Physicochemical Characteristics and Droplet Impact Dynamics of

May 27, 2014 - overall behavior is observed when a free-falling water droplet impinges SH-CNT arrays. Upon impact, the droplet deforms and spreads on ...
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Physicochemical Characteristics and Droplet Impact Dynamics of Superhydrophobic Carbon Nanotube Arrays Adrianus I. Aria and Morteza Gharib* Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: The physicochemical and droplet impact dynamics of superhydrophobic carbon nanotube arrays are investigated. These superhydrophobic arrays are fabricated simply by exposing the as-grown carbon nanotube arrays to a vacuum annealing treatment at a moderate temperature. This treatment, which allows a significant removal of oxygen adsorbates, leads to a dramatic change in wettability of the arrays, from mildly hydrophobic to superhydrophobic. Such change in wettability is also accompanied by a substantial change in surface charge and electrochemical properties. Here, the droplet impact dynamics are characterized in terms of critical Weber number, coefficient of restitution, spreading factor, and contact time. Based on these characteristics, it is found that superhydrophobic carbon nanotube arrays are among the best water-repellent surfaces ever reported. The results presented herein may pave a way for the utilization of superhydrophobic carbon nanotube arrays in numerous industrial and practical applications, including inkjet printing, direct injection engines, steam turbines, and microelectronic fabrication.

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INTRODUCTION It has been widely known that superhydrophobicity of a surface can be explained by three different states, i.e., the Cassie state, the Wenzel state, and the transition between these two states.1,2 For all states, the static contact angle for water is very high, typically higher than 150°.2,3 In the case of a Cassie state, there exist a liquid−vapor−solid interface between water and the surface such that only a tiny fraction of water is in a direct contact with the surface asperities. Generally, Cassie state superhydrophobicity can only exist if the surface is hierarchically structured or has a nanometer-scale surface roughness. Such a special type of surface topography is critical to ensure minimal contact between water and the surface.3,4 Typically, synthetic Cassie state superhydrophobic materials are fabricated using various techniques of micro- or nanolithography to create micro- or nanoscale surface patterns. These patterns have to be carefully designed to achieve an ideal superhydrophobicity. Unlike other types of synthetic Cassie state superhydrophobic materials, vertically aligned carbon nanotube (CNT) arrays have an inherent submicrometer surface topography (Figure 1a). Thus, they do not have to be patterned using lithography techniques to satisfy the surface roughness criteria of the Cassie state superhydrophobicity. The absence of prepatterning necessities makes CNT arrays as one of the most promising synthetic materials for numerous advanced practical applications. Note that CNTs in general are widely known as wonder materials due to their exceptional electrical, thermal, and mechanical properties. The presence of naturally occurring CNT entanglements at the top surface of the CNT arrays provides an additional height variation at nanometer length © 2014 American Chemical Society

Figure 1. Low magnification (a) and high magnification (b) scanning electron microscopy images of AG-CNT arrays used in this study. The vertical alignment of the nanotubes and the presence of some entanglements at the top of the array can be clearly seen from these images.

scale (Figure 1b). From surface roughness point of view alone, CNT arrays should definitely exhibit the Cassie state superhydrophobicity. Received: October 3, 2013 Revised: May 25, 2014 Published: May 27, 2014 6780

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However, our previous study has shown that the presence of oxygen adsorbates prevents as-grown CNT (AG-CNT) arrays to exhibit Cassie state superhydrophobicity.5 In fact, numerous previous works to make CNT arrays superhydrophobic involve the use of surface coating by nonwetting chemicals, such as polytetrafluoroethylene (PTFE), ZnO, and fluoroalkylsilane, and surface passivation by CF4, CH4, and NF3 plasma treatment.6−10 We for the first time reported that superhydrophobic CNT (SH-CNT) arrays can be easily fabricated by exposing the AG-CNT arrays to a vacuum annealing treatment.5 A vacuum annealing treatment performed at a moderate temperature is sufficient to remove a large amount of the adsorbed oxygen, which ultimately renders the AG-CNT arrays superhydrophobic. The resulting SH-CNT arrays exhibit the Cassie state superhydrophobicity. Numerous previous studies have reported that upon impact on a nonwettable surface a water droplet flattens and spreads on the surface and subsequently rebounds off of the surface.11−18 Depending on the type of the superhydrophobic surface, surface transition from Cassie state to Wenzel state may occur during the impact. The use of a nonideal superhydrophobic surface often results in the pinning of water droplet on the surface under a certain condition due to the transition from the Cassie state to Wenzel state.12,13,19,20 Such a transition can be avoided whenever an ideal Cassie state superhydrophobic surface is used. While the use of thermodynamically modified surfaces, e.g., superheated metal plates that exhibit the Leidenfrost effect, is not practical in many applications, they are a good example of ideal Cassie state superhydrophobic surfaces.11,21,22 While the droplet impact dynamics on various superhydrophobic surfaces has been intensively investigated in the past few decades, such behavior on SH-CNT arrays is yet to be characterized. The aim of this work is to provide an insight into the characteristics and performance of SH-CNT arrays that are relevant in many industrial and practical applications. Here, the physicochemical characteristics of SH-CNT arrays are examined and compared with those of AG-CNT arrays. These characteristics are obtained by performing analyses using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, zeta potential, and electrochemical impedance spectroscopy (EIS). The impact dynamics of impinging water droplet on Cassie state superhydrophobic SH-CNT arrays is also fully characterized and compared with those on other types of superhydrophobic surfaces. The droplet impact dynamics are presented in terms of critical Weber number (We1 and We2), coefficient of restitution (ε), spreading factor (β), and contact time (τC). This work also attempts to elucidate lingering questions related to the spreading behavior and loss of energy of water droplet during the impact. Because of the minimal interaction between water droplet and superhydrophobic surface, it was originally hypothesized that the impact behavior would satisfy the energy conservation principle.11 Considered to be too simplistic, this hypothesis was subsequently modified to take into account the effect of complex internal flows induced by the impingement.13,17,23 The result presented herein shows that such modification in the hypothesis is, in fact, unnecessary. As originally hypothesized, a simple energy conservation approach is found to be adequate to predict the droplet impact behavior.14−16,20,24−26

Article

EXPERIMENTAL METHODS

CNT Growth and Vacuum Annealing Treatment. The AG arrays used in this study were grown on catalysts coated silicon substrates using thermal chemical vapor deposition. The growth itself was performed in a 1 in. diameter quartz tube furnace (Lindberg/ BlueM single-zone tube furnace) under the 490 sccm ethylene gas (Matheson, 99.999%) and 210 sccm hydrogen gas (Airgas, 99.999%) at a temperature of 750 °C and a pressure of 600 Torr. This CVD technique resulted in densely packed multiwalled carbon nanotube arrays, with a typical nanotube diameter of 16−25 nm and an internanotube spacing of 50−80 nm. The overall growth quality, including the length and packing density of the array, was characterized under a field emission scanning electron microscope (ZEISS LEO 1550VP). Vacuum annealing treatments employed to fabricate SH arrays from AG arrays were conducted in a vacuum oven (VWR Signature Vacuum Oven) at a reduced pressure of 2.5 Torr and an elevated temperature of 250 °C for at least 3 h.27 Water Contact Angle. All contact angle measurements are conducted using a custom-made contact angle goniometer at standard room pressure and temperature. Samples of both AG-CNT and SHCNT arrays are placed on the contact angle goniometer sample table. Care must be taken to make sure that the sample table is perfectly level and not tilted toward one direction. A 5 μL deionized water droplet was dispensed slowly on the top surface of each CNT array using a 5 μL syringe (Hamilton 7105KH) equipped with a 31 gauge flat-tipped needle (Hamilton KF731). During this process, images of water droplet were taken for the advancing contact angle measurement. Once a water droplet has come to rest on the surface of the CNT array and the equilibrium condition was achieved, images of the water droplet were then taken for the static contact angle measurement. The water droplet was then redrawn slowly from the CNT array using the same syringe and needle. During this process, images of water droplet were taken for the receding contact angle measurement.The contact angles are then measured by processing the captured images with LBADSA software.28 The contact angle hysteresis was measured by subtracting the receding contact angle from the advancing contact angle. XPS, Raman Spectroscopy, Zeta Potential, and EIS. The surface chemistry characteristics of AG and SH arrays were assessed using X-ray photoelectron spectroscopy (Surface Science M-Probe XPS). A monochromatic 1486.6 eV Al Kα was used as the X-ray source. High-resolution scans were collected in C 1s region with a resolution of 0.065 eV and a spot size of 100 μm. All photoelectron spectra were obtained using ESCA25 Capture software (Service Physics, V5.01.04), and analyzed using CasaXPS software (Casa Software Ltd., V2.3.15). Deconvolution of the C 1s high-resolution spectra was performed using a Gaussian−Lorentzian peak shape fitting with Shirley baseline correction. The growth quality and defects of AG and SH arrays were characterized using micro-Raman spectroscopy (Renishaw M1000) equipped with an Ar ion laser. All AG and SH arrays were characterized without prior sample preparation. Raman spectroscopy characterizations were performed at an optical magnification of 50× or 100× in a nonpolarized mode at an excitation wavelength of 514.5 nm. The surface charge properties of AG and SH arrays were measured by means of their zeta potential using laser Doppler microelectrophoresis (Malvern Zetasizer Nano ZS). A HeNe laser operated at a wavelength of 632.7 nm was used as the light source. Prior to the zeta potential measurement, a small portion of each sample was scrapped from the growth substrate and ultrasonically dispersed in 1 mM KCl aqueous solution (Sigma-Aldrich) for 5 min at full power (Hielscher UP50H). The pH of the solution was adjusted by manual titration using 0.1 M HCl and 0.1 M KOH solutions (Sigma-Aldrich). The isoelectric point of CNT samples is defined as the pH value at which that particular type of CNT has a zero charge or in this case is indicated by the zero value of the zeta potential. The electrochemical characteristics of AG and SH arrays were assessed by electrochemical impedance spectroscopy using a potentiostat (Biologic SP-200). The electrochemical impedance was 6781

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measured at a frequency range of 10 mHz−7 MHz using threeelectrode configuration, where the AG or SH array was set as the working electrode, a platinum mesh (Sigma-Aldrich, 52 mesh, 99.9%) was used as the counter electrode, and a saturated calomel electrode (Biologic RE-2B) was used as the reference electrode. Droplet Impact Dynamics. All droplets were generated by a flattipped stainless steel needle connected to a syringe pump (Harvard Apparatus PHD 22/2000). The flat-tipped needle used here was varied from 24 to 33 gauge, depending on the volume of the droplet. Each droplet was formed using the pendant drop method where water with a specific volume was slowly pushed by the syringe pump through the needle until it detached under its own weight. The evolution of the droplet before, during, and after the impingement was captured using a backlight technique, where the CNT sample and the impacting droplet were placed in between a high-speed camera (Dantec Dynamics NanoSense MkIII) running at a frame rate of 2000 fps and a white LED (Bridgelux BXRA-C800) illumination source. A smooth PET light diffuser was placed in front of the LED to distribute the light evenly. All measurements regarding droplet dimensions were done manually using an open-source image processing software (NIH ImageJ). To minimize variability of the experimental results, the entire experiment was performed using deionized (DI) water with a resistivity of 18.2 MΩ·cm and a neutral pH in a controlled environmental condition at an ambient temperature and humidity of 20 °C and 50%, respectively. Since the influence of external air pressure to the impact behavior was considered insignificant,29 the ambient pressure was left uncontrolled at a standard atmospheric pressure. The interference from the ambient environment was further minimized by placing the needle, from which the droplet fell freely, inside transparent acrylic windscreens. The drop impact velocity itself was controlled by varying the distance between the tip of the needle and the surface (Figure S1 in the Supporting Information). The relation between the impact velocity and the fall height is described elsewhere.13,17

obviously seen on their surface whenever these SH-CNT arrays are submerged in water. In a shallow pool of water, the waterfree surface is deformed near the top surface of the SH-CNT array. Thus, walls of water are formed around it, leaving the SH-CNT array completely dry (Figure 2a). This phenomenon

Figure 2. (a) Top view of AG-CNT array (§) and SH array (‡) submerged in a shallow pool of DI water. The deformation of waterfree surface is indicated by the red arrow. (b) Top view of AG-CNT array (§) and SH-CNT array (‡) submerged in a deep pool of DI water. The SH-CNT array looks reflective because of the presence of a continuous film of trapped air at the interface.

infers that water is being repelled strongly by the surface of SHCNT arrays. In a deep pool of water, the SH-CNT array exhibits a silvery mirror-like surface, indicating the presence of liquid−vapor−solid interface between water and its top surface. A bright region on the surface of the SH-CNT array corresponds to a strong light reflection due to the presence of a thin air cushion trapped between water and the surface (Figure 2b).34,35 Judging from the size of the reflective area, it can be confirmed that the entire surface of the SH-CNT array is covered by a continuous layer of air. This trapped air cushion keeps the entire SH-CNT array completely dry, preventing a direct contact with water to be made anywhere on its surface. In fact, this air film is the signature interface of an ideal Cassie state superhydrophobic surface. It has previously shown on CNT nanocomposite film using confocal laser scanning microscopy technique that the thickness of such air cushion is in the order of ten micrometers.34 The presence of such thick layer of air cushion is imperative to prevent water penetration in order to keep the CNT arrays superhydrophobic. Indeed, this air film appears to be very stable throughout the length of the study and all SH-CNT arrays remain superhydrophobic for at least 2 weeks (Figure S2b,c in the Supporting Information). As a comparison, the widely used silica nanoparticles coating loses its superhydrophobicity in just 7 days of ambient air exposure or water immersion.32,36 SH-CNT arrays offer many advantages compared to other types of superhydrophobic materials. In terms of superhydrophobic robustness, SH-CNT arrays are among the best water-repellent surfaces ever produced. The robustness of a superhydrophobic surface can be approximated by a dimensionless robustness parameter (H*), which is given by H* = 2[(1 − cos θ)R + H]lcap/D2.37 R and H are the radius of individual CNT and the height of the CNT arrays, respectively, and D is the intertube spacing of the CNT arrays. θ and lcap are

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RESULTS AND DISCUSSION Physicochemical Characteristics. As mentioned earlier, the SH-CNT arrays used in this study are fabricated by chemically modifying the AG-CNT arrays using a vacuum annealing treatment. The AG-CNT arrays used in this study themselves are grown using a common thermal CVD method on catalyst coated silicon substrates. Note that no catalyst patterning process is performed prior to the growth. In order to ensure a successful superhydrophobic conversion using vacuum annealing treatment, the AG-CNT arrays have to be grown uniformly across their substrates without any large scale surface defects and irregularities. Oxygen adsorption may occur physically or chemically on the AG-CNT arrays during this growth process and the subsequent storage period. A superhydrophobic conversion is then performed by removing the adsorbed oxygen from the AG-CNT arrays using a vacuum annealing treatment. A vacuum annealing treatment performed at a moderate temperature of 250 °C for 4 h or more is sufficient to remove the majority of oxygen adsorbates from the AG-CNT arrays.5 After losing most of their oxygen content, the resulting SH-CNT arrays exhibit a very high static contact angle of about 171°, which is a significant increase from about 143° for the original AG-CNT arrays. In addition, they exhibit a very low contact angle hysteresis and a roll-off angle of less than 3° and 4°, respectively.5 These contact angle hysteresis and roll-off angle are comparable to those of the current state of the art superhydrophobic materials (Table S1 in the Supporting Information).30−33 Further, a liquid−vapor−solid interface, which is the main characteristics of a Cassie state superhydrophobicity, can be 6782

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Figure 3. (a) Time-lapse images of a water droplet sliding on a U-shaped SH-CNT array, which is fabricated by anchoring the SH-CNT array on a flexible PDMS substrate. (b) Plot of the position of the water droplet against time as it slides back and forth from one end of the array to the other.

200−400 °C, above which they become brittle and inflexible, a high annealing temperature has to be avoided. Thus, a moderate annealing temperature allows the anchoring polymer substrates to remain flexible, while the resulting SH-CNT arrays become superhydrophobic. Such ability can be observed from experiments using water droplet on a U-shaped SH-CNT array anchored on a flexible PDMS substrate. Here, the PDMS substrate retains its flexibility and does not break even at a radius of curvature of 10 mm. The water droplet itself slides and rolls from one end of the array to the other in such a fashion that is expected from a water droplet on a superhydrophobic surface (Figure 3a). Notice that although the friction between water and the SH-CNT arrays is minimal, it still cannot be completely neglected. After a few times sliding and rolling from one end of the array to the other, the water droplet loses its kinetic energy and drops off of the SH-CNT array (Figure 3b). A XPS analysis was conducted to confirm the removal of oxygen adsorbates from the AG-CNT arrays by the vacuum annealing treatment. Deconvolution of the C 1s XPS spectra of both AG-CNT and SH-CNT arrays shows seven distinct peaks associated with C−C sp2 (284.5 ± 0.1 eV, fwhm 0.9 eV), C−C sp3 (285.5 ± 0.1 eV, fwhm 1.3 eV), C−O (286.5 ± 0.1eV, fwhm 1.4 eV), CO (287.4 ± 0.2 eV, fwhm 1.4 eV), O−C O (289.1 ± 0.4 eV, fwhm 1.7 eV), O−C(O)O (290.3 ± 0.4 eV, fwhm 1.7 eV), and π−π* (292.1 ± 0.4 eV, fwhm 1.6 eV) bonds (Figure 4a).42,43 The C−C sp2 peak is the signature of the graphitic structure of CNT, and the C−C sp3 peak is typically associated with the presence of defects and functional groups. The C−O peak is typically associated with the presence of hydroxyl and epoxide groups on the CNT outer wall and cyclic ether at the edges of CNT defect sites. The CO peak indicates the presence of quinone and carbonyl groups at the edges of CNT defect sites. The O−CO and O−C(O)−O peaks indicate the presence of carboxyl and carbonate terminating groups, respectively. The π−π* peak is associated

the material contact angle and capillary length of water, respectively. It is found that the H* of 100 μm tall SH-CNT arrays is about 7.5 × 107, which is about 10 times, 100 times, and 100000 times higher than that of silicon nanoparticles, silicon nanopillars, and lotus leaf, respectively (Table S1 in the Supporting Information).32,37,38 In terms of fabrication process, the technique to produce SHCNT arrays is relatively cheap and simple due to the lack of surface patterning requirement. As a comparison, the intricate structures of superhydrophobic silicon nano- and micropillars can only be made using complicated and expensive microfabrication techniques, including photolithography patterning and chemical etching (Table S1 in the Supporting Information).38,39 Although this fabrication technique may not be as cheap and simple as dip-coating and sol−gel techniques,33,39,40 it allows the critical physical parameters of the surface to be decoupled and varied independently to achieve a higher value of H*.37 For instance, the height of the arrays, the diameter of the individual nanotube, and the internanotube spacing can be potentially tuned to further improve the stability and robustness of the liquid−vapor−solid interface of SH-CNT arrays. Furthermore, the fact that SH-CNT arrays can be anchored onto flexible polymeric substrates without losing their superhydrophobic capability allows them to be used in many applications with arbitrary geometries and shapes. The method to anchor SH-CNT arrays onto polymer substrates has been discussed in detail elsewhere.41 The anchoring process itself can take place either prior or subsequent to the vacuum annealing process. However, to ensure a successful superhydrophobic conversion, the anchoring process is usually performed prior to the annealing process. The moderate annealing temperature does play a crucial role in maintaining the flexibility and structural integrity of the polymer substrates. Since commonly used flexible polymers, e.g. PDMS or PI, always have a transition temperature of about 6783

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Figure 4. (a) High-resolution XPS spectra of C 1s peak of AG-CNT and SH-CNT arrays. Deconvolution of these spectra shows six components associated with C−C sp2, C−C sp3, C−O, CO, O−CO, O−C(O)−O, and π−π* bonds. (b) Raman spectra of AG-CNT and SH-CNT arrays, showing three distinct peaks associated with D, G, and G′ bands. (c) Zeta potential of AG-CNT and SH-CNT arrays as a function of pH of the dispersant. (d) Nyquist plot of AG-CNT and SH-CNT arrays at low-frequency range and high-frequency range (inset) in 1 M NaCl. (e) Double layer capacitance and charge transfer resistance of AG-CNT and SH-CNT arrays in in 1 M NaCl.

yet to be performed. In the case of oxygen reabsorption, the SH-CNT arrays are expected to lose their superhydrophobicity, making re-exposure to vacuum annealing treatment necessary to keep them superhydrophobic over an extended period of time. The oxygen desorption process can also be observed from the changes in Raman spectra before and after the vacuum annealing treatment. In general, Raman spectra of both AGCNT and SH-CNT arrays show three distinct peaks known as the D band (∼1350 cm−1), G band (∼1585 cm−1), and G′ band (∼2695 cm−1). The G band is the Raman scattering due to strain in the π-bond network of CNT. The D band is the Raman scattering associated with disorder and non-sp2 bonds of CNT. The G′ band is the resonance of the G band.44 The graphitic quality of the CNT arrays is commonly quantified by the ratio between the integrated area under the D band and the

with the graphite stacking between layers of the CNT wall and between neighboring CNT. Notice that the intensity of C−O, O−CO, and O−C(O)− O peaks of SH−CNT arrays are much lower than those of AGCNT arrays (Figure 4a). Elemental analysis obtained from survey-scan XPS spectra also shows that the oxygen-to-carbon ratio of SH-CNT arrays is about 4%, which is half of that of AG-CNT arrays (Figure S3a in the Supporting Information). These findings, along with the obvious difference in the profile of O 1s XPS spectra (Figure S3b in the Supporting Information), confirm that SH-CNT arrays are basically AGCNT arrays that have lost most of their oxygen content, and a vacuum annealing performed at a moderate temperature is enough to induce oxygen desorption from the AG-CNT arrays. A more detailed study to determine whether or not oxygen reabsorption occurs in a prolonged storage and usage period is 6784

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Figure 5. Time-lapse images of a water droplet impinging the surface of SH-CNT array at (a) We = 9.1, (b) We = 57.5, and (c) We = 151.8. Secondary droplets formed by the Worthington jet and prompt splash are indicated by the red arrows in (b) and (c).

G band (ID/IG). Since the value of ID/IG is highly influenced by the impurities concentration and defect density, it is actually expected that the ID/IG of AG-CNT arrays is higher than that of SH-CNT arrays. Notice that AG-CNT arrays have a ID/IG of about 0.84, while SH-CNT arrays have a ID/IG of about 0.64 (Figure 4b). The dispersions of AG-CNT and SH-CNT arrays in water with various pH were also investigated to further verify the difference in their wettability. Just like any other hydrophobic substances, the mildly hydrophobic AG-CNTs cannot be easily dispersed in water.43,45 At a neutral pH, the AG-CNT dispersion is unstable and they tend to form coagulations. The highly hydrophobic SH-CNTs are even more difficult to be dispersed in water. At a neutral pH, the SH-CNTs are always found to float on the surface of water, even after being subjected to ultrasonication.5 This unstable dispersion is reflected from their zeta potential measurement. At neutral pH, the SH-CNTs are almost neutrally charged, while the AG-CNTs are more negatively charged (Figure 4c). Such a difference in surface charge can be attributed to difference in concentration of carboxyl groups, where the concentration of carboxyl groups on AG-CNTs is higher than that on SH-CNTs. The plot of zeta potential as a function of pH of the solution also shows that the isoelectric point of AG-CNTs can be found at a pH of 3.8, while that of SH-CNTs is found at a pH of 5.7 (Figure 4c). It has widely accepted that a stable dispersion can only be achieved by a sample that has an absolute value of zeta potential larger than 30 mV.46,47 This suggests that a stable dispersion of AG-CNTs can be achieved at a pH higher than 8, while the SH-CNTs will always coagulate even in the extreme basic condition. The electrochemical characterization was conducted to provide additional insight on the difference between SHCNT arrays and AG-CNT arrays. It has been known that the impedance of CNT arrays in aqueous solution is dictated by their wettability.48 The Nyquist impedance plot of the electrochemical impedance spectroscopy (EIS) data in 1 M NaCl aqueous electrolyte shows that SH−CNT arrays yield a higher absolute impedance than AG-CNT arrays at the same frequency (Figure 4d). Indeed, at a low frequency of 12.6 mHz,

the absolute impedance of SH-CNT arrays is about 4 times higher than that of AG-CNT arrays. In addition, the transition frequency, at which the capacitive-resistive behavior is transformed into a pure resistive behavior, of SH-CNT arrays is found to be higher than that of AG-CNT arrays. The Nyquist plot in the high frequency regime shows that such transition occurs at a frequency of 108.6 kHz for SH-CNT arrays and 27 kHz for AG-CNT arrays. This finding suggests that in the low frequency regime the electrochemical behavior of AG-CNT arrays in 1 M NaCl is dominated by double layer capacitance, while that of SH-CNT arrays is more dominated by charge transfer resistance (Figure 4e). Indeed, the double layer capacitance of AG-CNT arrays is found to be around 1.4 × 10−4 F, which is about an order of magnitude higher than that of SH-CNT arrays that is found to be around 2.5 × 10−5 F. In contrast, the charge transfer resistance of AG-CNT arrays is measured to be around 6.6 × 104 Ω, which is about half of that of SH-CNT arrays that is measured to be around 1.7 × 105 Ω. Such behavior is actually expected because of the presence of a thin film of air at the interface between SH-CNT array and the aqueous electrolyte. This air film suppresses the electrons and proton transfer between the array and the electrolyte, while simultaneously reduces the effective contact area between the array and the electrolyte, which leads to a much smaller effective area of Helmholtz layer. This results in the dramatic increase of charge transfer resistance and significant decrease of double layer capacitance.48 Thus, it can be implied that the thickness of the air film correlates negatively to the charge transfer conductance and double layer capacitance. Although it is yet to be investigated in more detail, an exact correlation between these parameters can be potentially used to measure the thickness of the air cushion. As mentioned earlier, the presence of such air cushion can be easily recognized by the silvery appearance of SH-CNT arrays in 1 M NaCl aqueous electrolyte. Droplet Impact Dynamics. In general, when a free-falling water droplet hits a superhydrophobic surface, it deforms and spreads on the surface until it reaches a maximum diameter. Right after the droplet reaches its maximum spread diameter, it 6785

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Figure 6. (a) Probability of a water droplet to create a Worthington jet or a prompt splash upon impact. Blue and red lines are a curve fit of the experimental data using a logistic function. Transition from a full rebound to a rebound with a Worthington jet occurs at We1 = 11 ± 3, while transition from a rebound with a Worthington jet to a splash occurs at We2 = 106 ± 17. Log−log plots of (b) ε, (c) β, and (d) τC as a function of We of the droplet. Solid and dashed lines are the curve fit of the experimental data obtained in this current study and previously published study, respectively. In (b), (c), and (d), open and closed markers indicate experimental data obtained from superhydrophobic microstructured surfaces and superhydrophobic Leidenfrost surfaces, respectively. For clarity, error bars are omitted. Plots that include error bars can be seen in Figure S5 of the Supporting Information.

the first critical Weber number (We1) and is observed experimentally at We1 = 11 ± 3. This implies that at We < 8 the bouncing droplet stays intact during the entire rebound phase, while at We > 14 secondary droplets are always formed at the beginning of the rebound phase (Figure 6a). This observed We1 is comparable to that observed on other superhydrophobic surfaces, with We1 ≈ 15 and We1 ≈ 24 for fluorocarbon-coated and CNT-coated micropatterned silicon surfaces, respectively.49,52 Here, the intensity of the Worthington jet is found to be increasing with the increase of We. Since Worthington jet induces the formation of secondary droplets, the number of secondary droplets formed during the rebound phase is also found to be increasing with the increase of We. Note that secondary droplets formed by a Worthington jet are significantly larger than those formed by the aforementioned microjet. In addition, a more intense Worthington jet yields a higher rebound velocity of the secondary droplets. In general, the rebound velocity of the secondary droplets is higher than that of the primary rebounding droplet. In fact, it is often observed that one of these secondary droplets has a rebound velocity as high as the initial drop impact velocity. In a higher We regime, the droplet spreads on the surface of the SH-CNT array with a higher radial momentum. Such increase of radial momentum results not only in a dramatic increase of the maximum spread diameter but also in the formation of many secondary droplets. Subsequently, the

quickly recedes and finally rebounds off of the surface. Similar overall behavior is observed when a free-falling water droplet impinges SH-CNT arrays. Upon impact, the droplet deforms and spreads on the surface of the SH-CNT array. Depending on the We, there are at least three outcomes after the droplet reaches its maximum spread diameter. In the low We regime, the droplet recedes right after the maximum spread diameter is reached and rebounds off of the SH-CNT array surface without being pinned onto the surface (Figure 5a). In this regime, a trapped air bubble is often observed during the droplet retraction phase. Differences in receding velocity between the top part and the bottom part of the droplet have been considered as the main cause of such air bubble trapping.49 This trapped air bubble is then either squeezed out or burst before the droplet bounces off of the surface. The disappearance of this air bubble is often, although not always, followed by the formation of a microjet that is ejected from the primary droplet at a very high velocity. Qualitatively, this finding is in a good agreement with the behavior observed on other types of natural and synthetic superhydrophobic materials.17,29,50−52 In the moderate We regime, the droplet recedes right after the maximum spread diameter is reached and rebounds off of the SH-CNT array surface without being pinned onto the surface. In this regime, a Worthington jet can be observed at the beginning of the rebound phase (Figure 5b). The transition in behavior from normal bouncing to rebound with Worthington jet on the SH-CNT array surface is denoted as 6786

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ε = vr/vi = (hr/hi)1/2, where hi is the initial drop height of the droplet and hr is the maximum rebound height of the primary rebounding droplet. Under normal circumstances, all impinging droplets have ε < 1, which means that they lose a fraction of their energy upon impact due to deformation and viscous damping. During the impact, a fraction of the droplet kinetic and surface energy is transformed into internal motions and vibrations of the droplet.13 This internal motions and vibrations are then dissipated by the viscosity of the droplet. Although in this study the effect of viscosity is small compared to that of inertia, it is significant enough to dissipate the energy of the droplet. As the droplet deformation becomes more pronounced with the increase of We, the amount of energy dissipated from the droplet becomes increasingly higher. Therefore, a water droplet that impinges a surface at a higher We is expected to have a smaller ε. As expected, experimental data confirm that ε is close to unity at a very low We and starts to decrease with the increase of We in the higher We regime. Indeed, the average coefficient of restitution decreases from ε = 0.99 to ε = 0.35 with the increase of Weber number from We = 1.79 to We = 86.5 (Figure 6b). For We > 86.5, ε decreases at an even higher rate, where the average coefficient of restitution decreases from ε = 0.31 to ε = 0.14 with the increase of Weber number from We = 96.6 to We = 328. A curve fit of the experimental data using a power law yields the relation

remaining primary droplet rebounds off of the SH-CNT array surface without being pinned onto the surface (Figure 5c). The transition where the droplet starts to splash upon impact on the SH-CNT array surface is denoted as the second critical Weber number (We2) and is observed experimentally at We2 = 106 ± 17. This implies that at We > 123 prompt splash phenomena always occur, and secondary droplets are always formed right after the maximum spread diameter is reached (Figure 6a). This observed We2 is comparable to that observed on other superhydrophobic surfaces, with We2 ≈ 160 and We2 ≈ 74 for fluorocarbon-coated and CNT-coated micropatterned silicon surfaces, respectively.49,53 It is important to note that no droplet pinning on the surface of SH-CNT array is ever observed during the experiment at a very wide range of We. Further, all SH-CNT arrays do not exhibit any changes in wettability after the droplet impingement, even after being subjected to an impact at We ≈ 335. Static contact angle measurement confirms that subsequent to the droplet impingement at a very wide range of We, all SHCNT arrays still exhibit a very high contact of about 171°. The nanostructures of the SH-CNT arrays after the impact also appear to be very similar to those before the impact (Figure S2a in the Supporting Information). In fact, the same SH-CNT arrays have been used repeatedly in the experiment without any noticeable alteration in their wettability. This finding is in contrast to the reported observation on different types of superhydrophobic materials where partial pinning occurs in the moderate We regime. Since partial pinning phenomenon is heavily influenced by the actual structure and material of the surface, the exact We at which the pinning occurs is different from one surface to another. Droplet partial pinning on fluorocarbon-coated and CNTcoated micropatterned silicon surfaces, which feature sparse and shallow microstructures, has been reported to occur at We > 50 and We > 74.49,52 It has been known that partial wetting is more likely to happen on the surface with sparse and shallow microstructures.49,54 On the other hand, SH-CNT arrays feature nanostructures that are dense and deep. In addition, the three-phase contact line along droplet boundary during the droplet spreading or receding on SH-CNT arrays is very short because of the dense nanostructures and height variation at the surface of the SH-CNT arrays. Therefore, the droplet pinning force on SH-CNT arrays is extremely weak, and the force needed for depinning the droplet becomes negligible.55 To induce pinning and wetting to SH-CNT arrays, the dynamic pressure of the impinging droplet must be excessively high. Droplet partial pinning and total wetting are predicted to occur on SH-CNT arrays at a very high droplet impact velocity of vi = 4.1 m/s and vi = 100.7 m/s, respectively (see Supporting Information). If this prediction is true, it opens up the possibility of using SH-CNT arrays as a protective coating to completely repel raindrops. It has been reported that the average terminal velocity of 1 mm raindrops is about 4 m/s.56 In addition, SH-CNT arrays may also be used to avoid total wetting of the surface of aircraft wings and steam turbines, on which water droplets impinge at an average velocity of hundreds of m/s. The loss of energy of the droplet upon impact can be represented by coefficient of restitution (ε), which is quantified by simply taking a ratio between the velocity of such object after and before the impact. Assuming that the aerodynamic drag during the free-fall and rebound phases can be neglected,13,17 ε of an impinging droplet can be calculated as

−1/4 ⎧ , We < We2 ⎪ a We ε=⎨ ⎪ −2/3 , We ≥ We2 ⎩b We

(1)

with the fitting coefficients a and b are found to be 1.1 (±0.02; 95% confidence) and 6.7 (±0.24; 95% confidence), respectively. For droplets that undergo a recoverable deformation upon impact at We < We2, the loss of energy is mainly caused by the internal motions and vibrations of the droplets. As the deformation of the droplet becomes more pronounced with the increase of We, the internal motions of the droplet become more intense, resulting in an increase of energy dissipation by viscous damping.13 Here, the surface energy of the droplet is still sufficient to fully recover the deformation of the droplet. As the droplet recovers from the deformation, it regains most of its vertical momentum and bounces off of the surface. During this rebound phase, a fraction of the kinetic energy of the rebounding droplet is also converted into vibrational energy, which decreases the remaining kinetic energy of the rebounding droplet even further.57 Note that in the We1 ≤ We < We2 regime the droplet is fragmented during the rebound phase due to the presence of a strong Worthington jet. Thus, the vertical momentum of the rebounding droplet is split between the secondary droplets and the remaining primary droplet. Since an increase in We leads to an increase in size and number of secondary droplets formed by the Worthington jet, the effect of droplet fragmentation to the loss of energy of the primary droplet is expected to increase with the increase of We. For droplets that undergo an irrecoverable deformation upon impact at We ≥ We2, the loss of energy can be mainly attributed to the formation of secondary droplets by the prompt splash phenomena. Here, the surface energy of the droplet is no longer sufficient to overcome its radial momentum. Thus, the secondary droplets formed by the splash retain their radial momentum and break away from the 6787

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primary droplet radially. Since the total size of secondary droplets formed by the prompt splash is no longer small compared to that of the remaining primary droplet, most of the energy is lost before the remaining primary droplet recedes and bounces off of the surface. Therefore, in this regime, the effect of droplet fragmentation to the loss of energy may be more dominant than that of internal motions and vibrations. The contribution of fragmentation due to prompt splash to the loss of energy of the impinging droplet is reflected by the abrupt change in the decrease rate of ε near We2 (Figure 6b). Qualitatively, the rate of change of ε observed on SH-CNT arrays is similar to that on different types of superhydrophobic surfaces.13,21,49,58,59 Basically, there exists a transition We, below which ε ∝ We−1/4 and above which ε ∝ We−2/3 (Figure 6b). This transition We corresponds to the condition where a major droplet fragmentation occurs. For instance, the change in the scaling of ε for a CNT-coated micropatterned silicon surface is observed at We ≈ 20, which is close to the beginning of the partial pinning behavior at We = 24.49 Similarly, the change in the scaling of ε for a superheated Leidenfrost surface is observed at We ≈ 50, which corresponds to the beginning of the prompt splash behavior at We = 64.21 It is important to note that the SH-CNT array yields a higher ε than other types of superhydrophobic surfaces, even in the low We regime. In the moderate and high We regime, the difference in ε becomes more pronounced. This finding implies that SH-CNT arrays are better in preserving the kinetic energy of the impinging droplet compared to other types of superhydrophobic surfaces. Another important parameter that has to be included in the analysis of droplet impact dynamics is the droplet spreading factor (β), which is defined as β = ds/di, where ds is the maximum spread diameter of the droplet during impingement and di is the initial diameter of the droplet. For splashing droplet at We ≥ We2, ds is measured right before the droplet breaks into many smaller secondary droplets. As previously stated, a free-falling water droplet deforms upon impact with a surface, and such deformation becomes more pronounced with the increase of the kinetic energy of the droplet. Since a stronger droplet deformation corresponds to a larger spread factor, it is expected that β increases with the increase of We. As observed experimentally, β of SH-CNT arrays increases with the increase of We in a continuous fashion (Figure 6c). Here β increases from β = 1.15 to β = 5.23 with the increase of We from We = 1.78 to We = 328. Such increase in β as a function of We can be approximated by a scaling of β = [1/12We(1 + ε 2) + 1]1/2

The experimental data obtained from the SH-CNT arrays are in a good agreement with the results obtained from other types of superhydrophobic surfaces (Figure 6c). In fact, the rate of increase in β is exactly the same for all types of superhydrophobic surfaces, including micropatterned silicon surfaces and superheated Leidenfrost surfaces.13,21,23,52,61 This finding shows that β may not be unique to each type of superhydrophobic surface, but instead it follows a universal scaling law. The agreement between eq 2 and the experimental data obtained from different types of superhdyrophobic surfaces suggests that a simple energy conservation approach may be adequate to predict the droplet spreading behavior on a superhydrophobic surface over a wide range of We. The time at which the impinging droplet is in contact with the surface before it finally bounces off of the surface, known as the contact time (tC), is also an important parameter in the analysis of droplet impact dynamics.62 Based on the definition, tC includes both the droplet spreading time and the droplet retraction time. It has to be noted that the time needed by the droplet to spread on the surface does not necessarily the same as that to retract. Thus, tC cannot and should not be used to determine either the droplet spreading time or the droplet retraction time. Each time parameter should be determined by direct measurements. Since tC is predicted to scale as tC ∝ (ρdi3/σ)1/2,63 the droplet contact time can be better represented by a normalized contact time (τC), which is given by τC = tCvi/di.52 Here τC increases from τC = 0.5 to τC = 9 with the increase of We from We = 1.78 to We = 328 (Figure 6d). Such increase in τC can be approximated by τC = c We1/2

(3)

with the fitting coefficients c is found to be 0.42 (±0.03; 95% confidence). Such scaling law is consistent with that observed on superhydrophobic micropatterned silicon.52 Remarkably, eq 3 is found to be very consistent with results obtained in the picoliter regime.33 For the same droplet density, surface tension, and impact speed, a decrease of volume from microliter to picoliter results in a decrease of contact time by 3 orders of magnitude. Thus, tC of a picoliter droplet is approximated to be in the microseconds regime, which is in a good agreement with the previous observation.33 Moreover, eq 3 has been previously observed on other types of superhydrophobic surfaces,13,21,49 although with a different empirical constant. The empirical constant for these other materials was observed to be 0.84 (Figure 6d), which is twice as large as the empirical constant observed on SH-CNT arrays. This minor disagreement in τC is actually unexpected since β is observed to be similar for all types of superhydrophobic surfaces (Figure 6c). Thus, it suggests that the spreading and retraction velocities of the impinging droplet on SH-CNT arrays are higher than those on other types of superhydrophobic surfaces. It has to be noted that the droplet spreading velocity does not necessarily the same as the droplet retraction velocity. Hence, the exact value for these parameters cannot be determined just from the correlation between τC and β. Direct measurements during both spreading and retraction phases have to be performed to determine the spreading and retraction velocities.

(2)

with ε is given by eq 1. This approximation is actually based on the calculation of the radius of the droplet when its kinetic energy is fully converted into surface energy.11,60 However, such energy conservation approach has been considered to be overly simplistic since it does not take into account the energy dissipation. A more precise approximation can be made by including the energy dissipation of the droplet into the equation of energy conservation. Here, the energy dissipation is determined empirically from the experiment (eq 1). As expected, the approximation given by eq 2 agrees with the experimental data over a wide range of We. A different approach to approximate β using the balance of gravity and surface forces has actually been suggested in the past to counter the energy conservation approach.13,17 However, the validity of such approach is still debatable (Figure S4 in the Supporting Information).20,26 6788

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CONCLUSION In conclusion, a vacuum annealing treatment performed at a moderate temperature is sufficient to remove a large amount of adsorbed oxygen from the as-grown CNT (AG-CNT) arrays. As characterized by XPS, this treatment leads to a significant decrease in the surface concentration of hydroxyl and carboxyl groups. As a result, the wettability of the CNT arrays changes from mildly hydrophobic to superhydrophobic. In fact, the resulting superhydrophobic CNT (SH-CNT) arrays exhibit a Cassie state superhydrophobicity, which is apparent from the presence of a continuous air film on their surface whenever they are being submerged in water. The disappearance of hydroxyl and carboxyl groups from the CNT arrays can also be observed from their zeta potential measurement. At a neutral pH, the AG-CNTs are found to be negatively charged, while the SHCNTs are almost neutrally charged. As a result, the electrochemical characteristic of SH-CNT arrays in aqueous electrolyte is dominated by charge transfer resistance instead of double-layer capacitance. The impact dynamics of impinging water droplet on SHCNT arrays is analyzed and compared with those on other types of superhydrophobic surfaces. Here, the droplet impact dynamics are presented in terms of critical Weber number (We1 and We2), coefficient of restitution (ε), spreading factor (β), and contact time (τC). The formation of Worthington jet is observed at We1 = 11 ± 3, while the prompt splash behavior is observed at We1 = 106 ± 17. The rate of change of ε, β, and τC as a function of We of SH-CNT arrays is found to be similar to other types of superhydrophobic surfaces. Note that no droplet pinning on the surface of SH-CNT array is ever observed during the experiment at a very wide range of We. These findings may pave a way for the implementation of SH-CNT arrays in numerous industrial and practical applications, including inkjet printing, direct injection engines and steam turbines, and microelectronic fabrication.

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California Institute of Technology. The authors also acknowledge C. Y. Shu and A. Ghosh for their valuable assistance in conducting the droplet impact dynamic experiments, B. J. Lyon for the assistance in conducting the superhydrophobic lifetime measurement, and Prof. George Rossman for providing an access to the Raman spectrometer.

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ABBREVIATIONS AG-CNT, as-grown carbon nantotubes; SH-CNT, superhydrophobic carbon nanotubes; We, Web number.

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ASSOCIATED CONTENT

S Supporting Information *

Parameter space of the experiment, derivation of equations, and experimental setup. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.G.). Present Address

A.I.A.: Department of Engineering, University of Cambridge, Cambridge, UK. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The Office of Naval Research under grant #N00014-11-1-0031 and The Fletcher-Jones Foundation under grant #9900600.The authors gratefully acknowledge support and infrastructure provided for this work by the Charyk Laboratory for Bioinspired Design at the California Institute of Technology, the Kavli Nanoscience Institute at the California Institute of Technology, the Molecular Materials Research Center of the Beckman Institute at the California Institute of Technology, and the Analytical Facility Division of Geological and Planetary Sciences of the 6789

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