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Exceptional Superhydrophobicity and Low Velocity Impact Icephobicity of Acetone-Functionalized Carbon Nanotube Films Liqiu Zheng,*,† Zhongrui Li,† Shawn Bourdo,† Khedir R. Khedir,† Madhu P. Asar,† Charles C. Ryerson,‡ and Alexandru S. Biris*,† †
Nanotechnology Center, Applied Science Department, Chemistry Department, University of Arkansas at Little Rock, Arkansas 72204, United States ‡ Terrestrial and Cryospheric Sciences Branch, Cold Regions Research & Engineering Laboratory, Engineer Research and Development Center, U.S. Army Corps of Engineers, Hanover, New Hampshire 03755-1290, United States
bS Supporting Information ABSTRACT: We present a simple method to produce carbon nanotube-based films with exceptional superhydrophobicity and impact icephobicity by depositing acetone-treated single-walled carbon nanotubes on glass substrates. This method is scalable and can be adopted for any substrate, both flexible and rigid. These films have indicated a high contact angle, in the vicinity of 170°, proved both by static and dynamic analysis processes. The dynamic evaporation studies indicated that a droplet deposited on the treated films evaporated in the constant contact angle mode for more than 80% of the total evaporation time, which is definitely a characteristic of superhydrophobic surfaces. Furthermore, the acetone-functionalized films showed a strong ability to mitigate ice accretion from supercooled water droplets (8 °C), when the droplets were found to bounce off the films tilted at 30°. The untreated nanotube films did not indicate similar behavior, and the supercooled water droplets remained attached to the films’ surfaces. Such studies could be the foundation of highly versatile technologies for both water and ice mitigation.
’ INTRODUCTION One of the most economically damaging solid-water interactions at low temperatures is ice formation, which can have an extremely negative impact on aeronautics, energy generation, and power distribution networks.13 As a result, lately there have been reported a number of approaches4 to mitigate ice formation on surfaces that include surface roughness and morphology control in parallel with altering the surface energy.5 The morphologic control down to the nanoscale holds great promise for the development of multifunctional surfaces that have both water and ice phobicity. It has been reported6 that superhydrophobicity is a primary prerequisite of icephobicity and that optimizing the surface conditions of materials in order to reach extremely high contact angles (CA) close to 170° could result in the nonwetting/nonicing of surfaces. Very few studies have presented durable hydrophobic surfaces with contact angles even close to 170° coupled with icephobicity because of two major needs that inevitably arise when designing materials with such requirements: (1) a morphologically rough surface (that has the ability to trap air) and (2) a top surface coated with low surface energy species that are chemically stable. Most of the current strategies for fabricating a textured surface with high roughness characteristics are primarily based on template synthesis,7 lithography,8 or electrohydrodynamics9 and require either very complicated techniques or costly, sophisticated devices. Even so, hydrophobicity can be maintained r 2011 American Chemical Society
with difficulty.10 Second, the need to optimally surface treat a starting material with low surface energy materials in an efficient and scalable manner is still an issue. Therefore, successfully overcoming these limitations has become a major challenge. We have previously presented the development of tungsten nanorods with pyramidal-shaped tips coated with highly hydrophobic polymers (silane and Teflon) for water mitigation.1113 Here, it was clearly shown that the accurate control of the spacing between these metallic nanorods can result in a CA varying from 120° to over 160°, holding great promise for possible icephobic applications. During all of these studies, the surface chemistry remained similar, but the roughness and the solid fraction were modified based on the accurate control of the tungsten nanorods’ growth and morphology. Lately, the development of various methods for carbon nanotube (CNT) synthesis, processing, and deposition has made these unique materials superb 1D structures with major applications in a number of areas that include: energy generation,14,15 conductive coatings,16,17 sensors,18 and bionanomedicine.19 In this work, exceptional surface superhydrophobicity (CA ∼ 169° ( 2°) coupled with resistance to impact ice formation was achieved, with a remarkably straightforward and simple method: Received: April 27, 2011 Revised: July 1, 2011 Published: July 11, 2011 9936
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Langmuir single-walled carbon nanotubes (SWNTs) were treated in acetone (99%) under ultrasonication for about 3 h. The resulting films of these nanotubes were tested for CA and dynamic water impaction studies at both room and in freezing conditions (substrate at 8 °C and the water supercooled at 8 °C). It was found that the superhydrophobic films presented an enhanced impact icephobicity that was not observed for the nanotube films that were not acetone treated (unfunctionalized) or for an aluminum surface, used as a control. A systematic experimental investigation—including scanning electron microscopy (SEM), wettability and icephobicity measurement, Fourier transform infrared (FTIR) spectroscopy, and Raman scattering spectroscopy—was subsequently conducted to validate the effectiveness of this approach. Although this study was intended only as a proof of concept, this approach could be used to create water and icephobic surfaces. Nevertheless, a number of issues remain to be addressed, such as adhesion of the films to the subsurface and their stability in various environments.
’ MATERIALS AND METHODS CNT Films Fabrication and Characterization. For the control film samples, 1 g of slightly oxidized SWNT powder (SWeNT Inc., SG65) was dispersed in 50 mL of dimethylformamide (DMF). The appropriate amount of as-obtained solution was air-brushed onto the top surface of glass slides, which were heated to 150 °C in order to remove the DMF solvent from the coatings to obtain the untreated control samples. For the acetone functionalization, we followed the simple procedure: 0.1 g of SWNT powder was dispersed in 50 mL of acetone (99%) and kept under sonication for about 3 h. The resulting dispersion was then air-brushed onto glass slides and allowed to dry under ambient conditions. The nanotubes formed a network coating onto the top surface of the glass substrate attached mainly through van der Waals forces between the two interfaces.20 The acetone treatment was performed in order to lower the surface energy of the nanotube samples and to surface functionalize them with CHx functional groups, which have one of the lowest surface energy reduction capabilities reported.21 The CH3 groups have been reported to have lower surface energy compared to the CH2 ones.21 Raman scattering studies of the nanotube films were performed using a Horiba Jobin Yvon LabRam HR800 equipped with a charge-coupled detector and a HeNe laser (632.8 nm) as an excitation source. The laser beam intensity measured at the sample was kept at 0.5 mW. Raman shifts were calibrated with a silicon wafer at the peak of 521 cm1. The Fourier transform infrared (FTIR) spectra were collected on a Nicolet MAGNA-IR 550 Series 2 spectrometer with resolution of 8 cm1. The reported spectra were an average of 32 scans. The water contact angles of the nanotube films were measured on a VCA-Optima system. The morphology of the nanotubes was monitored with a JEOL 7000F high-resolution scanning electron microscope. For the dynamic impaction behavior a simple water repellence test was performed. Water droplets were allowed to fall over a 30o tilted substrate under test, as produced by a stainless steel needle fixed at a constant height above the surface. The dropping process was video recorded continuously at high speed. The camera used for high-speed recording was the model HiSpec-1 from Fastech. This camera provides a good compromise between form factor, light sensitivity (monochrome version, ISO = 3200), and high-speed capabilities. Although it is capable of very high speeds at lower resolutions, we chose to use the highest frame rate of 506 frames/s. That still provides the full resolution of 1280 1024 pixels. The lens chosen for this application was the Nikon Macro Zoom Nikkor AF (2485 mm f/2.84.0 D) lens. Icephobicity Analysis. A temperature-controlled environmental chamber was custom-designed for the hydrophobic impaction studies.
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Figure 1. The morphologies of (a) the untreated and (b) the acetonetreated SWNT films as analyzed by SEM. The scale bars are 100 nm in size. The water droplets were delivered from a 5 cm3 syringe positioned above the substrate. A special substrate holder was designed that can be tilted at any angle ranging from 0° and 90° (Figure S1, Supporting Information). The temperatures of both the substrate and the syringe were controlled independently through Peltier cooling modules in order to reach temperatures as low as 10 °C for the water droplets and 20 °C for the substrate. The cooling plates, including the syringe cooler, were acoustically isolated from vibrations of the heat sink cooling fans in order to prevent premature nucleation of the supercooled water. The adjustable impaction height of the droplets was set at 25 mm for these experiments. The chamber volume can be adjusted by adding or removing foam insulation as needed. In these experiments, the empty volume of the chamber was reduced to 5080 cm3 to minimize the cooling load of the ambient air in the chamber. The relative humidity (RH) was reduced to less than 20% with the use of a desiccant placed inside the chamber. It was imperative that the humidity be reduced to prevent frost formation on the substrate.
’ RESULTS AND DISCUSSIONS Superhydrophobic Properties of Acetone-Coated Carbon Nanotube Films. The surface morphology and chemical com-
position essentially govern the wettability of a solid. First, we investigated the morphology of both samples (control and acetone-treated films). The acetone ultrasonication treatment scarcely changed the morphology of the random network of SWNTs, as shown in the SEM images (Figure 1a,b) of the untreated and treated SWNT films. The networks were formed by laying the SWNTs on the substrate with plenty of empty interspaces. Meanwhile, many bundles of SWNTs were assembled in both cases, typically attributed to van der Waals forces.22 The diameters of bundles were found to range from approximately 1 to 70 nm, which provided a large degree of roughness to the films’ top surfaces (see Figure S2 in Support Information). It is worth pointing out that the roughness of the SWNT network is higher by several orders of magnitude than that of graphene films, which simply depend on the interconnection between individual sheets and a few wrinkles at the edge of some flakes.23,24 Although the treated and untreated samples share similar morphologies, water droplets behave very differently on them, suggesting that the chemical composition (controlling the surface energy) of the surfaces is quite different. Superhydrophobicity can be attained only when a textured structure combines with the proper chemical composition on a surface, since increased surface energy would result in a more hydrophilic behavior.25 The wettability properties of the films made from as-produced (acid-treated and therefore slightly oxidized) and acetone-treated 9937
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Figure 2. The water droplet CAs on (a) the untreated and (b) the acetone-treated carbon nanotube films.
Figure 3. Dynamic behavior during the evaporation of water droplet CAs over the acetone-treated carbon nanotube film.
nanotubes were evaluated by water CA measurements. Untreated carbon nanotube films demonstrated a relatively hydrophilic behavior, as seen in Figure 2a. When a water droplet is deposited on the surface, it instantly disappears, leaving the trace of a liquid droplet behind. The acetone-treated SWNT film exhibits superhydrophobic behavior with an extremely high CA of 169° ( 2° (Figure 2b). The water droplet maintains an almost spherical shape due to the strong water repellency of the surface. Ultralow contact-angle hysteresis (defined as the difference between the advancing and receding CAs) was observed, which means that a water droplet would easily bounce off. Further evidence for the low contact angle hysteresis is provided in Figure 3, which shows the dynamic evaporation behavior of the droplets deposited onto the acetone-functionalized nanotube films. Both high static water contact angle and low contact angle hysteresis are the prerequisites for any surface to be identified as water repellent. Therefore, monitoring the water droplet dispensed on the substrate as a function of time, diffusion of water molecules into the ambient environment, and consequent decrease in the size of water droplet would give valuable information regarding the kinetics of interfacial interaction between the solid surface and water droplet. Studies have shown that the kinetics of water droplet evaporation over the solid surface can be described as a function of both the contact angle (CA) and the baseline diameter (BLD).26 The decrease in the size of the water droplet dispensed on a superhydrophobic surface, due to the evaporation process and the consequent variation of both CA and BLD, has shown two important stages of evaporation. The first stage is called the constant baseline diameter mode (CBLD) with simultaneous decrease in the CA as evaporation occurs, as the result of the pinning of the baseline on the patterned surfaces. The second mode is known as the
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constant contact angle (CCA) mode during which the shrinkage in BLD from both sides allows the water droplet to maintain its spherical shape.27 The CA (stable CA) at which the transition from CBLD to the CCA mode occurs is considered the receding CA. A direct relationship has been proposed between initial CA and stable CA in the case of evaporation and advancing and receding CAs in the case of contact angle hysteresis (CAH) measurements. Therefore, the difference between initial and stable CAs is equivalent to the difference between advancing and receding CAs.28 At the very small size of a water droplet before vanishing, both CA and BLD decrease dramatically until the droplet vanishes; this is described as the mixed mode.29 After monitoring the evolution of water droplet evaporation over the treated samples, it was determined that CCA was the dominant mode of evaporation. As shown in Figure 3, the evaporation process started with the CCA mode with a decrease in the BLD accompanying a quasiconstant CA, which is the sign of low CAH, and the subsequent easy motion of water droplet BLD over the superhydrophobic surface of CNTs.30 Moreover, the droplet retained its spherical shape with evaporation evolution to keep the CA unchanged at the expense of a decrease in BLD. The mixed mode occurred after the droplet evaporated in the CCA mode for more than 80% of the total evaporation time. Interestingly, no transition from CCA mode to CBLD mode was observed during the entire evaporation process. Therefore, the analysis of water droplet evaporation and the dynamic BLD motion over the functionalized carbon nanotube surfaces indicated a very low CAH of less than 3°, whereas the evaporation started with CCA mode with both initial and stable CAs of around 169° (the observed CA). As mentioned above, this is equivalent to an advancing CA/receding CA of almost 169°/169°. The high advancing CA of more than 150° and low CAH of less than 10° combined with the bouncing phenomenon of water droplets are intrinsic properties of superhydrophobic surfaces.31 The only regime (nonsticky regime) with these properties is the Cassie regime, in which the water bridges over the surface texture, sitting mostly on air pockets. All of these findings clearly show that the water droplet has adopted the Cassie regime of wetting for the acetone-treated nanotube films. The collapse of the water droplet at the final stage of the evaporation process, as shown in Figure 3, resulted in a dramatic increase in the BLD. This provides additional concrete evidence that the droplet had been in the Cassie regime before switching to the Wenzel regime at the very end of the evaporation process. At the very end of evaporation process and due to the very small size of water droplet comparable to the size of surface features, the transition would occur from Cassie regime to Wenzel regime and consequently wetting the texture by the BLD expansion. These strong hydrophobic properties can be explained by the possible chemical surface alterations of the nanotube films since, as indicated by the microscopy analysis, there were no significant morphological and/or roughness surface changes between the acetone-treated and untreated control nanotube films. As previously shown,24 acetone is strongly chemisorbed onto the surface of the nanotubes via C(acetone)OC(nanotube) bonds and mostly attaches to the surface defects present in the graphitic structure of the nanotubes. To explore the chemical composition change that occurs at the nanotube surfaces after acetone treatment, the samples were scrutinized by Raman-scattering spectra. As shown in Figure 4 (left), both samples demonstrate a peak (G band) at about 1590 cm1, which is the characteristic peak of any graphitic-based structure 9938
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Figure 4. The Raman scattering spectra (left) and the FTIR spectra (right) from the untreated and acetone-treated SWNT samples.
due to the in-plane E2g vibrational mode. The multicomponent spectral feature of the G band is associated with the tangential displacements of the CC bond stretching vibrations.23,32 After the acetone treatment, the intensity of the G band considerably decreased because the intrabundle interactions, determining the relative intensity,33 were drastically decreased by the adsorption of acetone. The D peak positioned at around 1305 cm1 represents the disorder in the untreated nanotubes, which is mostly attributable to structural defects. As reported,34 acetone functionalization causes a large number of (CHx) groups to be adsorbed onto the exposed SWNT surfaces and grooves, as well as possibly accessible interstitial channels of the bundled SWNT system, thus significantly increasing the intensity of the D band (1337 cm 1), as shown in the spectra of the treated SWNT. Notably, the blue shift (32 cm1) of the D band after the treatment further suggests that the acetone treatment induced new types of surface alterations like attachment of CH2/CH3 groups rather than morphological defects. This can be explained because the dangling bonds could be saturated by hydrogen bonds and structural defects diminished by taking in new functional groups.35 These changes of the Raman modes substantiate that chemical modifications of the nanotube surface take place during the acetone treatment of the nanotubes. Therefore, Raman spectra analysis revealed that the surface chemical composition of the treated sample was altered and could be responsible for the observed superhydrophobic behavior of the nanotube films. To further corroborate the different surface chemical composition of samples, FTIR spectroscopy was used to characterize the species introduced onto the nanotubes’ surface due to acetone treatment and functionalization. Sonication-induced changes in temperature and pressure result in chemical interactions of acetone with the SWNTs.36 As seen in Figure 4 (right), after the acetone treatment, the spectrum manifests absorption peaks at 2964 and 2922 cm1 corresponding to unhindered asymmetric stretching vibration modes of CH3 and CH2, respectively. Symmetric stretching vibration modes of CH3 and CH2 are present at 2875 and 2850 cm1. In addition, four more absorption peaks were observed at 1368, 1218, 1081, and 838 cm1, which are twistingrocking and wagging progression CH2 modes, consistent with the results of multiwalled carbon nanotubes treated in acetone.37 All of these (CHx) groups are generated through acetone adsorption and functionalization and can significantly lower the surface energy of the SWNTs films.23 Nevertheless, such absorption peaks are not present in the FTIR
spectra of graphene films treated with acetone,34 which illustrates that the SWNTs can be more efficiently functionalized than flat graphene flakes. The first reason is that the reactivity of carbon materials with curved geometry is primarily driven by the significant strain induced by the pyramidalization given by the curvature of the graphitic surface and the corresponding misalignment in carbon atoms’ π-orbitals. These effects are expected to make carbon nanotubes more reactive than flat graphene sheets.38 Second, the relatively large surface areas of SWNTs are studded with various defects, such as oxygen functionalities, Stone Wales defects, and vacancies that are essential sites for acetone to chemically bind to the nanotube surfaces.3941 Even the bundled structure of SWNTs induces multiple sites for acetone adsorption, such as endohedral sites, interstitial sites, grooves, and external walls.42 Moreover, sonication in organic solvents induces dangling bonds onto the nanotube surfaces that provide additional chemical reactivity.43 As the FTIR spectra indicates, there is an additional CO absorption peak at 1709 cm1 appearing in the treated sample, other than the one at about 1633 cm1 present in both spectra. Therefore, we can deduce that this group comes from the fragments of adsorbed acetone. Basically, there are no CHx peaks in the spectrum of the untreated sample other than two extremely low intensity peaks corresponding to hydrophilic groups CO (1633 cm1) and OH (not shown) that stem from the oxidative purification treatment.44 These hydrophilic groups account for the strong tendency of water to get absorbed on the untreated SWNT film. Accordingly, the FTIR spectra tenably support the fact that the different chemical composition of the films resulted in quite different surface energy values, which induces the different wettability despite their similar morphologies. Furthermore, the highest adsorption energy estimated for acetone on SWNTs is ∼110 kJ mol1, which implies that adsorption is very strong and the chemical modification remains stable until high temperatures.43 The nanotubes’ stable graphitic walls’ acetone modification is also reflected by the lack of aging effects of the corresponding hydrophobic characteristic for the treated samples. After about 6 months, there has been no major degradation, and the CA has not been found to decrease significantly (data not shown here). The hydrophobicity results presented in Figure 2 can be explained by the CassieBaxter model,45 which indicates that surfaces with structured nanomorphologies and a low surface energy are needed for enhanced superhydrophobicity. The effective contact angle (θ*) for a droplet on such a surface is given 9939
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by the following equation: cos θ ¼ fA cos θA þ fB cos θB
ð1Þ
where fA,B are the surface area fractions of components and θA,B are the Young contact angles of the components [cos θ = (γSV γSL)/γLV, in which γSV, γSL, γLV are the interface tension of solid/vapor, solid/liquid, and liquid/vapor, respectively]. If textured structures on a surface can entrap air pockets, the effective contact angle is considerably increased since the contact angle θBof the air component with water is 180°. Then, the CassieBaxter equation takes a simplified form: cos θ ¼ fA ðcos θA þ 1Þ 1
ð2Þ
Thus, the ability to capture air bubbles in its structures is the first optimal condition. The perfect configuration of the random nanotube networks allows the entrapment of air in its interspaces, which exactly meets the first requirement. To further optimize an effective contact angle θ*, a smaller fA and a larger θA of a solid component are required according to the simplified CassieBaxter eq 2. The high aspect ratio of carbon nanotubes in the network drastically decreases the contact area of a droplet with the nanoprotrusions on the surface and results in a nearly spherical droplet. In other words, a smaller area fraction, fA, was obtained, satisfying the second optimal requirement. The third optimal condition is a larger θA. Since θA is governed by its surface energy,25 a larger value can be achieved merely by functionalizing the solid with low surface energy groups. The high reactivity of carbon nanotubes makes it possible to efficiently functionalize them with low surface energy groups. Hence, the significantly low surface energy of the network after coating greatly increases the θA, which fulfills the third optimization condition. Superhydrophobicity can therefore be ensured due to the unique geometry configuration and the chemical composition of the treated sample, which provides a precondition for icephobicity. From an analysis of the micrographs presented in Figure 1, it is possible to estimate the surface area fraction, fA to be used in eq 2. Consequently, from the measured contact angles, θA in eq 2 can also be estimated. The apparent contact angle on the prepared surface can be predicted using the Cassie model. To predict the observed CA (θ*) for the functionalized CNTs with both CH2 and CH3 groups using the Cassie model, the values of both solid fraction fA of CNTs at the interface and intrinsic CA (θA) of the smooth surface covered with both CH2 and CH3 groups are required. Image analysis was used to estimate the solid fraction by considering the white areas as solid areas and black areas as voids.13 The determined fA was found to be around 0.1, and the θA for the smooth surface of CH2 and CH3 groups was estimated to be around ∼100°.46 After substituting these values in the eq 2, the predicted value of θ* was found to be on the order of 157°. The predicted value of θ* is relatively lower than the observed one, which was around 169°. The primary cause could be errors in the estimation of the solid fraction, where the overlap of CNTs caused the image analysis technique to add some white areas of CNTs from the underlayer areas that have no contribution at the interface. Low Velocity Impact Icephobicity of the CNT Surfaces. To examine the icephobic properties, the samples were tested under icing conditions for impact adhesion. The environmental conditions included maintaining both the water droplets and the substrates at below freezing temperatures, in our case 8 °C.
Figure 5. Superimposed water droplets’ interaction with the acetonetreated carbon nanotube films at room temperature (A) and while under icing conditions (B). Icing conditions include maintaining both the substrate and the droplets at 8 °C. The substrates were tilted at 30°. The images were obtained from high-speed-camera frames and by superimposing the same drop at various time frames. The actual highspeed movies used for this analysis are provided in the Supporting Information.
The substrate was tilted at 30°, and single droplets of 10 ( 1 μL sizes were allowed to impact the substrate from a height of 25 mm. The release of the water droplet from this height as a free fall object will produce an impact velocity of about 0.7 m/s. The nondimensional Weber number,47 which is the measure of impact strength (the ratio of the water droplet’s inertia to its surface energy), for the water droplet of 1.3 mm radius is about 8.8. At this relatively small Weber number, the water droplet can retain its integrity after impact without fragmentation. The humidity inside the chamber was kept at a relative humidity of 20%. Figure 5 shows that the superimposed droplet sequences indicate that the droplet bounced off from the acetone-treated carbon nanotube substrate both at room temperature (A) and under icing conditions (B). The change in the trajectory and kinetic energy of the droplet between room temperature and icing conditions could suggest an approach for a possible and very accurate measurement of the adhesion energy at low temperatures for any surface. Further research will include such studies for films of nanotubes treated under various conditions. On the other hand, similar studies performed for untreated nanotube samples (both room temperature and icing conditions) and aluminum substrates (icing conditions) indicate that the droplets remained attached to the substrates and froze shortly after the impact; as a result, they did not bounce off, as shown in Figure 6AC. In this work, the icephobicity of the prepared surfaces was measured only when both the substrate and the water droplet were at room temperature and at 8 °C. More studies regarding the icephobicity of superhydrophobic surfaces with various conditions are still under investigation and yet to be published. In the current paper, we only wanted to present the fact that acetone-treated nanotubes have superhydrophobic and icephobic properties. These studies clearly indicate that superhydrophobicity could be a premise for icephobicity, but more studies need to be performed in order to fully understand the ice adhesion over substrates of various surface energy and roughness values.48,49 Since the samples share similar surface morphology, the conclusion can be drawn that the morphology of surfaces, alone, is of 9940
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Figure 6. Superimposed water droplets’ interaction with the untreated (oxidized) carbon nanotube films at room temperature (A), while under icing conditions (B), and with an aluminum surface used as a control under icing conditions (C). Icing conditions include maintaining both the substrate and the droplets at 8 °C. The substrates were tilted at 30°. The images were obtained from high-speed-camera frames and by superimposing the same drop at various time frames. The actual high-speed movies are provided in the Supporting Information. The scale bars are 3 mm in size.
limited use in ice-prevention technologies. The combination of surface morphology and proper surface chemistry could efficiently prevent droplets from freezing even at supercooled substrate temperatures. It has been proposed that the icesolid surface contact areas are directly associated with the water solid contact areas at the sample interfaces, which, in turn, determines the strength of ice adhesion.50 In essence, the low wetting hysteresis truly reflects the ice repellent property of the surface. The presence of entrapped air underneath a water surface could be responsible for slowing down the droplet solidification.5 Furthermore, if the triple phase line (TPL) is unstable, the water droplet can freeze easily because the heat transfer rate at the TPL region is much higher than for other conditions, and solidification of a droplet is initiated at the TPL instead of the inner part of the droplet. The different TPL of water droplets on the surfaces is responsible for the different wetting properties and even icephobicity. The TPL on a rough surface is metastable, which favors freeze prevention and the solidification of the liquid water at the TPL region, thus inhibiting ice formation. Therefore, the generation of rough surfaces with low surface energies is the most desirable requirement to reduce ice formation and water icing on impact. As a result, superhydrophobic materials can be designed to remain entirely ice free down to 8 °C due to their ability to repel impacting water before ice nucleation occurs. Recent studies have further shown a strong correlation between icephobicity and the wettability of the surfaces.49 The key parameter involved in this relationship is the receding contact angle of the water droplet over the various surfaces. A linear relationship has been reported between ice adhesion strength on superhydrophobic surfaces and their practical work of adhesion. The systematic study of icephobicity in terms of the wetting properties of smooth and textured surfaces (features with various geometries and sizes) has been previously reported by Mishchenko et al.6,50,51 It has been shown that a continuous,
10-min stream of supercooled water droplets was entirely repelled by superhydrophobic surfaces at temperatures ranging between 20 and 25 °C, while the water freezing and ice formation was quite significant over hydrophilic surfaces and relatively lower over hydrophobic surfaces. However, freezing occurred for the supercooled water droplets even on superhydrophobic surfaces when the surface temperature was lowered to less than 25 °C. In another work on the fabrication of fluoropolymer surfaces with various contact angle hysteresis, Kulinich and Farzaneh6 confirmed the correlation between ice adhesion strength and CAH. They also demonstrated that the correlation between ice repellency and CA of the prepared surfaces was only true for surfaces with low CAH. Therefore, superhydrophobic surfaces with low CAH would be the most promising candidates for minimizing or eliminating ice accumulation by causing supercooled water droplets to bounce off before they can freeze on the surface.51 However, there has been some controversy on the limited applicability of superhydrophobic surfaces for icephobic usage in the presence of high humidity and frost.5255 Our results also suggest that the correlation between cooled water droplets/ice repellency and CA of the prepared surfaces is only true for surfaces with low CAH. Therefore, superhydrophobic surfaces with low CAH would be the most promising candidate to minimize or eliminate the ice accumulation by bouncing off the impinged supercooled water droplets before they can freeze on the surface.
’ CONCLUSIONS The functionalization by acetone of single-walled carbon nanotubes with low surface-energy groups and the generation of continuous nanotube films by air-spraying has proven to be a promising method to mass produce long-lasting superhydrophobic coatings (CA close to 170°) at relatively low cost. Reduced 9941
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Langmuir surface contact time and contact area of the impinging water droplets on properly designed surfaces can leave the surface icefree, as the droplets were found to bounce off during ice conditions before ice nucleation could occur. This simple method owes its effectiveness to the unique configuration of the nanotube random networks, as well as the stable and efficient chemical modification of the graphitic nanotubes in the presence of acetone and under sonication. Acetone-treated carbon nanotube films have both the required roughness and low surface area to present water repellency at both room temperature and in icing conditions. Such approaches could become the foundation for ice-fighting technologies with important implications for the economic aspects of various industrial fields, including aeronautics, power generation and distribution, and defense.
’ ASSOCIATED CONTENT
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Supporting Information. Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT This research was partially supported by the DOD (Grant No. W912HZ-09-2-0008). Also the financial support of the Arkansas Science and Technology Authority (ASTA) grant # 08-CAT-03 is highly appreciated. The editorial assistance of Dr. Marinelle Ringer is also acknowledged. We also acknowledge the work done by Mr. Harry L. Maddox in creating the composite drop sequence images along with image enhancements from the individual JPEG frames that were derived from the high-speed videos. ’ REFERENCES (1) Jafari, R.; Menini, R.; Farzaneh, M. Appl. Surf. Sci. 2010, 257, 1540–1543. (2) Menini, R.; Farzaneh, M. Surf. Coat. Technol. 2009, 203, 1941–1946. (3) Ryerson, C. C. Cold Reg. Sci. Technol. 2011, 65, 97–110. (4) Farzaneh, M.; Ryerson, C. C. Cold Reg. Sci. Technol. 2011, 65, 1–4. (5) He, M.; Wang, J.; Li, H.; Jin, X.; Wang, J.; Liu, B.; Song, Y. Soft Matt. 2010, 6, 2396–2399. (6) Kulinich, S. A.; Farzaneh, M. Langmuir 2009, 25, 8854–8856. (7) Zhang, G.; Wang, D.; Gu, Z.; Mchwald, H. Langmuir 2005, 21, 9143–9148. (8) Liu, B.; He, Y.; Fan, Y.; Wang, X. Macromol. Rapid Commun. 2006, 27, 1859–1864. (9) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338–4341. (10) Onda, T.; Shbuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125–2127. (11) Khedir, K. R.; Kannarpady, G. K.; Ishihara, H.; Woo, J.; Ryerson, C.; Biris, A. S. Phys. Lett. A 2010, 374, 4430–4437. (12) Kannarpady, G.; Sharmaa, R.; Liu, B.; Trigwell, S.; Ryerson, C.; Biris, A. S. Appl. Surf. Sci. 2010, 256, 1679–1682. (13) Khedir, K. R.; Kannarpady, G. K.; Ishihara, H.; Woo, J.; Ryerson, C.; Biris, A. S. Langmuir 2011, 27, 4661–4668. (14) Wang, S.; Khafizov, M.; Tu, X.; Zheng, M.; Krauss, T. D. Nano Lett. 2010, 10, 2381–2386.
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