we describe the nucleation rate (in units of nuclei s−1 in a certain volume) of the water droplet on the cooled surface, building upon the framework developed by Alizadeh et al. and Zobrist et al.46,51

R(T ) = R bulk(T )V + R lg(T )S lg + R sl(T )Ssl

(5)

where R(T), Rbulk(T), Rlg(T), and Rsl(T) are the corresponding temperature-dependent total, bulk, liquid−gas, and solid−liquid interface nucleation rates, respectively. V, Slg, and Ssl represent the volume of the liquid, the liquid−gas contact area, and the solid−liquid contact area, respectively. Also, Rbulk(T) can be expressed by R bulk(T ) = n

⎡ ΔQ (T ) ⎤ ⎡ ΔG*(T ) ⎤ kT exp⎢ − ⎥exp⎢ − ⎥ ⎣ h kT ⎦ ⎣ kT ⎦

(6)

−23

where k and h are the Boltzmann (1.38 × 10 J/K) and Planck (6.626 × 10−34 J s) constants, T is the absolute temperature, and n is the volume number density (3.1 × 1028 m−3) of water molecules. The Gibbs free energy ΔG*(T) and diffusion activation energy ΔQ(T) are provided by ΔG*(T ) =

ΔQ (T ) =

16πγ 3Tm 2 3ΔHm,v 2(Tm − T )2 kT 2E (T − T0)2

(7)

(8)

where E and T0 are the fit parameters and the values of T0 = 118 K and E = 892 K have been determined via the experimental results by Kay and Smith.52 We assumed that the nucleation occurring at the interface of liquid/solid is only dominated by the heterogeneous nucleation. Rsl(T) can be given by 10802

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Figure 3. Bulk (black symbols), liquid/gas interface (red symbols), solid/liquid interface (blue symbols), and total (pink symbols) nucleation rates as a function of the temperature for (a) Surf 1, (b) Surf 2, (c) Surf 3, (d) Surf 4, and (e) Surf 5, respectively.

R sl(T ) = n

⎡ ΔQ (T ) ⎤ ⎡ ΔG*(T )f (θ ) ⎤ kT exp⎢ − ⎥ ⎥exp⎢ − ⎣ ⎦ h kT ⎦ ⎣ kT

nucleation. As a consequence, Rlg(T) approximately equals Rbulk(T), except for the value (×1019 m−2) of n. Consequently, combining our experimental results of the 4 μL water droplet on the above sample surfaces, we give the ice nucleation as a function of the temperature, as illustrated in Figure 3. We can obviously find that the ice nucleation of the water droplet on the smooth substrate (Surf 1) is almost completely dominated by the solid/liquid interface nucleation from the observation (complete superposition of blue and pink symbols) of Figure 3a. This furthermore demonstrates that the

(9)

In this case, n (×1019 m−2) is the number density of water molecules at the interface and f(θ) is the wetting parameter, determined by the chemical component, ranging from 0 (complete heterogeneous nucleation) to 1 (homogeneous nucleation). For the nucleation occurring at the interface of liquid/gas, we assumed that it was the homogeneous 10803

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Figure 4. (a) Total nucleation rate and (b) solid−liquid nucleation rate of the water droplet on the five sample surfaces as a function of the temperature.

with the actual solid−liquid contact area. Thus, we conclude that, in comparison to other sample surfaces, the superhydrophobic surface can greatly reduce the solid−liquid interface nucleation rate, owing to the extremely low actual solid−liquid contact area caused by the trapped air pockets, leading to the bulk nucleation dominating the entire ice nucleation at lower temperatures. 3.4. Growth Process. As we know, ice nucleation occurs, inevitably accompanied by growth at the same time. We macroscopicly observed the growth process of ice in the water droplet, as shown in Figure 5. It can be found that ice grows

hydrophilic smooth substrate surface greatly decreases the energy barrier required for the ice nucleation, resulting in the water droplet freezing rapidly. After fluoration of the smooth substrate surface, the contributions of the solid/liquid interface and bulk to the total nucleation rate of the water droplet on the Surf 2 are equally important (see Figure 3b). Moreover, which nucleation is dominated depends upon the supercooling temperature. At a lower temperature, the bulk nucleation plays a significant role compared to the solid−liquid interface nucleation, which is due to the high nucleation rate of bulk nucleation being easily initiated with the high energy introduction at the lower temperature. As seen from panels c−e of Figure 3, the black symbols representing bulk nucleation and the pink symbols representing total nucleation are almost completely superposed, indicating the ice nucleation on Surf 3, Surf 4, and Surf 5 being controlled by the bulk nucleation. Furthermore, the roles played by the solid−liquid interface nucleation gradually weaken, owing to the reduction of the actual water−solid contact area from Surf 3 to Surf 5. As expected, the microstructures on superhydrophobic surfaces (Surf 4 and Surf 5) trap a large amount of air pockets underneath the water droplet, leading to the contribution of solid−liquid interface nucleation to total nucleation being negligible. However, at the mild temperature, the total nucleation rate can be dramatically reduced because of the bulk nucleation being not easily triggered with a low energy supply. To compare conveniently, we take the total nucleation rate and the solid−liquid nucleation rate ofthe water droplet on the five sample surfaces as a function of the temperature, as shown in Figure 4. In comparison to the smooth substrate surface (Surf 1), the total nucleation rates of the water droplet on the other sample surfaces (i.e., Surf 2, Surf 3, Surf 4, and Surf 5) have a remarkable reduction, owing to the layer molecular membrane of low surface free energy and the constructed microstructures increasing the energy barrier of ice nucleation. However, some differences ranging from Surf 2 to Surf 5 can also be seen in Figure 4a, which shows the total nucleation rate slightly decreasing. This is mainly attributed to the highest bulk nucleation rate (approximately 4 orders of magnitude) being far greater than the other nucleation rates. From Figure 4b, we clearly observe that the solid−liquid nucleation rate gradually decreases ranging from Surf 1 to Surf 5, being well consistent

Figure 5. (a) Macroscopical growth process of the ice in the water droplet at −10 °C (superhydrophobic surface) and (b) corresponding schematic diagram.

from the bottom to the top after a cooling period of time. In comparison to the cooling time, the time required for the ice growth is significantly short (orders of magnitude), which is also consistent with the reported results.50 However, it is still necessary to investigate this growth process to understand fully the anti-icing potential of the superhydrophobic surface. Essentially, this growth process refers to the ice nucleus− water interface gradually moving to water, i.e., the water molecules continually accumulating at the interface under the supercooling action.53−55 Thus, under the same supercooling, 10804

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the composite micro−nanoscale hierarchical structures trapping air pockets, leading to the bulk nucleation dominating the entire ice nucleation at lower temperatures. Furthermore, ice on the superhydrophobic surface possessed a lower macroscopical growth velocity as a result of the less ice nucleation rate and the insulating action of the trapped air pockets.

the main factors influencing the ice growth are the number of the ice nucleus (i.e., the ice nucleation rate) and the ability of the water molecules to move to the ice nucleus−water interface. Figure 6 shows the measured macroscopical growth velocity of the ice on the five sample surfaces at different temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-25-52112911. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Science Foundation of China (51202112 and 51475231), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Jiangsu Innovation Program for Graduate Education (KYLX_0261), the Open Fund of Jiangsu Key Laboratory of Materials and Technology for Energy Conversion (MTEC2015M04), and the Fundamental Research Funds for the Central Universities (NJ20150027).

Figure 6. Relationship between the macroscopical growth velocity of the ice in the water droplet and temperature.

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REFERENCES

(1) Cao, L.; Jones, A. K.; Sikka, V. K.; Wu, J.; Gao, D. Anti-icing Superhydrophobic Coatings. Langmuir 2009, 25, 12444−12448. (2) Dou, R.; Chen, J.; Zhang, Y.; Wang, X.; Cui, D.; Song, Y.; Jiang, L.; Wang, J. Anti-icing Coating with an Aqueous Lubricating Layer. ACS Appl. Mater. Interfaces 2014, 6, 6998−7003. (3) Wang, Y.; Xue, J.; Wang, Q.; Chen, Q.; Ding, J. Verification of Icephobic/Anti-icing Properties of a Superhydrophobic Surface. ACS Appl. Mater. Interfaces 2013, 5, 3370−3381. (4) Boinovich, L. B.; Emelyanenko, A. M. Anti-icing Potential of Superhydrophobic Coatings. Mendeleev Commun. 2013, 23, 3−10. (5) Shen, Y.; Tao, H.; Chen, S.; Zhu, L.; Wang, T.; Tao, J. Icephobic/ Anti-icing Potential of Superhydrophobic Ti6Al4V Surfaces with Hierarchical Textures. RSC Adv. 2015, 5, 1666−1672. (6) Peng, C.; Xing, S.; Yuan, Z.; Xiao, J.; Wang, C.; Zeng, J. Preparation and Anti-icing of Superhydrophobic PVDF Coating on a Wind Turbine Blade. Appl. Surf. Sci. 2012, 259, 764−768. (7) Xiao, J.; Chaudhuri, S. Design of Anti-Icing Coatings Using Supercooled Droplets as Nano-to-Microscale Probes. Langmuir 2012, 28, 4434−4446. (8) Parent, O.; Ilinca, A. Anti-icing and De-icing Techniques for Wind Turbines: Critical Review. Cold Reg. Sci. Technol. 2011, 65, 88− 96. (9) Chernyy, S.; Järn, M.; Shimizu, K.; Swerin, A.; Pedersen, S. U.; Daasbjerg, K.; Makkonen, L.; Claesson, P.; Iruthayaraj, J. Superhydrophilic Polyelectrolyte Brush Layers with Imparted Anti-Icing Properties: Effect of Counter ions. ACS Appl. Mater. Interfaces 2014, 6, 6487−6496. (10) Roseen, R. M.; Ballestero, T. P.; Houle, K. M.; Heath, D.; Houle, J. J. Assessment of Winter Maintenance of Porous Asphalt and Its Function for Chloride Source Control. J. Transp. Eng. 2014, 140, 04013007. (11) Schutzius, T. M.; Jung, S.; Maitra, T.; Eberle, P.; Antonini, C.; Stamatopoulos, C.; Poulikakos, D. Physics of Icing and Rational Design of Surfaces with Extraordinary Icephobicity. Langmuir 2015, 31, 4807−4821. (12) Maitra, T.; Tiwari, M. K.; Antonini, C.; Schoch, P.; Jung, S.; Eberle, P.; Poulikakos, D. On the Nanoengineering of Superhydrophobic and Impalement Resistant Surface Textures below the Freezing Temperature. Nano Lett. 2014, 14, 172−182. (13) Kim, Y.; Lee, S.; Cho, H.; Park, B.; Kim, D.; Hwang, W. Robust Superhydrophilic/Hydrophobic Surface Based on Self-Aggregated

The reduction of the temperature inevitably results in the growth velocity of the ice increasing, yet the growth velocities of the ice on the superhydrophobic surfaces (Surf 4 and Surf 5) change only slightly in comparison to the other sample surfaces, which is attributed to the slowly increasing ice nucleation rate on the superhydrophobic surfaces. Also, regardless of the temperature, the macroscopical growth velocity of the ice on the Surf 5 is the lowest among those on the five sample surfaces. According to the above analysis, except the influence of the least ice nucleation rate on the Surf 5, the ability of the water molecules to move to the ice nucleus−water interface also plays an important role in this growth process. Among the five sample surfaces, the micro−nanoscale hierarchical structures on Surf 5 trap a large amount of air pockets between the surface and the supercooled water droplet, showing an insulating action. During the ice nucleation and growth stages, the continuous supercooling is necessary to promote the growth of the ice. However, the insulating action of the trapped air pockets retards the generation of the supercooling, increasing the difficulty of the water molecules to move to the ice nucleus−water interface. Thus, ice on the micro−nanoscale hierarchical structured superhydrophobic surface (Surf 5) possesses the lowest macroscopical growth velocity among the five sample surfaces.

4. CONCLUSION We took the various surfaces ranging from hydrophilic to superhydrophobic as the research objects and discussed the cooling, ice nucleation, and growth processes of water droplets on these surfaces. The micro−nanoscale hierarchical structured superhydrophobic surface displayed a robust icing-delay performance with the delay time reaching approximately 750 s at −10 °C. Also, in combination with the classical nucleation theory, it was found that the superhydrophobic surface could greatly reduce the solid−liquid interface nucleation rate, owing to the extremely low actual solid−liquid contact area caused by 10805

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Langmuir

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DOI: 10.1021/acs.langmuir.5b02946 Langmuir 2015, 31, 10799−10806