Instant Tuning of Superhydrophilic to Robust Superhydrophobic and

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Surfaces, Interfaces, and Applications

Instant Tuning of Superhydrophilic to Robust Superhydrophobic and Self Cleaning Metallic Coating: Simple, Direct, One-Step and Scalable Technique O.S. Asiq Rahman, Biswajyoti Mukherjee, Aminul Islam, and Anup Kumar Keshri ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19045 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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ACS Applied Materials & Interfaces

Instant Tuning of Superhydrophilic to Robust Superhydrophobic and Self Cleaning Metallic Coating: Simple, Direct, One-Step and Scalable Technique $O.

S. Asiq Rahman, $Biswajyoti Mukherjee, Aminul Islam, Anup Kumar Keshri*

Metallurgical and Materials Engineering, Indian Institute of Technology Patna, Bihta, Bihar, 801106, India $O.

S. Asiq Rahman and Biswajyoti Mukherjee have contributed equally to this work.

Abstract We present a simple, direct, one-step, scalable technique for instant tuning of all the different states of wetting characteristics using atmospheric plasma spray (APS) technique. We observed that, just by changing the process parameters in APS technique, the wetting characteristics of an intrinsically hydrophilic aluminium metallic surface can be tuned to superhydrophilic (CA: 0°), hydrophilic (CA: 19.6°), hydrophobic (CA: 97.6°) and superhydrophobic (CA: 156.5°) surfaces. Also, tuned superhydrophobic surface showed an excellent self-cleaning property. Further, we demonstrated that these surfaces retain their superhydrophobic nature even after exposure at elevated temperatures (upto 773 K) and on application of mechanical abrasion. Manipulation in different wetting behaviour was possible mainly due to the presence of varying degree of smooth surface as well as micro-pillars, which incorporated the multi-scale roughness to the surface. “Re-entrant” like microstructures such as mushroom, cauliflower and cornet microstructures was observed in the case of tuned superhydrophobic surface, which is well known for achieving the excellent water repellency over the hydrophilic surface.

Keywords: Wettability, Metal, Plasma spraying, Self-cleaning, Robust *Corresponding

Author: Ph: +91-612-3028184. Email address: [email protected] (A.K. Keshri)

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1. Introduction Superhydrophobic surface can be achieved by the introduction of either low surface energy materials and/or suitable nano/micro-roughness, which was first pointed out by Barthlott et al. in 19971. Since then, the principle behind the interplay between surface roughness and surface energy has triggered a scientific boom and was expanded to superwetting states (i.e., superhydrophilic)2-3. Superhydrophilic and superhydrophobic surfaces, since then have gained tremendous interest from scientific communities and industries owing to their fascinating wetting properties. As an example, superhydrophilic surface has intriguing demand in antifogging, heat exchanger, plates for offset printing or in biological systems like cell activity and cell proliferation etc, where water droplet needs to be adhered to the surface4-5. Whereas, superhydrophobic surfaces find its applications as water repellent, oil/water separation, corrosion resistance, self-cleaning, anti-icing and antibacterial surfaces to name a few6-7. To date, several different protocols such as lithography, chemical vapor deposition (CVD), inkjetprinting, etching, electrodeposition, sol-gel, micro/nanoparticles deposition has already been developed for the fabrication of these superhydrophilic and superhydrophobic surfaces individually8-10. Whilst, most of these work strategies provide excellent wetting characteristics, the fabrication of these surfaces involves highly sophisticated technique and multiple steps, thereby limiting them only to laboratory scale preparation. Moreover, majority of these strategies involves polymeric surface modifiers11. The limited mechanical and thermal durability of polymers could destroy the microscopic roughness or remove the hydrophilic/hydrophobic layer upon mechanical or thermal contact, thereby causing a decline in their wetting properties12-13. As a result, only a few commodities prepared through these approaches are available for practical application mainly due to the low scalability, high cost and poor robustness. Furthermore, it is also a challenging task to manipulate the wetting


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behaviour of a surface without the addition of surface modifiers or application of UV light which might limit its application14.

The

Holy

Grail

for

the

design

and

fabrication

of

an

efficient

superhydrophilic/superhydrophobic surface is the achievement of appropriate wetting instantly, by a simple, direct, one-step and scalable technique. In addition, significant interest has also been given to create these surfaces, which can maintain its integrity at harsh environment. Now that, a lot of advancement has taken place in manipulating the wetting characteristics of the surface, researchers are in pursuit of finding a one-step and scalable technique just by changing the process parameters. Of course, emphasis is being put to fabricate thermally and mechanically robust surface. In this effort, Liang et al. attempted manipulating the two extreme end of wetting of metallic titanium (Ti) by single step anodization and fluorination, which they considered to be a facile and simple technique15. However, the major concern of their work is the unavailability of the data regarding the robustness of the fabricated surface. Further, even if they claim their fabrication process is one step, the process involved traditional multi-step process i.e., creating the appropriate roughness followed by the application of surface modifiers. In addition, fluorinated chemicals such as fluoroalkyl-silanes possess potential threat to human health and natural environments16. Therefore, it is highly desirable to synthesize a surface without these modifiers that can work even at harsh environment. Bae et al. presented a one-step method to synthesize a superhydrophobic metallic surfaces from an intrinsic hydrophobic surface (Al 7075 alloy, γ = 30.65 mJ/m2) using scalable wire electrical discharge machining (WEDM) technique17. They were successful in achieving high static water contact angle (CA) of 156° and low water contact angle hysteresis (3°). However, the case becomes extremely difficult if the wetting characteristics has to be manipulated using a starting material, which is intrinsically hydrophilic (150°) just by changing the gas environment from O2 to Ar or H2 during the laser processing18. We applaud the work of Li et. al., however, they choose an exotic material i.e. graphene for manipulating the wetting characteristics which falls behind in scalability and the robustness. Motivated by this, an attempt has been made to manipulate the conventional hydrophilic metallic surface of the coating into all the different states of wetting instantly. We have used industry benign APS technique, equipped with an inert atmosphere shroud to manipulate the surface of the coatings. Process parameters such as type of the primary gas in the plasma as well as application of shroud were varied during the tuning of the surfaces using APS. Emphasis has also been given to investigate the self –cleaning property, thermal robustness (upto 773K) and abrasion resistance of the superhydrophobic surface.

2. Experimental Section 2.1 Tuning the Metallic Surface of the Coatings We procured light-weight aluminium powder (density: 2.6 g/cm3) from the Trixotech Pvt. Ltd. having the purity of 99.99 %, for fabricating the surface of the coating. Spray drying was done to improve the flowability of the powder during plasma spraying. The size of the starting powder prior to spray drying was 0.7±0.2 m. Atmospheric plasma spray (Oerlikon Metco, Switzerland) equipped with a 9 MB gun was used to tune the metallic surface of the coatings. A conventional inert atmosphere shroud was attached in front of the plasma gun for the 4 ACS Paragon Plus Environment

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controlled atmosphere, which is shown in Figure 1. The inert gas, Nitrogen (N2) was fed at a constant flow rate of 40 psi through a coaxial gas shroud (dia: 40 mm) around the plasma plume. It is expected that the shroud (shown as inset in Figure 1) will affect the degree of melting of the particle in the plasma as well as to lower down the oxygen content around the plasma plume. In order to tune the surface of the coating, changing the combination of two key parameters was mainly focussed viz. primary gas of plasma and the application of shroud during spraying. Four different combinations were used to tune the different wetting characteristics, which is as follows. (i) superhydrophilic surface: Nitrogen as primary gas and without shroud, (ii) Hydrophilic surface: Argon as primary gas and without shroud, (iii) Hydrophobic surface: Argon as primary gas with shroud and (iv) Superhydrophobic surface: Nitrogen as primary gas with shroud. These individual combinations will be referred as parameters P1, P2, P3 and P4 and the surfaces fabricated using the parameters were referred as W1, W2, W3 and W4. All the plasma sprayed coatings were deposited on a glass substrate (60 mm × 20 mm × 2 mm). The coatings were well adhered to the glass substrates and has the thickness of 50-70 μm, which is shown in the supporting Figure S1. Prior to the coating, the glass substrates were cleaned thoroughly with acetone in a water bath ultrasonicator. Immediately after the fabrication of these coatings, we executed a simple water repellency test in our laboratory to see the wetting characteristics of the manipulated surfaces and the results were encouraging in term of wetting and de-wetting. A video of this test has been provided in supporting video (Movie S1). Additionally, the excellent water repellent surface demonstrated the complete rolling of water droplets on tilting at certain degree as shown in supporting video (Movie S2).



Figure 1: illustrates the 9 MB plasma gun integrated with an inert atmosphere shroud. Inset shows the top view of the shroud.

2.2 Characterizations Powder morphology and microstructure of all the tuned surfaces were evaluated using a Field Emission Scanning Electron Microscope (FE-SEM) (Zeiss, Sigma HD, UK) operating at a voltage of 10 kV. Phase analysis of the powder was carried out using an X-ray diffraction (XRD) (Rigaku, TTRAX III, Japan) of Cu Kα (1.54 Å) radiation. The surface roughness and 3D optical imaging of all the tuned surfaces were obtained using the Nano Map-D optical profiler (AEP Technology, USA). The surface elemental composition and stoichiometry analysis of all the surfaces were carried out using the X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Prob II, FEI Inc. Japan). Static and dynamic water contact angle of the tuned surfaces were performed at room temperature using 2 L droplets of Milli-Q water on a contact angle goniometer (OCA-35, DataPhysics, Germany). The contact angle measurement was obtained using the circle fitting method. Advancing and Receding contact angles were measured using the adding and suction of water droplets in/from the surface. The highest 6 ACS Paragon Plus Environment

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contact angle achieved during addition of water droplets in the surface was recorded as the advancing contact angle while the lowest contact angle achieved during suction of droplets from the surface was recorded as receding contact angle. The contact angle hysteresis (CAH) is the difference between the advancing and receding contact angle. The sliding contact angles were measured using a tilted base with the addition of 5 L volume of water droplets. All the contact angle measurements were done at 27.9C and 63% humidity.

2.3 Self Cleaning Property, Thermal and Mechanical Durability Test The self-cleaning ability of the surface was demonstrated by the removal of alumina (Al2O3) particles by rinsing the surface with water. To scrutinize the self-cleaning property of the W4 surface, the surface was placed on a petridish at a certain tilt angle. Alumina particle was used as a model contaminant. For comparison, we have also included the self-cleaning ability of an uncoated glass substrate in the supporting video (Movie S6). Water droplets were dropped on the adulterated surface and bare glass substrates using a syringe. For investigating the thermal robustness, the static CA of the W4 surface was measured after the heat treatment at different temperatures between room temperature and 773 K for 30 minutes. The mechanical durability or abrasion test was performed on the W4 surface, according to the method reported in the literature19. The abrasion test was performed by scratching the W4 surface in one direction against the P800 grit size SiC abrasive papers. This test was performed with a varying applied load (0 – 100 g) and the static CA was measured after the ten numbers of abrasion cycles.

3. Results and Discussion Figure 2a shows the low magnification FE-SEM images of the spray-dried Al powder, while Figure 2b illustrates the corresponding high magnification images of a single spray dried Al particle. It is observed that all the particles were spherical in structure. Particle size distribution 7 ACS Paragon Plus Environment


of the spray dried Al powder is shown as histogram (Figure 2c), which shows the particles has wide size variation i.e., 10-100 μm. Inset shows the cumulative powder particle size which illustrates that D50 is 26μm and D90 is 72μm. Hence, this powder can be referred as dual-size powder. Figure 2d shows the X-ray diffraction pattern of spray dried Al powder showing sharp crystalline peaks of cubic structure Al (JCPDS Card No. 98-004-3423). Absence of any other foreign peaks other than Al peaks, suggest that the powder was free from any contamination.

Figure 2: (a) shows the low magnification FE-SEM images of the spray dried Al particle and (b) its corresponding highly magnified images of the single spray dried Al particle (c) Particle size distribution graph of the spray dried Al Powder, inset shows the cumulative particle size distribution curve showing D50 and D90 value of powder particles, (d) XRD spectra of the spray dried Al Powder. 8 ACS Paragon Plus Environment

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Figure 3a shows the static contact angles for all the surfaces prepared under different condition, while the inset shows their respective contact angle hysteresis. The mean CA of the water droplets on the surface of the coatings were determined by measuring them at different positions of the surface. As observed from Figure 3a, the manipulated surfaces of W1, W2, W3 and W4 showed CA of 0˚, 19.6˚, 97.6˚ and 156.4˚ respectively. The extremely low CA of W1 surface prepared using process parameter P1 confirms the superhydrophilic nature of the surface. It can also be seen in supporting information (Movie S3) that the water droplet immediately spreads and completely wets the surface of W1. Slight increase in contact angle of the W2 surface (19.6˚) compared to W1 (0˚) was achieved by using process parameter P2, which shows the hydrophilic nature of the surface. In contrary, the surface which was tuned using process parameter P3 showed relatively higher CA of 97.6˚. Additionally, the surface also showed higher CAH (56.6˚), indicating the hydrophobic nature of the surface. The presence of higher CAH in W3 surface might be due to the pinning effect, which has been discussed later in this paper. More interestingly, when the surface prepared using plasma parameter P4, the CA reached as high as 156.5˚. In addition, the surface also demonstrated very low CAH (1.4˚) and sliding angle (~5˚), representative of an excellent superhydrophobic nature of the surface, similar to a lotus leaf. This shows the instant tuning of the wetting characteristics in all four coatings.

The stability of surfaces showing water repellency (i.e. W3 and W4) were evaluated with the function of time. Figure 3b shows the dynamic CA of surfaces for nearly 120s and the inset shows the digital images of water droplets over the surface after the test. Slight decreasing trend in the dynamic CA of W3 surface was observed, which hints to the de-pinning of water



droplet with course of time. On the contrary, the water droplet over W4 surface maintains almost constant CA throughout the test for 120s.

Figure 3: (a) Static contact angle of the four different surfaces, i.e., W1, W2, W3 and W4, while inset shows their respective contact angle hysteresis. (b) Dynamic contact angle of W3 and W4 surface, with the digital photographs of the water droplets over the coatings after 2 mins as inset.


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Again, in order to reconfirm the nature of the hydrophobic and superhydrophobic surfaces, water bouncing experiment from a certain height (8 mm) was conducted over the tilted surface at 3˚. The water droplet falling on W3 surface, adheres to the surface immediately with CA greater than 90˚ as shown in supporting video (Movie S4), representing a sticky hydrophobic or a typical case of Wenzel state20. On the other hand, Figure 4 shows the step by step bouncing of the water droplet over W4 surface with the function of time, showing excellent water repellency. The droplet bounces off elastically without leaving any residual traces of droplets on the surface and ultimately comes to rest, maintaining CA of above 150˚, shown in supporting video (Movie S5).

Figure 4: Series of snapshots depicting a water droplet (5μL) bouncing on the W4 surface with function of time.

Now, we demonstrate the ability of this superhydrophobic surface as a potent self-cleaning surface. Figure 5a-c demonstrates the sequential photographs of the droplets, showing the cleaning of nearly all the dust particle from the path of the rolling water droplet. Subsequent water drops clean the surface thoroughly, without leaving any trace of water droplets or dirt over the surface as demonstrated in the supporting video (Movie S6), showing an excellent self-cleaning behaviour of the surface. This visual evidence of the self-cleaning action suggests its potent applications as self-cleaning coatings in cements, textiles industries, blades of wind turbine, steam turbine and condenser tubes of thermal and hydro power plants.



Figure 5: (a-c) sequential photographs of the droplets showing the self-cleaning of dust from the superhydrophobic surface (W4).

Above findings indicates the manipulation of different wetting characteristics of the surface as well as self cleaning properties is possible just by changing the combination of plasma parameters. Now, in order to be industrially acclaimed, these manipulated surfaces should retain its integral wetting characteristics even at harsh environment, where the polymer modified metal and ceramic fails to satisfy. In order to demonstrate the robustness, the manipulated superhydrophobic surface was exposed at three different temperatures starting from room temperature (RT) to 773K. It can be seen from Figure 6a that the superhydrophobic surface displays almost constant contact angle even after exposure at 773K. The inset in Figure 6a shows the digital images, illustrating the water droplets over the surface immediately after exposing the surface at the different temperatures scale. Further, the mechanical durability was also analysed by an abrasion test. We found that the surface displays an almost constant contact angle upon mechanical abrasion and spherical water droplets are still observed on the abraded surface as shown in Figure 6b.


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This represents that our manipulated surface has the potential to retain their water repellency even after the exposure at harsh thermal and mechanical environment. It can be recalled that the wettability of any surface is governed either by its surface energy (chemistry) and/or the surface roughness of the material1. Since time immemorial, there has been debate regarding the intrinsic hydrophobicity of metals21. However, subsequent studies have proven that metals are intrinsically hydrophilic but rapidly adsorbs hydrocarbon from atmosphere, resulting in an increased CA with respect to time. For example, Li et al. deposited nickel-chromium (Ni-20Cr) coating by plasma spraying which showed superhydrophilic nature immediately after the fabrication of the coating22. However, the same coating showed superhydrophobicity after exposing it to ambient air for 60 days. They concluded that the airborne hydrocarbon gets adsorbed over the surface which eventually leads to the transformation of superhydrophilic to superhydrophobic surface.



Figure 6: Static contact angle after (a) heat treatment of W4 surface at varying temperature, i.e. RT, 373K, 573K and 773K and (b) abrasion testing of W4 surface by varying the load 0g, 20 g, 50g and 100g (referred to as 0G, 20G, 50G and 100G) using SiC P800 grit paper.


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However, our manipulated surface showed different state of wetting, immediately after fabrication. Nonetheless, we performed XPS analysis to see any contribution of surface contamination/hydrocarbon towards the wetting behaviour in our tuned surfaces. Representative XPS spectra of all the surfaces are presented in Figure 7. Photoelectron peaks of Al, C and O are detected on the all manipulated surface. The XPS spectrum reveals presence of carbon peak in all the surfaces. This peak is often referred to “adventitious carbon” peak, originating due to the adsorption of hydrocarbons on the surface. This “adventitious carbon” is often regarded as the source of hydrophobicity upon exposing a hydrophilic surface to atmosphere for an extended duration of time21. Preston et al. have shown that gold, which is also an intrinsically hydrophilic material, contains ~25 at.% carbon after 1 hour exposure in laboratory atmosphere 21. However, these accumulated surface hydrocarbons did not affect the CA of the metal. This refers that surface has to be exposed for longer time to show the hydrophobic character. In the present study, the manipulated surface demonstrated the wetting characteristics instantly after the fabrication of the coatings. Additionally, the superhydrophobic surface demonstrated no change in CA even after thermal treatment in oxidizing atmosphere at 773K for 30 min. It has previously been shown that heating a hydrocarbon contaminated surface in an oxidizing environment removes adsorbed hydrocarbons without affecting the surface roughness. Therefore, it is obvious that that the wetting behaviour of our surface is not governed by the adsorption of surface hydrocarbon. Additionally, we have also deconvoluted the Al 2p spectra and quantified the percentages of Al, Al-O and Al-N (see supporting information Table-S1) to understand the effect of an inert atmosphere shroud on the surface chemistry of the surfaces, which will be discussed in later section.



Figure 7: XPS spectra of all the tuned surfaces showing the photoelectron peaks of Al 2p, Al 2s, C 1s and O 1s.

Figure 8a-b shows the optical profiling (OP) images and the respective FE-SEM images of the W1 surface respectively. Two distinct features i.e., smooth region and randomly oriented micro-pillars are visible in the OP image of the surface (Figure 8a), which was manipulated using process parameter P1. Formations of both these features are mainly responsible for the multi-scale roughness in the surface. The manipulated surface shows root mean square (rms) roughness in the range of 0.9150.40 m, which arose due to both the features. Similar characteristics of smooth region and micro-pillars were also seen in the FE-SEM image of the coated surface (Figure 8b). Smooth region is the result of melted particles in plasma, while the micro-pillars (17%) are the result of un-melted particles. Since, the powder particles are of dual-size range, melting of the 10-50 m sized powders will be relatively easier and form the smoother region. However, the larger particles (50-90 m) will undergo partial melting due to temperature gradient inside the particle. Since, plasma spray is a layer-by-layer deposition 16 ACS Paragon Plus Environment

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technique, these un-melted particles will keep depositing one over another and form the micropillars or hillocks. Hence the population density of these micro-pillars will be directly related to the increase in the amount of un-melted particles in plasma and it will be higher for increased amount of un-melted particles. Now, the extremely low CA of this surface can be explained using the Wenzel model23. This model predicts that both hydrophilicity and hydrophobicity can be rendered on to a surface by introducing roughness, according to the following relation. 𝐶𝑜𝑠 𝜃𝑟𝑜𝑢𝑔ℎ = 𝑟 𝐶𝑜𝑠 𝜃𝑓𝑙𝑎𝑡

(1)

Where, 𝜃𝑟𝑜𝑢𝑔ℎis the apparent CA on a rough surface, 𝜃𝑓𝑙𝑎𝑡 is the ideal CA on a flat homogeneous surface and r is the roughness factor, which is defined as the ratio of actual surface area over projected area. It is evident from the relation that for an intrinsically hydrophilic material introduction of surface roughness will lead to the decrease in the apparent CA of the surface. We measured the contact angle of grinded and polished flat aluminium surface, which was found to be 51.6 (i.e., 𝜃𝑓𝑙𝑎𝑡). (More details are shown in supporting information as Figure S2). Apart from this, the deconvoluted Al 2p XPS result revealed the presence of high surface energy compounds AlN (γ ≈ 1000 mJ/m2)24 and Al2O3 (γ ≈ 970 mJ/m2)25 on the surface, which is shown in Figure 8c. This might be the additional reason for achieving the extremely low CA (close to 0˚).

Figure 8d shows the OP image of the W2 surface manipulated using process parameter P2. In this case, Ar was used as the primary gas instead of N2, while the remaining parameters were maintained constant as in process parameter P1. The surface of the coating prepared using process parameter P2 also showed two distinct features i.e., smooth region and randomly oriented micro-pillars. The rms roughness of this manipulated surface is 0.8260.04 m. Similar characteristics of micro-pillars and smooth surface were also seen in the FE-SEM 17 ACS Paragon Plus Environment


image of the surface (Figure 8e). However, we attribute the slight decrease in roughness to the use of Ar as primary gas. Since, Ar is a monoatomic gas, it requires lesser energy to ionize and forms plasma26. Hence, in this case, a large amount of thermal energy is available, compared to N2, to melt the powder particle, which assists the formation of smooth surface with relatively lower density of micro-pillars (12%), and hence lower surface roughness. However, unlike W1, which is superhydrophilic, the CA of W2 is in the hydrophilic regime (~19.6). This was attributed to the slight decrease in surface roughness. Though, presence of lower intensity peak of Al2O3 is seen over the surface (as shown in Figure 8f), the complete absence of high surface energy compound AlN is also an additional reason for the hydrophilic surface

Now, in case of W3 surface, changing the process parameter, i.e., using Ar as primary gas, along with an inert atmosphere shroud resulted in an exponential increase in the number of micro-pillars. Figure 8g demonstrates the OP image indicating higher population density of micro-pillars (40%) compared to W1 and W2 surfaces. As a result, this surface also showed relatively higher root mean square (rms) roughness (1.030.01 m). We attribute the increase in number of micro-pillars to the use of (i) inert atmosphere shroud and (ii) dual-size powder particles. The inert atmosphere shroud contributes in significantly lowering down the plasma plume temperature. Additionally, the shroud was designed in such a way, that the powder particle is fed perpendicular to the plasma plume and away (~25 mm) from the nozzle exit, where the temperature is almost 2000 K-3000 K lesser than that at the nozzle exit. Drop in temperature due to the usage of shroud has been explained using the schematic which is shown in supporting Figure S3. Now again, since the powder size distribution is also higher, it is anticipated that the powder particles with larger size will not melt uniformly and hence this will be involved in forming the micro-pillars. The formation of these micro-pillars gives rise to a hierarchical roughness, as shown in Figure 8h. In addition, the deconvoluted Al 2p XPS 18 ACS Paragon Plus Environment

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result revealed the presence of high surface energy compounds AlN and the lower intensity peak of Al2O3 on the W3 surface which is shown in Figure 8i. Even though the high surface energy compound i.e. AlN is present over this surface, the hydrophobicity (WCA > 90˚) of this surface was dominated by the presence of the hierarchical micro-pillars, where the droplets was getting pinned. It is assumed that the liquid fills up the space between the protruded micropillars on the surface and demonstrates higher CAH (56.6˚) due to such pinning.

Figure 8j shows the OP image of the W4 surface fabricated using process parameter P4, where N2 was used as the primary gas along with the use of an inert atmosphere shroud, while remaining other parameters were kept identical. The population density of micro-pillars (82%) in this case is higher compared to W3 and has higher root mean square (rms) roughness (1.33  0.24 m). We attribute the formation of these highly dense micro-pillars to the following parallel phenomenon, i.e., use of (i) N2 as primary gas, (ii) inert atmosphere shroud and (ii) dual-size of the powder particles. FE-SEM image (Figure 8k) further illustrates the visual evidence of higher number of micro-pillars with no smooth surface. These micro-pillars are hierarchical and replicate the behaviour of naturally obtained superhydrophobic material like lotus leaf microstructures patterns. However, investigation of the Al 2p peak revealed the presence of AlN over the surface (Figure 8l). AlN, being a higher surface energy compound, is expected to render hydrophilicity to the surface. However, interestingly, the surface showed very high CA in the superhydrophobic regime (i.e. 156.5).



Figure 8: (a-b) shows the 3D optical profiler images of W4 surface and its corresponding FESEM and (c) deconvoluted Al 2p peak of the W4 surface showing the Al-N and Al peaks.

Therefore, in order to investigate the reason behind the superhydrophobic behaviour of the surface, the morphology of the micro-pillars was studied. High magnification FE-SEM images of these micro-pillars (Figure 9a-c) revealed an interesting set of microstructures, similar to mushroom, cauliflower and cornet shape. Tiny irregular-shaped particles (i.e. nanograins) in 20 ACS Paragon Plus Environment

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nanometer range were observed over each micro-pillars, which is shown in Figure 9a-c. These nanograins over the micro-pillars gave rise to hierarchical structures, which is known to be favourable in forming a stable superhydrophobic surface27. It is to be noted that these hierarchical microstructures were absent in the W1 surface (See supporting Figure S4). In the case of W4, (Nitrogen as primary gas; with shroud), the powder is being fed far away from the nozzle exit (as explained previously for the W3 coating), which results in drop in temperature. In addition, the shroud nitrogen gas also lowers down the temperature of the flame and this will lead to the increase in amount of un-melted particle in the plasma. We attribute the formation of the special nanograins to the splashing of the fully melted particles in the nanoregime over the islands formed by the un-melted micron sized particles. These special kinds of micro-structures were fabricated by other researchers and known as “re-entrant geometry”, which confer superhydrophobic properties to hydrophilic surfaces27-30 by hanging the water droplet between these special kind of microstructures. It is worth mentioning here that these re-entrant structures were fabricated using sophisticated and multi-step technique. However, we have managed to achieve the same using a simpler and industry benign technique. The role of these re-entrant with hierarchal microstructures in conferring superhydrophilic property to an intrinsically superhydrophilic surface can be explained by the schematic, as shown in Figure 9d. Figure 9d represents a water droplet sitting over the surface containing the several re-entrant structures (Mushroom, cauliflower, and cornet). Firstly, these re-entrant microstructures acts as a solid overhangs, which assist the water droplet to remain suspended, without piercing these overhangs. In addition, nanograins will act as air-trapping pocket between the droplet and solid overhangs. This results in the water droplets sitting on a composite surface of air and coating material and hence, makes the Cassie–Baxter model 31(Equation 2) valid for these structures 𝐶𝑜𝑠 𝜃𝑟𝑜𝑢𝑔ℎ = ∅𝑠 𝐶𝑜𝑠 𝜃𝑓𝑙𝑎𝑡 ― (1 ― ∅𝑠)


(2)


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Where 𝜃𝑟𝑜𝑢𝑔ℎ is the apparent contact angle on a rough surface, 𝜃𝑓𝑙𝑎𝑡 is the ideal contact angle of water on a flat/smooth homogeneous surface and ∅𝑠 is the area fraction of the solid surface in contact with liquid. Therefore, the hierarchical structures, otherwise known as re-entrant has been shown to be beneficial in forming a stable superhydrophobic surface with an excellent water repellence capability. Hence, the present work showed that it is well possible to manipulate the conventional hydrophilic metallic surface into all the different states of wetting instantly by the simple, direct, one-step and scalable plasma spraying technique. The superhydrophobic

surface

showed

excellent

water

repellency,

self-cleaning

and

thermal/mechanical robustness, which could find its application in several industries. More work is in progress in order to see and evaluate its compatibility in medical applications and in the highly corrosive environment.


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Figure 9: (a-c) high magnification FE-SEM image showing the presence of Re-entrant geometry structures with hierarchical micro-pillars and (d) schematic illustrates the drop of water in contact with these special kinds of micro-pillars.

4. Conclusion: In conclusion, we have demonstrated the tunable wetting characteristics of the surface of the coatings, by the industrially benign atmospheric plasma spraying technique. Use of Argon as primary gas leads to the water contact angle 0⁰, which switched to 19.6⁰ upon changing the primary gas from argon to nitrogen. Now, on the application of inert nitrogen shroud as well 23 ACS Paragon Plus Environment


as argon as primary gas, surface showed the contact angle 97.6⁰, which indicates the hydrophobicity of the surface. Surface turns to superhydrophobic (i.e. contact angle 156.5⁰) by just using the nitrogen as primary gas and the inert nitrogen shroud. Different kind of wetting characteristics was mainly attributed to the presence of smooth region and micro-pillars, which resulted the hierarchical structure over the surface. Special kind of re-entrant geometry like cauliflower, mushroom and cornet shape was an additional factor for achieving the superhydrophobic surface. Excellent self-cleaning property was observed over the superhydrophobic surface. Further, there was no change in contact angle upon exposing the superhydrophobic surface at a harsh environment of elevated temperature (upto 773 K) and mechanical abrasion. It is further envisioned that this technique presented here will find many uses in preparing superhydrophilic/ superhydrophilic metallic surfaces for more diverse applications with harsh environments.

Conflict of Interest The authors declare no conﬂict of interest.

Supporting Information Coatings cross sectional images as well as temperature distribution in shroud plasma spray has been included

Acknowledgements Authors O.S. Asiq Rahman and Anup Kumar Keshri acknowledges the financial support provided by the Indian Institute of Technology Patna. Aminul Islam acknowledge the financial support from DST, Government of India, Grant No. DST/TSG/AMT/2015/264. Authors also


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acknowledges the Advanced Centre for Material Science, IIT Kanpur for the XPS characterization.

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