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A dynamic study of liquid drop impact on supercooled cerium dioxide: Anti-icing behavior Sin Pui Fu, Rakesh Prasad Sahu, Estefan Diaz, Jaqueline Rojas Robles, Chen Chen, Xue Rui, Robert F. Klie, Alexander L. Yarin, and Jeremiah T. Abiade Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00847 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 15, 2016

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A dynamic study of liquid drop impact on supercooled cerium dioxide: Antiicing behavior Sin-Pui Fu1†, Rakesh P. Sahu1†, Estefan Diaz1, Jaqueline Rojas Robles2,#, Chen Chen1, Xue Rui3, Robert F. Klie3, Alexander Yarin1,* and Jeremiah T. Abiade1,* 1

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor St., Chicago IL 60607-7022, USA 2

Department of Bioenginering, University of Illinois at Chicago, 851 S. Morgan St., Chicago IL 60607-7022, USA 3

Department of Physics University of Illinois at Chicago 845 W. Taylor St., Chicago, IL 60607-7022, USA

Abstract This work deals with the anti-icing behavior at sub-freezing temperatures of CeO2/polyurethane nanocomposite coatings with and without a stearic acid treatment on aluminum alloy substrates. The samples ranged from superhydrophilic to superhydrophobic depending on surface morphology and surface functionalization. X-ray photoelectron spectroscopy was used to determine the surface composition. The anti-icing behavior was studied both by importing fog into a chamber with controlled atmosphere at sub-zero temperatures and by conducting experiments with drop impact velocities of 1.98 m/s, 2.8 m/s, 3.83 m/s and 4.95 m/s. It was found that the icephobicity of the ceramic/polymer nanocomposite coating was dependent on the surface roughness and surface energy. Water drops were observed to completely rebound from the surface at sub-freezing temperatures from superhydrophobic surfaces with small contact angle hysteresis regardless of the impact velocity, thus revealing the anti-icing capability of such surfaces. * To whom correspondence should be addressed. E-mail: E-mail: [email protected]. Phone: +1(312) 355-2155 and [email protected]. Phone: +1(312) 996-3472. Fax: +1(312) 413-0447 † Equal Contribution ACS Paragon Plus Environment

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Introduction. Ice accretion on surfaces such as airfoils, power line cables, wind turbines, marine vessels, highway roads, airport runaways and commercial and residential appliances pose a serious problem in terms of safety and efficiency. Regular operation of several major sectors such as transportation, power generation, power transmission, telecommunications, and home and office buildings are challenged by it.1, 2 The current techniques of removing accumulated ice from a surface are either chemical, thermal or physical which are energy intensive and ecologically unsafe. Icephobicity is the property of a surface that prevents formation of ice and reduces its adhesion to the surface. Icephobicity has been of interest for decades.3 Removing the ice from a surface is more difficult in comparison to avoiding and delaying the formation of ice on the same surface and hence researchers have focused on developing materials that could possibly delay and avoid ice formation. The inspiration for the development of icephobic surfaces can be found in observing how water droplets behave on living organisms.4-5 Significant progress in fabrication of superhydrophobic surfaces (SHPS) that could potentially be used as anti-icing surfaces has been made after being inspired by the self-cleaning properties of lotus leaves.6-9 Several methods have been used to create superhydrophobic surfaces such as phase separation, sol-gel processing, electrospinning, chemical vapor deposition, wet chemical reaction, crystallization control, electrochemical deposition and lithography.10, 11 Branched hydrocarbon chains along with the nanoparticles were used to prepare superhydrophobic surfaces as a replacement to hazardous fluorocarbons.12

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Surface superhydrophobicity is associated with the interplay between the surface energy and texture. The dynamics of drop impact onto patterned surfaces has been explored and the threshold velocity for the transition from the fully non-wetting Cassie-Baxter state to the fully wetting Wenzel state was found for different textured surfaces.13 Coalescence of condensing droplets on a superhydrophobic surface results in reversal of the Wenzel state to the CassieBaxter state due to the release of the surface energy resulting in self-bouncing of the coalesced drop.14 The rationale behind using superhydrophobic materials as the icephobic materials is that the drop supported on the chemically-modified micro/nano-textured surface (i.e. in the CassieBaxter state) will have a low surface contact area with the supercooled surface and thus the heat transfer will be delayed. As a result, the ice nucleation and hence the freezing of the drop will also be delayed. Moreover, bouncing of impacting drops off the surface will decrease the contact time, which also contributes to delays in freezing of the droplet and ice formation. However, superhydrophobicity of a surface does not necessarily imply that it is icephobic at low temperatures. For example, it was shown that there is a critical drop size for surface superhydrophobicity, and another critical size for its icephobicity.15 A detailed knowledge of the surface chemistry and morphology is required to tackle this question. Superhydrophobic coatings reveal retardation in the freezing time and reduced ice adhesion strength for sessile drops at the surface.16-19 The correlation between the ice adhesion strength and the wetting hysteresis was established and it was experimentally shown that the strength is not correlated to the contact angle.17 The oscillation of the surface wettability on the superhydrophobic surface causes the delay in the frost formation while the constant increase of surface

hydrophobicity

on

the

hydrophobic

surfaces

facilitates

frost

formation.18

Superhydrophobic surfaces show promising behavior in preventing ice formation at low 3 ACS Paragon Plus Environment

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temperatures.15, 20 Icephobicity of both treated and untreated surfaces was previously tested by pouring supercooled water on a supercooled surface and measuring the freezing delay on those surfaces.15 The mechanism of ice nucleation and freezing of supercooled sessile drops on surfaces has been discussed.21-22 The freezing of a supercooled droplet on a superhydrophobic surface is initiated with homogenous nucleation at the gas-liquid interface that is thermodynamically favorable to the heterogeneous nucleation at the liquid-solid interface. Excellent properties, such as anti-icing robustness and low adhesion strength on micro-nano textured surfaces at sub-zero temperatures have been reported for sessile drops.23-24 Ice accretion and removal results in damage of the surface microstructure and a subsequent loss of the antiicing properties of the surfaces. The variation of ice adhesion strength with different wettability has also been discussed.25 The dynamic behavior of water drops over supercooled hydrophilic, smooth hydrophobic and nano-textured superhydrophobic surfaces at -25°C at an impact velocity of 1.4 m/s were studied, and drop repulsion was observed before the onset of ice nucleation.26 A comprehensive study using classical nucleation theory combined with the heat transfer and wetting dynamics was conducted.21,26-27 Various fragmentation modes during the impact of ice particles of different sizes and impact velocities onto solid surfaces have been thoroughly studied both experimentally and theoretically.28 Freezing delays were studied on both hydrophilic and hydrophobic surfaces in the absence of water vapor in the drop impact chamber, and it was concluded that both freezing delay and liquid shedding ability should be considered for best anti-icing materials.27 The drop impact dynamics on superhydrophobic surfaces is significantly affected by solidification of the supercooled drops impacted onto substrates below the freezing point.29 Slippery liquid infused into porous surfaces (SLIPS) were observed to reduce the ice adhesion 4 ACS Paragon Plus Environment

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strength by an order of magnitude compared to the state-of-the-art materials and thus to reduce the ice buildup to a great extent.30,31 Modifying the surface chemistry restricts ice-prevention technology to only delaying the freezing process instead of fully avoiding it. It has recently been demonstrated that superhydrophobic materials lose their superhydrophobicity at low temperatures as the contact angle changes with surface temperature.32-34 The mechanism for water and ice adhesion on a surface was shown to be different as the ice formed on the superhydrophobic surface supports the shear force at the interface in contrast to water drops that practically do not support any shear force at the interface. The presence of voids in the surface topography also reduces the shear strength and thus helps in removal of ice.35 It was demonstrated that hierarchical structures are affected by several icing / deicing cycles because of low abrasion resistance.24, 36 The preceding highlights a demands for superhydrophobic surface with excellent abrasion resistance. Here we present our study on the anti-icing behavior of cerium dioxide-coated nanocomposite surfaces under static and dynamic conditions at room temperature and supercooled freezing temperatures. The effect of the surface texture and the surface chemistry on the antiicing behavior has been demonstrated. Cerium dioxide nano-composite surfaces that are superhydrophilic, hydrophobic and superhydrophobic depending on the surface treatment have been used in the present study because of such attractive properties of cerium dioxide like good corrosion resistance, high hardness, high thermal stability, high wear resistance and high chemical resistance.

Experimental


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Materials: Cerium dioxide (CeO2) nano-particles were synthesized by the hydrothermal method.37 Cerium(III) nitrate hexahydrate was purchased from STREM and used as is. Hexamethylenetetramine (HMT) was purchased from Alfa Aesar and stearic acid (STA) from Sigma-Aldrich was used to lower the surface energy of the cerium dioxide. For the fabrication of superhydrophobic cerium dioxide nano-composite coatings, polymer polyurethane (PU) purchased from MINWAX was used. The 6061 mirror-like Al alloy substrates were purchased from McMasterr Carr. Grouping of Samples. Three different cerium dioxide nano-composite samples were used that can be grouped as follows: cerium dioxide nano-composite by hydrothermal synthesis with STA coating (HMT-STA), cerium dioxide nano-composite by hydrothermal synthesis without STA coating (HMT), cerium dioxide nano-composites with polyurethane on smooth and etched aluminum alloy substrates. Stearic acid treatment of cerium dioxide coating obtained using hydrothermal method. Cerium dioxide superhydrophobic coatings were deposited on 6061 aluminum alloys by the hydrothermal method37 followed by stearic acid treatment as shown in Figure S1(c.f. Supplementary info). Circular samples of 6061 aluminum alloy substrates of 1.75” in diameter and 0.063” thickness were prepared to grow the cerium dioxide nano-structure. The aluminum alloy substrates were also polished using silicon, grinding paper of grade 400, 600 and 1200 to remove the native oxide layers. Then, the polished alloy substrate was ultrasonically cleaned in acetone for 10 min at room temperature. A primary solution was prepared using 2.17 g of cerium(III) nitrate hexahydrate and 5.8 g of hexamethylenetetramine that was dissolved in 60 ml of DI water. The polished alloy substrate


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was then immersed into the primary solution with the polished surface face up. Aliquots of each solution were sealed in 120 ml of fluoropolymer digestion vessel (Savillex, MN, USA) and placed into a 70 ℃ water bath separately for 1.5, 3, 12 and 24 hours. The CeO2 nano- structure was grown on the alloy surface and then cleaned by rinsing in DI water. The cleaned CeO2 sample was then placed over a hot plate for drying for 1 h at 80 ℃. Chemical modification to lower the surface energy was done by immersing the sample in the stearic acid solution for 1 h. The stearic acid solution was prepared by dissolving 0.01 g of stearic acid powder in 10 ml of ethanol. Finally, the treated CeO2 sample was cleaned by rinsing in DI water and dried in air. Cerium dioxide/polyurethane nano-composite on smooth mirror-like or etched aluminum alloy substrates. The cerium dioxide nano-composite superhydrophobic surfaces were fabricated using a two-step process: the synthesis of nano-flower like particles by the hydrothermal method, and preparation of the nano-composite coatings as shown in Figure S2 (c.f. Supplementary info). For the cerium dioxide nano-structure synthesis process, 2.17 g of cerium(III) nitrate hexahydrate and 5.8 g of hexamethylenetetramine were dissolved into 60 ml of DI water. The solution was then stirred for 10 min at 300 rpm at 60 ℃. Then, the solution was sealed into a 120 ml fluoropolymer digestion vessel (Savillex, MN, USA) and placed in a hot water bath for 6 h at 70 ℃. The white powder was collected by centrifugation in DI water for 5 min at 4000 rpm, which was repeated for 4 cycles. The cleaned powder was dried in an oven at 60 ℃ for 12 h. The cerium dioxide nano-composite coating was prepared by mixing the dry cerium dioxide nano-particles with the polyurethane polymer. Two different substrates were used: mirror-like smooth aluminum alloys and etched aluminum alloys of 1.5 cm x 5 cm (and 0.008” thickness). The mirror-like substrate was cleaned in an ultrasonic bath with acetone for 10 min. The etched 7 ACS Paragon Plus Environment

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aluminum substrate was prepared by immersing it in 4 M HCl solution for 5 min and then by rinsing it in boiling DI water for 2 h. For coatings on the smooth aluminum alloy samples, the ratio of polymer to dry cerium nano-particles was approximately 1:1 (0.4 g polyurethane : 0.4 g nano-particles), which was dissolved in 40 ml of acetone. For coatings on the etched aluminum alloy substrate, the ratio of polymer to dry cerium nano-particles was about 1:1.5 (0.1 g polyurethane : 0.15 g nano-particles), which was dissolved in 20 ml of acetone. The ratio of polymer to powders for the smooth and etched substrates was modified to optimize water repellency and adhesion. We tailored the ratio of polymer to powders to achieve the highest water contact angle and mechanical durability using the cerium dioxide nano-composite coating on both smooth and etched substrates. The polymer was dissolved in acetone and place in the ultrasonic bath for 5 min. The cerium dioxide nanoparticles were added to the polymer solution and placed in the ultrasonic bath for 30 min. The coating was then sprayed onto the aluminum alloy surface at 80 ℃ using an airbrush with a 0.3 mm diameter nozzle at a pressure of 60 psi.

Sample Surface Characterization Contact angle measurement. The static contact angle and the contact angle hysteresis were determined for each sample. A Nikon D3100 DSLR camera with macro-focusing teleconverter lens (Vivitar 2X) and Nikon 35 mm f/2.0-22.0 lens was used to record contact angle measurements, which is shown in Figure S3 (cf. Supporting info). A fixed volume of 5 μl DI water was gently placed on the surface at room temperature and humidity of 40±5%. Then the static contact angle was measured using images of the drop on the horizontal surface. The contact angle hysteresis was determined by calculating the difference between the advancing and receding angles. The roll-off angle was also determined by the tilting plate method. The droplets 8 ACS Paragon Plus Environment

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were initially placed on a horizontal surface. The plate was then gradually tilted and the roll-off angle was measured when the droplets started to move. Ten different measurements were recorded at different locations on each sample to ensure reproducibility and to account for variability in the sample coating. Surface morphology characterization: Scanning electron microscopy (SEM) (Raith 100) was was used to determine the surface roughness with a scan size of 0.45 mm × 0.60 mm. used to determine the surface morphology. An optical profilometer (BRUKER CONTOURGT-K) was used to determine the surface roughness with a scan size of 0.45 mm × 0.60 mm. The measured roughness average (Ra) was averaged over 7 different spots. The roughness average represented by Ra is the average of the absolute values of the surface deviation from the reference plane. The X

three-dimensional approximation of Ra is R a =

Y

1 ∑∑ Z where X, Y are the number of data XY i=1 j=1 ij

points in the x- and y- directions and Z is the surface elevation relative to the reference plane. It is used to measure the surface roughness. X-ray Photoelectron Spectroscopy (XPS) surface characterization. XPS (Thermo Scientific ESCALAB 250Xi) was used to determine the elemental composition, chemical state, and valence state of cerium and oxygen near the surface. The samples were analyzed using aluminum Kα radiation with the excitation energy of 1486 eV at 10-9 Torr base pressure. The results were collected at 90° with 50 eV pass energy and a 500 μm spot size. The software package Avantage 5.9 was used for fitting the peaks and determination of the surface stoichiometry. A static Shirley background was used to fit the XPS results. The binding energy of the C 1s peak (284.6 eV) was used as a reference for the peak shifts.


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The two most common oxidation states of cerium, Ce (IV) and Ce (III) can be resolved into five features in the Ce 3d5/2 and Ce 3d3/2 core level peaks. V and U indicate the spin-orbit coupling for the Ce 3d5/2 and Ce 3d3/2, respectively. Ce (IV) has been matched to six peaks listed below V (BE ≈ 882.6 eV), V’’ (BE ≈ 888.85 eV), V’’’ (BE ≈ 898.4 eV), U (BE ≈ 901.05 eV), U’’ (BE ≈ 907.45 eV), and U’’’ (BE ≈ 916.7 eV). V, V’’ and V’’’ were assigned, respectively, to 3d9 4f2 Vn-2, 3d9 4f1 Vn-1 and 3d9 4f0 Vn CeIV final states. Ce3+ has been fitted with 4 peaks, V0 (BE ≈ 880.6 eV), V’ (BE ≈ 885.45 eV), U0 (BE ≈ 898.9 eV) and U’ (BE ≈ 904.05 eV). V0 and V’ were assigned, respectively, to 3d9 4f1 Vn-1 and 3d9 4f0 Vn CeIII final states following.38, 39 The valence states of Ce (III) and Ce (IV) are labeled on the curve-fitted regions of the Ce 3d spectrum in Figure S4 (cf. Supporting info). There are three main features for the O 1s spectrum in figure S5 for the cerium dioxide samples. O1 (~ 529 eV) is associated with O2- anions in the lattice of cerium dioxide. O2 (~531 eV) is a combination of surface hydroxyl and carbonates. O3 (~535 eV) is associated with adsorbed water on the surface40 as shown in Figure S5 (c.f. Supporting info). The C 1s spectrum can be fitted to three main features in the cerium samples. In Figure S6 (cf. Supporting info) C1 (~284.6 eV) is assigned to the carbon or hydrocarbon chains (C-C). C2 (~286 eV) is assigned to the carbon bonded to one oxygen atom as in alcohols or ether groups. C3 (~288 eV) is assigned to the carbon double bonded with oxygen (O-C-O, C=O) as in aldehydes or ketones. C4 (~289 eV) is assigned to the carboxyl or ester groups (O=C-O).40,41 Transmission Electron Microscopy (TEM) TEM (JEOL JEM-3010) was used to evaluate the size of the cerium dioxide nano-particles. The dry cerium dioxide powder was suspended in isopropyl alcohol (IPA) and placed in an ultrasonic


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bath for 10 min. A holey carbon grid was then dipped into the suspension and dried under a filament lamp for 5 min. The TEM sample was analyzed by TEM under 300 KV and 120 µA. These images were viewed and recorded by DigitalMicrograph.

Experimental Methods and Apparatus Ice formation experiment for evaluation of anti-icing coatings. The samples were cooled down inside a custom aluminum chamber at -10 ℃ (40 ± 5% RH) for 1 h. A Sayon cooled incubator (MIR-154) was used to control the temperature and humidity conditions within the custom aluminum chamber as shown in Figure 1. Fog was generated using a humidifier and imported into the chamber for 5 min. Ice crystals started to accumulate on the surface of the samples. The experiment was completed by defrosting the surface at room temperature (40 ± 5% RH) to remove the ice layer. The icing cycles were repeated 3 times for each sample. We also made 3 samples for each experimental group.


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Figure 1. Experimental setup with the incubator and the custom-made aluminum chamber inside the incubator for icing experiments. Drop impact experimental setup. The drop impact experiments were performed at two different surface temperatures, room temperature and -25 °C (40 ± 5% RH). Figure 2 shows the schematic of the experimental setup employed for the drop impact experiments. A custom- made Styrofoam chamber of 18” x 12” x 11” was made using 1.5” thick Styrofoam board to control the temperature during the drop impact experiment. The 5 μL DI water droplets were impacted onto the samples at velocities of 1.98 m/s, 2.8 m/s, 3.83 m/s and 4.95 m/s. The diameter of the droplets was approximately 2 mm. A high-speed camera (phantom V210) was used to record the droplet impact dynamics at 4100 fps.


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Figure 2. Schematic of the drop impact experimental setup.

Results and Discussion Wetting behavior characterization. Table 1 lists the data on the contact angle, contact angle hysteresis and the surface roughness for the different samples. The first group of samples (the samples coated with cerium dioxide nano-particles with STA treatment) have a very high water repellency with ~160° static contact angle and ~ 3° contact angle hysteresis. The cerium dioxide nano-particles samples were superhydrophilic before the STA treatment with static contact angles less than 10°. Clearly, the STA coating on the cerium dioxide nano-particle coating reduces the surface energy to a level sufficient for superhydrophobicity. For the samples with cerium dioxide nano-composite coating on the mirror-like aluminum alloy substrates, superhydrophobicity was achieved by the increase in surface roughness when coated by the cerium dioxide nano-particles. The static contact angle is ~155° and the contact hysteresis is ~12°. The mirror-like aluminum alloy and the polyurethane coating on the mirror-like aluminum


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alloy have similar wetting behavior with ~93° static contact angles and ~35° contact angle hysteresis. The cerium dioxide nano-composite coatings on etched aluminum alloy substrates has very good superhydrophobicity with ~150° static contact angle and ~23° contact angle hysteresis. The etched aluminum surface was initially hydrophilic with ~ 10° static contact angle. However, it became hydrophobic with ~124° static contact angle after a thin polyurethane coating was applied. These results suggest that the surface roughness and surface chemistry play an important role regarding the wetting behavior. The hydrophobicity increases when the surface roughness is increased and the surface energy is reduced. Table 1. Wetting and surface roughness parameters for different samples.

Sample Group

CeO2 nanocomposite with STA

CeO2 nanocomposite without STA

CeO2 nanocomposite with polyurethane on flat Al

CeO2 nanocomposite with polyurethane on etched Al

160±1.5 160±1.7 158±3.3 162±1.6 8±2.3 7±3.2 7±1.8 8±1.4 93±7.5

Contact Angle Hysteresis (°) 3±1.9 2.4±1.2 2.5±1.1 4.3±1.3 35.1±11.1

3.94±0.9 5.07±0.7 2.81±0.5 2.68±0.2 2.47±0.2 3.73±0.7 2.36±0.3 2.37±0.2 0.051±0.005

92±1.3

35.7±11.9

0.26±0.02

155±4.6

11.8±7.6

20.4±2.7

10±2.1

-

8±0.2

124±4.4

41.8±6.3

4.81±0.4

150±4.1

23.3±3.5

7.37±0.3

Contact Angle (°)

Sample Name 1.5 h HMT-STA 3 h HMT-STA 12 h HMT-STA 24 h HMT-STA 1.5 h HMT 3 h HMT 12 h HMT 24 h HMT Flat Al Polyurethane on flat Al Nano-composite coating on flat Al Etched Al Polyurethane on etched Al Nano-composite coating on etched Al


Surface Roughness (µm)

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Surface morphology. Figure 3 shows the surface morphology of the samples of 1.5 h HMTSTA, 24 h HMT-STA and the nano-composite coating on the mirror-like aluminum and etched aluminum substrates. The SEM images show that the flower-like cerium dioxide nano-structures were coated on the Al alloy surface for the series of samples synthesized using HMT. However, cerium dioxide nanoflakes were observed on the mirror-like aluminum and etched aluminum substrates. Figure 4 is a TEM image that shows the primary particle size of the cerium dioxide nano-particles after 3 h synthesis time. The average size of the cerium dioxide nano-particles is ~ 6 nm. Together, the SEM and TEM images show that the CeO2 nanoflakes consist of aggregates of smaller particles with average size ~ 6 nm.


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Figure 3. SEM images of different samples showing the surface morphology, (a-d) cerium dioxide nano-composite with STA treatment and different number of hours of HMT, (e) nanocomposite coating on mirror-like aluminum alloy, and (f) nano-composite coating on etched aluminum alloy. The scale bar is 1 µm in panels a, b, d, e and 2 µm in panels c and f.

Figure 4. TEM image showing the size of cerium dioxide nano-particles after 6 h synthesis by the hydrothermal method. The scale bar is 5 nm. XPS characterization. XPS was used to determine the surface chemistry and stoichiometry. Figure 5 shows the XPS spectra for C 1s, O 1s, Al 2p and Ce 3d before and after the STA treatment on the samples processed for 3 h using HMT. The peak assignments for the C 1s, O 1s and Ce 3d spectra are shown in the Supporting info file. The cerium dioxide nano-particles are initially superhydrophilic after the hydrothermal synthesis process. The cerium dioxide nanoparticles can become superhydrophobic after the stearic acid treatment with ~160° contact angle. 17 ACS Paragon Plus Environment

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The C 1s spectra (Figure 5a) shows that the hydrocarbon peak (~ 284.6 eV) becomes the dominant peak in the C 1s spectra after the STA treatment. In our previous study we showed that hydrocarbon species play an important role in determining the hydrophobicity at oxide surfaces45. For the O 1s spectra (Figure 5b) the peak intensity slightly decreases after the STA treatment, but the peak position is roughly the same. For the Al 2p spectra (Figure 5c) the main peak (~74.3 eV Al2O3) slightly decreases after the treatment. For the Ce 3d spectra (Figure 5d) similar behavior was observed. The cerium dioxide peaks slightly decrease after the treatment. This is likely due to the surface sensitive nature of XPS. The STA coating results in decreased intensity of photoelectrons from the CeO2 nanoparticles buried below STA. The stearic acid layer bonds to cerium dioxide nano-particles. The cerium dioxide nano-particles with multiple hydroxyl groups bind with stearic acid. We expect the bonding of stearic acid to cerium dioxide nanoparticles to follow a similar mechanism to that of the bonding of stearic acid to aluminum oxide. 42 The surface stoichiometry was determined before and after the STA treatment. The surface contains 16.2 % carbon, 57.5 % oxygen, 19.4 % cerium and 7 % aluminum before the treatment. However, the surface contains 54.2 % carbon, 33.2 % oxygen, 9.4 % cerium and 3.2 % aluminum after the STA treatment. STA is a saturated fatty acid with an 18-carbon chain with a water repellent tail. The function of the STA treatment is to reduce the surface energy on the nanostructured cerium dioxide particles. The increased carbon concentration and reduction in the concentration of aluminum and cerium is because the STA layer prohibits the photoelectrons associated with those elements from escaping, hence the surface sensitivity of the measurement technique. XPS characterization was also performed on the cerium dioxide nano-composite samples coated on the mirror-like aluminum alloy and the etched aluminum alloy substrates. Figures 6 18 ACS Paragon Plus Environment

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and 7 show the survey scan of the samples, and the C 1s spectra and O 1s spectra for the cerium dioxide nano-composite coating on different aluminum alloy substrates. For the survey scan, no cerium signal was detected at the surface for either sample. It is believed that the polyurethane coating on the cerium dioxide nano-particles is too thick with more than 6 atomic layers to observe a signal from the nanoparticles. However, the C 1s spectra suggest that the concentration of hydrocarbon species is very high at the surface, which corresponds to increased water repellancy. For the O 1s spectra, the results do not show a peak associated with the oxygen lattice of cerium. Therefore, the XPS results suggest that the cerium dioxide nano-particles are completely coated by the polyurethane only contributing roughness to the nanoparticle coating as described in SI figure 2.


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Figure 5. XPS results for the sample processed in HMT for 3 h before and after the stearic acid treatment. (a) C 1s spectra. (b) O 1s spectra. (c) Al 2p spectra. (d) Ce 3d spectra.


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Figure 6. XPS results for the superhydrophobic cerium dioxide nano-composite sample coated on mirror-like aluminum alloy substrates. (a) Survey scan. (b) C 1s spectra. (c) O 1s spectra.


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Figure 7. XPS results for the superhydrophobic cerium dioxide nano-composite sample coating on etched aluminum alloy. (a) Survey scan. (b) C 1s spectra. (c) O 1s spectra. Ice formation experiment. It is evident from the contact angle measurements listed in Table 1 that the series of the HMT samples with STA coating has the highest water repellency with high static contact angles and low contact angle hysteresis. This group of samples was selected for the ice formation experiment. The anti-icing results in Figure 8 show that the formation of ice crystals depends on the surface roughness and surface adhesion. The icing and defrosting cycle was repeated 3 times for consistence. Spherical ice deposits form on the 3 h HMT-STA surface, while the other 4 samples had flat ice coatings. The 3 h HMT-STA samples possessed the 22 ACS Paragon Plus Environment

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highest surface roughness. In the ice formation experiment, several stages were observed: condensation, growth, coalescence and jumping.14 The high humidity environment was generated by the humidifier. The fog condensed on the cold superhydrophobic surface and the small water droplets grew in the gap between the nano-structures. The small droplets then coalesced into one big drop that subsequently jumped off the surface. The droplets started to crystallize and become a film of ice. For the samples with 1.5 h HMT-STA, 12 h HMT-STA and 24 h HMT-STA processing times, no spherical ice deposits were formed. The small droplets may have crystallized before jumping off the surface during coalescence processes. The ice adhesion to the surface also depends on the surface contact area. The spherical ice deposits have lower contact area on the surface compared to flat ice, which covers the entire surface. Therefore, the spherical ice deposits with less contact area can be removed easier than a flat layer of ice. However, the anti-icing properties of the superhydrophobic surface are still not guaranteed because a thin frost layer easily forms on the surface as a precursor to the larger ice coating. The results show that the 3 h HMT-STA sample possesses the best anti-icing properties, which may be associated with the high surface roughness and low surface energy. The 3 h HMT-STA has the maximum average roughness and thus low contact angle hysteresis as mentioned in Table 1. For immersion times less or greater than 3 h, the surface roughness is lower even though the thickness of the nanostructures varies proportionally to the immersion time. The decrease in surface energy due to the presence of the thin STA coating over the nanostructures results in the best anti-icing properties amongst all samples.


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Figure 8. The anti-icing behavior of the cerium dioxide superhydrophobic surface with stearic acid treatment.


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Drop impact experiments. To investigate the effect of surface roughness and surface energy on the anti-icing behavior of the different surfaces that were synthesized and listed in Table 1, water drop impacts onto such surfaces were studied in detail. Water drops of approximately 2 mm were impacted onto different surfaces both at room temperature (25 °C) and at -25 °C with different impact velocities: 1.98 m/s, 2.8 m/s, 3.83 m/s and 4.95 m/s. The Weber number (We) and the Reynolds number (Re) of the impacting drops varied from 120 to 750 and from 4000 to 11000, respectively. Drop impact at room temperature. Impacts of liquid drops onto a solid substrate are followed by spreading, receding or bouncing depending on the impact velocity, drop size, liquid properties and surface properties (roughness and wettability).43 The main dimensionless groups that govern 2 drop impact dynamics are the dimensionless Weber and Reynolds numbers We = ρD0V / σ and

Re = ρVD0 / µ , respectively, where ρ, µ and σ are the liquid density, viscosity and surface tension respectively, and D0 and V are the impacting drop diameter and velocity, respectively. The Ohnesorge number, Oh = µ / (ρσD)1/ 2 = We1/ 2 / Re

is also used.

An additional

dimensionless group of importance in such cases is K= We1/2Re1/4 which determines the splashing threshold.43, 44 It has been demonstrated that at high We and Re values the maximum spreading diameter is independent of the surface wettability and roughness cf. the reviews in.43, 45 An improved model for predicting the maximum spreading ratio has been recently proposed, especially for low impact velocities.46 However, the rate of receding of the spread-out liquid lamellae after the impact does depend on the surface wettability. Drop impacts onto a solid surface result either in deposition, prompt splash or corona splash, receding breakup, or partial or complete rebound of the drop off the surface depending on the surface and the impact conditions.


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Figure 9 illustrates the water drop impact dynamics following the impact onto various substrates with an impact velocity of 3.83 m/s at 25 °C. The static contact angle on the etched Al substrate is 10° with a average roughness, Ra ~ 8 µm. Accordingly, the water drop impact onto the etched Al substrate results in its spreading and prompt splash (Figure 9a). The drop does not recede back due to the surface hydrophilicity. With the deposition of the polymer coating and the cerium dioxide nano-composite, the surface becomes hydrophobic and superhydrophobic, respectively, which results in receding breakup with partial rebound of the drop as seen in Figure 9a. Both the flat Al and polymer-coated Al surfaces are hydrophobic with similar wetting parameters as listed in Table 1. Accordingly, the water drop spreads and recedes back without any splash. With an increase in surface hydrophobicity after the deposition of the cerium dioxide nano-structured coating, the water drop undergoes splashing and receding breakup. This shows that the surface wettability and roughness do affect the splashing threshold. The surfaces that were treated with HMT result in deposition and prompt splash (1.5 h HMT, Figure 9c) and only deposition (3 h HMT, Figure 9c). As the STA coating was applied to the samples treated with HMT, both partial and total rebound of water drops was observed. In these cases, after the maximum drop spreading was achieved, it was followed by a rim atomization, as is evident in Figure 9d.


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Figure 9. Impact of 2 mm water drops onto various solid substrates with an impact velocity of 3.83 m/s at room temperature. The results reveal deposition, prompt splash, rim atomization, as 27 ACS Paragon Plus Environment

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well as partial and total drop rebound for coatings on (a) Etched Al substrate with cerium dioxide nano-composite and polyurethane coating. (b) Flat Al substrate with cerium dioxide nanocomposite and polyurethane coating. (c) Flat Al with 1.5 h and 3 h of HMT and no STA coating. (d) Flat Al with 1.5 h and 24 h of HMT with STA coating. The values of the dimensionless parameters We, Re and K are 448, 8426 and 203, respectively.

As the drop spreads over the surface after impact, the lamella increases, reaches a maximum radius, and if the drop recedes on hydrophobic and superhydrophobic surfaces, the lamella radius starts decreasing. Figure 10 shows how the dimensionless spread factor ξ=D D0 (with D0 being the drop diameter before the impact and D being the current lamella diameter after the impact) changes in time on different substrates. As the lamella radius increases, ξ increases and its variation could be linked to different morphologies labeled in Figure 10. Drop deposition is indicated by a constant plateau value of ξ , whereas the decrease in ξ after the maximum spreading indicates the receding motion of the drop. If the drop undergoes total rebound, as in Figure 9d (1.5 h HMT-STA), D becomes zero and thus ξ is zero and hence the corresponding curve in Figure 10 reveals a sharp drop. In the case of partial rebound, the drop is attached to the surface and ξ decreases after maximum spreading similar to the case of receding.


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Figure 10. Retraction of the liquid lamella after 2 mm water drop impact onto various solid substrates with an impact velocity of 3.83 m/s at room temperature. Variation of the spread factor ξ=D D0 . Different regimes corresponding to drop deposition, receding and rebound are observed here. (a) Etched Al substrate with cerium dioxide nano-composite and polyurethane coating. (b) Flat Al substrate with cerium dioxide nano-composite and polyurethane coating. (c) Flat Al with 1.5 h, 3 h, 12 h and 24 h of HMT and no STA coating. (d) Flat Al with 1.5 h, 3 h, 12 h and 24 h of HMT with STA coating.


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Drop impact at freezing temperature of -25 °C. The drop impact experiments were conducted on the same substrates as listed in Table 1 inside a Styrofoam chamber. Dry ice was used to bring the local temperature down to that of -25 °C. The sample surface temperature was ensured to reach the freezing temperature. After that, water drops were dripped under gravity onto the surface. The surface temperature was allowed to stabilize for 2 to 3 min before the subsequent water drops were allowed to impact the surface. All the substrates were kept horizontal and the drop impacts were applied normal to the surface. Figure 11 shows the impact dynamics at different instances on a flat Al surface, an Al surface covered with PU, and an Al surface coated with cerium dioxide nano-composite. Both the flat Al and Al coated with PU are partially wettable surfaces with a high contact angle hysteresis which is evident from the receding motion of the drop at room temperature (Figures 11a.1 and 11a.2; 11c.1 and 11c.2), whereas the cerium dioxide-coated surface is superhydrophobic, but still with a high contact angle hysteresis that results in prompt splash with receding motion. At freezing temperatures, the receding motion and the prompt splash are suppressed compared to the corresponding cases at room temperature. This reveals that the cerium dioxide-coated surface needs further optimization for enhanced anti-icing protection. In addition, the impacting drop could undergo transition from the Cassie Baxter to Wenzel state, as the roughness value is high, which pins the contact line of the drop facilitating early freezing and thus restricting its receding motion. Similar phenomena were observed for etched Al samples both with the PU coating and also with the cerium dioxide nano-composite coated over the surface as shown in Figure 12. The receding motion was severely affected at the freezing temperature and the drop freezes without showing any signs of self-cleaning. 30 ACS Paragon Plus Environment

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Figure 11. Sequence of drop impact images with an impact velocity of 1.98 m/s (a-b), and 4.95 m/s (c-d). Room temperature for panels (a) and (c) and the freezing temperature of -25 °C for panels (b) and (d). The substrates used are the flat aluminum (denoted as 1 in the panels), flat aluminum coated with PU (denoted as 2 in the panels) and CeO2 -coated flat aluminum (denoted as 3 in the panels).


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Figure 12. Sequence of drop impact images with an impact velocity of 1.98 m/s (a-b), and 4.95 m/s (c-d). Room temperature for panels (a) and (c) and the freezing temperature of -25 °C for panels (b) and (d). The substrates used are the etched aluminum (denoted as 1 in the panels), etched aluminum coated with PU (denoted as 2 in the panels) and cerium dioxide- coated etched aluminum (denoted as 3 in the panels).


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The HMT samples without the STA coating were hydrophilic and the drops upon impact and spreading were deposited and frozen on the surfaces. All surface samples behaved similarly as is seen in Figure 13.

Figure 13. Sequence of drop impact images with an impact velocity of 1.98 m/s (a-b), and 4.95 m/s (c-d). Room temperature for panels (a) and (c) and the freezing temperature of -25 °C for panels (b) and (d). The substrates used are the 1.5 h HMT coated Al (denoted as 1 in the panels) and 3 h HMT coated Al (denoted as 2 in the panels).

The HMT treatment with the additional STA coating results in a reduced surface energy at the coating surface, making the surface superhydrophobic. Moreover, both the contact angle hysteresis and the surface roughness are very small. At room temperature, the drops upon impact, 33 ACS Paragon Plus Environment

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completely rebound, as shown in Figures 14a and 14c. The drops do not undergo any transition from the Cassie-Baxter to the Wenzel state because of the small roughness value, and hence completely rebound. Even at the freezing temperature at both low and high impact velocities the drops completely rebound from the surface after spreading which reveals the anti-icing properties of these types of surfaces. It should be emphasized that after any drop impact experiment, the substrates were taken for deicing and then used again. Thus, they were subjected to several icing/deicing cycles, and even after that revealed complete rebound of the impacting water drops at low and high impact velocity. This shows that the chemically modified surface is not only icephobic but also resistant to the icing/deicing cycles and very durable.

Figure 14. Sequence of drop impact images with an impact velocity of 1.98 m/s (a-b), and 4.95 m/s (c-d). Room temperature for panels (a) and (c) and the freezing temperature of -25 °C for


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panels (b) and (d). The substrates used are the 1.5 h HMT with STA coating on Al (denoted as 1 in the panels) and 3 h HMT with STA coating on Al (denoted as 2 in the panels).

The maximum spread factor at different temperatures. The maximum spread factor of a drop impacting on a solid impermeable surface is defined as ξ max = Dmax D . An empirical correlation has

been

proposed

ξ max = 0.61( We Oh )

for

0.166

the

maximum

spread

factor

on

solid

surfaces,

namely

.47 The values corresponding to this expression, as well as the

experimentally measured values of the maximum spread factor on different surfaces listed in Table 1 both at room temperature and at freezing temperature are listed in Table 2. The comparison between the experimental maximum spread factor at room temperature and the freezing temperature is shown in Figure 15. It is evident from the figure that the maximum spread factor at room temperature is lower than that at the freezing temperature for the impact velocities of 1.98 m/s and 2.8 m/s. On the other hand, the maximum spread factor is higher at the room temperature than that at the freezing temperature for the impact velocity of 4.95 m/s. For the impact velocity of 3.83 m/s the maximum spread factor is similar at both temperatures.

Table 2: The splash threshold (the K-group) and the experimentally measured maximum spread factor and the one predicted by the correlation ξ max ,corr = 0.61( We Oh )

0.166

for different impact

velocities and different substrates at room and freezing temperatures. Impact height and the corresponding impact velocity

20 cm (V=1.98 m/s)

40 cm (V=2.8 m/s)

75 cm (V=3.83 m/s)

125 cm (V=4.95 m/s)

Maximum spread factor ξ max

3.65

4.09

4.54

4.94

K-parameter value

89

Sample

T: 25°C

Flat Al PU-Flat Al

3.27 3.42

137 T: -25°C 3.52 3.8

T: 25°C 3.97 4.1

203 T: -25°C 4.02 4.12


T: 25°C 4.47 4.56

280 T: -25°C 4.36 0

T: 25°C 4.95 4.96

T: -25°C 4.55 4.27

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CeO2 –Flat-Al Etch-Al PU-Etch-Al CeO2 –Etch-Al 1.5 hrs HMT 3 hrs HMT 12 hrs HMT 24 hrs HMT 1.5 hrs HMT-STA 3 hrs HMT-STA 12 hrs HMT-STA 24 hrs HMT-STA

3.37 3.16 3.06 3.15 3.04 3.46 3.58 3.61 3.49 3.4 3.22 3.37

3.5 3.25 3.46 3.35 3.49 3.87 3.69 3.75 3.65 3.57 3.56 3.57

3.7 3.86 4.09 3.93 3.85 3.9 3.96 3.91 4.16 4.12 4.11 4.19

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3.69 3.98 4.12 4.09 4.02 4.06 4.19 4.07 4.31 4.23 4.07 4.12

3.55 3.47 3.88 4.06 3.91 3.93 4.48 4.49 4.53 4.43 4.45 4.39

3.61 3.51 4.11 3.89 4.18 4.12 4.23 4.48 4.57 4.39 4.21 4.28

4.11 4.22 4.62 4.58 4.1 4.13 4.91 4.91 4.7 4.96 4.83 5.01

4.28 3.85 4.31 4.53 3.97 3.4 4.7 4.36 4.7 4.98 4.29 4.65

Figure 15. The experimentally measured maximum spread factor compared with the one predicted by the correlation ξ max ,corr = 0.61( We Oh )

0.166

for different substrates listed in Table 1.

The colored filled symbols are for the data points for the impacts onto substrates at room temperature (R.T) and the hollow symbols are the data points for the impact onto substrates at freezing temperatures (F.T).


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In the case of drop impacts with a velocity of 4.95 m/s the thickness of the lamellae at the maximum spread of the drop is estimated from the volume balance as about 53 × 10-4 cm. The thickness of the frozen lamella on the freezing-temperature substrate is estimated as h= αt where α is the thermal diffusivity of water (~ 10-3 cm2/s) and t is the time after impact to reach the maximum lamella spread (t ~ 4.8 × 10-4 s). Accordingly, h is ~ 7 × 10-4 cm. The estimated value of h is lower than that of the measured final lamella thickness (53 × 10-4 cm), which implies that the lower part of the drop upon impact starts cooling down and thus its viscosity increases. The latter can be responsible for a reduction of the maximum spread factor compared to that at room temperature observed in some cases in Table 2. At low impact velocities the thickness of the lamellae increases and is of the order of 0.1 mm, while the t ~ 2.4 × 10-3 s. Accordingly, h ~ 1.5 × 10-3 cm, still significantly lower than 0.1 mm, which means that at the lower impact velocities still only the lower part of the drop can be cooled down during the impact.

Summary and Conclusions A facile process was used to fabricate cerium dioxide/polyurethane coatings on aluminum substrates. Coatings ranging from superhydrophilic to superhydrophobic were obtained. Extreme water repellency was achieved due to the surface roughness and surface functionalization by stearic acid. The presence of STA was confirmed by x-ray photoelectron spectroscopy. The tendency of the coated surfaces to inhibit ice formation during freezing or shed ice upon defrost was found to be a function of both surface roughness and wettability. Drop impact experiments were performed at various water drop impact velocities and at room temperature and freezing temperature of the surface. Drop behavior after the impact (spreading, receding or bouncing) is dependent on the impact velocity, droplet size, surface roughness and wettability and the liquid properties. The maximum spread factor was determined as a function 37 ACS Paragon Plus Environment

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of all the parameters involved. A dependence on the impact velocity was observed at room temperature except for an impact velocity of ~ 4.0 m/s. The stearic acid-coated samples retained their superhydrophobicity even at freezing temperatures and also displayed icephobicity over several freeze/defrost cycles. Thus, a very simple and low cost process of preparing icephobic surfaces using nanostructures and short non-branched hydrocarbon chains has been demonstrated.

References (1) Gent, R. W.; Dart, N. P.; Cansdale, J. T. Aircraft icing. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 2000, 358 (1776), 2873-2911. (2) Laforte, J. L.; Allaire, M. A.; Laflamme, J. State-of-the-art on power line de-icing Atmospheric Research 1998, 46 (1), 143-158. (3) Raraty, L.; Tabor, D. The adhesion and strength properties of ice. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. The Royal Society 1958, June 3 245 (1241), 184-201. (4)Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography Advanced Materials , 2007, 19 (17), 2213-2217. (5) Gao, X.; Jiang, L. Biophysics: water-repellent legs of water striders. Nature 2004, 432 (7013), 36-36. (6) Saito, H.; Takai, K.; Yamauchi, G. Water-and ice-repellent coatings. Surface Coatings International Part B: Coatings Transactions 1997, 80 (4), 168-171 (7) Nakajima, A. Design of hydrophobic surfaces for liquid droplet control. NPG Asia Materials 2011, 3 (5), 49-56. (8) Ganesh, V. A.; Raut, H. K.; Nair, A. S.; Ramakrishna, S. A review on self-cleaning coatings. Journal of Materials Chemistry 2011, 21 (41), 16304-16322. (9) Lv, J.; Song, Y.; Jiang, L.; Wang, J. Bio-inspired strategies for anti-icing. ACS nano 2014, 8 (4), 3152-3169. (10) Guo, Z.; Liu, W.; Su, B. L. Superhydrophobic surfaces: from natural to biomimetic to functional. Journal of colloid and interface science 2011, 353 (2), 335-355. (11) Asmatulu, R.; Ceylan, M.; Nuraje, N. Study of superhydrophobic electrospun nanocomposite fibers for energy systems. Langmuir 2010, 27 (2), 504-507. 38 ACS Paragon Plus Environment

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