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1 Superamphiphobic aluminum alloy surfaces with micro and nanoscale hierarchical roughness produced by a simple and environmentally friendly technique Zubayda S. Saifaldeen, Khedir R. Khedir, Mehmet F. Cansizoglu, Taha Demirkan & Tansel Karabacak Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN DOI /s x 1 23

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3 DOI /s x Superamphiphobic aluminum alloy surfaces with micro and nanoscale hierarchical roughness produced by a simple and environmentally friendly technique Zubayda S. Saifaldeen Khedir R. Khedir Mehmet F. Cansizoglu Taha Demirkan Tansel Karabacak Received: 16 July 2013 / Accepted: 8 November 2013 Ó Springer Science+Business Media New York 2013 Abstract In this study, superamphiphobic (SAP) metallic aluminum (Al) alloy 2024 surfaces were produced with water and oil contact angles (CAs) of more than 150 and sliding angles of less than 10. The two simple and environmentally friendly techniques of mechanical sanding and boiling water treatment were used to introduce micro and nanoscale roughnesses, respectively, which resulted in a hierarchical morphology. Surface energy of the rough surfaces was reduced by coating them with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane agent. SAP property was absent for samples with micro or nanoroughness only, and it emerged only after both kinds of roughnesses were introduced. The highest CAs approaching 158 for water, 156 for ethylene glycol, and 154 for peanut oil were obtained after forming hierarchical structures involving shapes of microgrooves obtained by one-directional sanding and nanograss by immersing in boiling water for 1 min. The effects of the two approaches of random and onedirectional sanding using various sandpaper grit sizes, and different time periods of treatment with boiling water on the wettability of surfaces were also investigated. In addition, fundamental wetting models were used to explain the experimental results obtained. SAP Al surfaces could see a wide range of applications in fields such as selfcleaning, anti-icing, anticorrosion, oil transportation, energy harvesting, and microfluidics. Z. S. Saifaldeen K. R. Khedir (&) M. F. Cansizoglu T. Demirkan T. Karabacak Department of Applied Science, University of Arkansas at Little Rock (UALR), Little Rock, AR 72204, USA krkhedir@ualr.edu Introduction Solid surfaces that can strongly repel both water and oils are known as superamphiphobic (SAP) surfaces. When a solid s surface only super-repels water, it is called superhydrophobic (SHP), while those that can super-repel only oils are known as superoleophobic (SOP). Thus, SAP property is the combination of both SHP and SOP properties. Such surfaces show liquid contact angles (CAs) of more than 150 and sliding angles (SAs) of less than 10 for both water and oils. The key parameters in developing such surfaces are as follows: (1) surface roughness with micro, nano, or hierarchical-scales having both, (2) low surface energy, and (3) re-entrant structures. In the case of SHP property, the first two parameters are adequate, while for imparting SOP property, the last parameter is also crucial [1 5]. The wettability of the flat-solid s surfaces described by Young s model is represented in the liquid s intrinsic CA (h) at the point of intersection between the three phases of solid liquid vapor as follows [6]: h ¼ cos 1 c sv c sl c lv ð1þ where c is the interfacial surface tension, and s, l, and v stand for solid, liquid, and vapor phases, respectively. According to Young s model, when a specific liquid is dispensed on a flat-solid s surface depending on surface tensions of both solid and liquid, the liquid will spread over the surface minimizing the solid s surface energy until thermodynamic equilibrium is reached at the meeting point between the three phases of solid liquid vapor. Therefore, the decrease in liquid s surface tension c l will lead to more spreading or a decrease in its intrinsic CA of h, while the solid s surface energy is constant. On the other hand, when

4 a solid s surface energy c s increases, the liquid s intrinsic CA h decreases accordingly. It is well known that surface roughness amplifies the surface wetting properties of a corresponding flat-solid s surface. The liquid s interaction with rough solid s surfaces in terms of its CA or apparent CA (h * ) is described independently by both Wenzel and Cassie Baxter models. The Wenzel model, also known as the homogeneous wetting (solid liquid) model, describes the state of wetting when a liquid invades the rough structures and comes fully into contact with a solid s surface. This model is the function of both the solid s surface intrinsic CA h and surface roughness (r) [7]: h ¼ cos 1 ðr cos hþ ð2þ According to the Wenzel model, Eq. (2), for liquids with intrinsic CA of h \ 90, the roughening of the solid s surface will enhance liquid spreading (h * \ h), while for liquids with h [ 90, roughness enhances surface repellency toward the liquids (h * [ h), all multiplied by the magnitude of surface roughness. Composite wetting (solid vapor liquid) of rough solid s surfaces, in which vapor pockets are trapped between the gaps in rough structures, is described by the Cassie model. It is the function of both the solid s intrinsic CA h and the solid s fraction area (f) in contact with the liquid and is formulated as follows [8]: h ¼ cos 1 f 1 þ r f cos h 1 ð3þ where r f represents the roughness on the top of rough structures of solid s fraction area f. Unlike the Wenzel model, the apparent CA h * for liquids with intrinsic CA of h \ 90 also increases if the composite condition of wetting is achieved. In terms of liquid s adhesion to rough solid s surfaces, the Wenzel wetting state promotes high adhesion to liquids, also known as the adhesive or sticky state of wetting, due to the increase in solid s area that touches the liquid and the pinning of the liquid s contact line between the gaps. On the other hand, the Cassie wetting state is considered a nonadhesive or slippery state of wetting due to the decrease in solid s surface area in contact with liquid and the bridging of the liquid s contact line over the top of rough structures. Therefore, for the fabrication of SAP surfaces with high apparent CA and low SA, the adoption of the Cassie state of wetting is required for both water and oil. The transition from the Wenzel to the Cassie state is examined by equating their corresponding Eqs. (2, 3) for the critical value of the intrinsic CA (h c )[4]: h c ¼ cos 1 f 1 ð4þ r f Because (r [ 1 [ f), the transition will occur only if (h c [ 90 ), and this is the reason that oils or low surface tension organic liquids with intrinsic CA of (h \ 90 ) are not anticipated to transition into the Cassie state with h * C 150 and low SA. However, it has been reported that even the waxy material on a lotus leaf is weakly hydrophilic (h * 74 ), and still the plant s hierarchical rough surface manages to exhibit SHP property [9]. This intermediate state of wetting is known as the Cassie metastable state that arises from the re-entrant or overhang design of rough structures that promote localized CA less or equal to the liquid s intrinsic CA on the side wall of structures. The metastable Cassie state is always prone to transition to the Wenzel state depending on the energy barrier between the two states of wetting compared to its robust counterpart of thermodynamically stable Cassie state [10 12]. The first but premature attempt to develop SOP metallic surfaces was carried out by Shibuichi et al. [13, 14], at the same time that SHP surfaces were reported in 1997 [15]. Then, surprisingly, the fabrication of SOP surfaces underwent a kind of hibernation period for almost a decade due to several limitations in developing such surfaces. One of the major challenges in developing SOP surfaces is due to the low surface tension of oils (\35 mn/m), where weak cohesion forces among oil molecules can be easily overcome by the adhesion forces at the liquid solid interface. Consequently, the oil wets the solid surfaces coated even with the lowest known surface energy materials and shows CA values of less than 90, which makes the surface oleophilic. However, on the same surface, water droplets can show a hydrophobic property with CA angles [90 and reach as high as angles of about 120 on a flat surface [16] or up to about 180 for a roughened substrate [17]. Hydrophobic or superhydrophobic property in these studies was achieved by surface morphology engineering and due to water s high surface tension of 72 mn/m. Thus, with the two parameters of an optimal roughness and low surface energy materials, SHP surfaces can be produced. A breakthrough in the fabrication of SOP surfaces was achieved in 2007 by Tuteja et al. [2]. In their systematic study, the authors showed that re-entrant or over-hang surface structures are critical and act as the third key prerequisite for introducing a SOP property. Thereafter, efforts to fabricate SAP surfaces increased. Over the past few years, a number of SAP metallic surfaces utilizing hierarchical roughness have been reported [18 24]. It was proven that closed boundary hierarchical structures can also provide re-entrant properties due to the existence of nanoscale structures on the top and side walls of micro structures. More details about the design and potential applications of SAP surfaces can be found in the following literature reviews [25, 26].

5 Aluminum (Al) and its alloy metallic surfaces are widely used in industry due to their light weight and mechanical robustness. Imparting SAP property to Al and its alloys would significantly enhance and widen its range of applications. Recently, few studies have addressed the fabrication of SAP Al surfaces using various techniques. Meng et al. [20] have treated Al and some other metals in perfluorocarboxylic acid for days to develop SAP surfaces with water and oil CAs of around 150. In another study by Ohkubo et al. [27], SHP (CA *158 ) but highly oleophobic (CA *140 ) Al surfaces were developed by combining both the sandblasting technique via single grain of #30 for imparting micro roughness and post electrolyte etching to obtain nanoscale roughness. Yang et al. [24] combined both chemical etching and post treatment with boiling water for 20 min, and SAP Al surfaces with water and oil CAs of and 157.6, respectively, were obtained after immersion of rough surfaces in perfluorooctanioc acid. Anodization has also been used by Wu et al. [21] to create alumina nanowires; after modification with perfluorosilane, SAP Al surfaces with water CA of and rapeseed oil CA of 155 were obtained. Very recently, Ji et al. [23] have combined both the grinding of Al surfaces via grit sizes of 400, 1000, and 2000 and post acid etching to develop SAP Al surfaces. After surface chemistry modification of the obtained hierarchical surfaces, SAP surface with water CA [ 160 and hexadecane CA *150 for 1000 grit size and 10 min etching was obtained. For the introduction of hierarchical roughness, chemical etching is one of the most common techniques used in the above mentioned studies. However, chemical etching can involve acidic or toxic solutions, which lead to a higher cost of fabrication and hazardous conditions. A combination of two simple and environmentally friendly techniques such as mechanical sanding and treatment with boiling DI water to introduce a hierarchical surface with both micro and nanoscale roughness characteristics has not been reported yet. In addition, the effects of various sandpaper grit sizes, isotropy/anisotropy of roughness, and various time periods of treatment with boiling water on the water and oil CA have not been investigated. In recent years, a few studies have used mechanical sanding to roughen solid s surfaces for modifying their wetting properties [23, 27, 28]. Mechanical sanding as a simple and environmentally friendly technique that has been used for centuries primarily for smoothening surfaces, as well as roughening, has been overlooked in spite of its potential in roughening surfaces at micro and submicron scales. In addition, surfaces were usually randomly sanded, and the effects of directional-sanding as well as sandpaper grit sizes on the shape of microstructures have not been addressed yet. Treatment of Al and its alloy (Al-alloy) with warm DI water and the formation of a nanograss -like structure on their surfaces has been previously investigated [29 32]. The technique was originally utilized to enhance the bond strength of epoxy Al joints as well as their interfacial adhesion. The production of nanoroughness on Al and Alalloy with a simple technique of immersion in warm DI water has only recently received attention for modifying their surfaces wetting properties. Jafari and Farzaneh [33] have reported SHP Al alloy 6061 surfaces with water CA as high as 164 and SA as low as 4 after immersion in boiling water for less than 5 min and post sputtering with Teflon. Zhou et al. [34] treated Al surfaces with hot water at 90 C for various times; then the surfaces were coated with PFDTS agent. As a result, SHP surfaces were obtained with a high CA of 166 and low SA of 3 for the sample immersed in hot water for 1.5 h. Hozumi et al. [35] have developed SHP and also oleophobic Al surfaces with water and hexadecane CAs of 170 and 117, respectively, after their treatment with boiling DI water and subsequent surface chemistry modification. In this study, simple, industrially scalable, and environmentally friendly techniques of mechanical sanding and treatment with boiling DI water were combined to engineer SAP Al alloy 2024 surfaces. The surface energy reduction was carried out by exposing the surfaces to vaporized 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFDTS) agents inside an oven. SAP properties with the highest CA approaching 158 for water, 156 for ethylene glycol, and 154 for peanut oil were obtained only after forming hierarchical structures involving shapes of microgrooves obtained by one-directional sanding and nanograss by immersing in boiling water for 1 min. The effects of two approaches, random and one-directional sanding using various sandpaper grit sizes, and different time periods of treatment with boiling water on the wettability of surfaces were also investigated. In addition, fundamental wetting models were used to explain the experimental results obtained. Materials and method Schematic illustration of the experimental procedure is presented in Fig. 1. Commercially available Al alloy 2024 was mechanically sanded to introduce microroughness using sandpapers with grit sizes of 36, 60, 120, 400, and The sanding process was carried out for 20 s with an applied pressure of 3.0 ± 0.2 kg/cm 2. To investigate the effect of isotropy-sanded topography, two different sets of microroughness samples were prepared by random and one-directional sanding of the surfaces. To introduce nanoroughness, first, Al alloy surfaces were polished with

6 Fig. 1 Schematic representation of the experimental procedure used to introduce a microroughness, b nanoroughness, and c hierarchical surfaces having both micro and nanoroughness. Images are not up to scale sandpapers of large grit sizes of 1500 and 3000, respectively, for removal of surface contaminants; next, the flatpolished surfaces rinsed with DI water were immersed in boiling DI water 1, 5, 20, 60, and 120 min. The hierarchical structures, which have micro (by sanding) and nanoroughness (by boiling), were engineered by first sanding the surfaces with grit sizes ranging from 36 to 1000, and then immersion in boiling DI water for different times of 1, 5, and 120 min. Ultimately, the roughened surfaces were ultrasonicated with acetone, isopropanol, and DI water for 10 min each and purged with nitrogen gas. For their surface energy reduction, roughened alloy samples and 5 % of PFDTS solution of toluene were put in a sealed bottle inside an oven at 90 C for 3 h. Then, the chemically modified rough Al alloy surfaces were left in an ambient environment for more than 12 h to be completely dried. The surface morphology of the prepared surfaces was characterized using scanning electron microscopy (SEM, JEOL 7000F). For wetting analyses, CA measurements were carried out using a VCA Optima goniometer. Liquid droplets of 5 ± 1 ll size of three liquids DI water, ethylene glycol, and peanut oil were gently dispensed on the surfaces. The captured digital images were then analyzed for their CA measurements using the Sessile Drop method available with the software. The sliding angle over the prepared hierarchical surfaces for the three liquids with droplets of 25 ± 3 ll in size was measured using a custom-made setup. First, the droplet was gently dispensed over the surfaces, and then the surfaces were tilted until the droplet started to slide. The tilting angle of the surface at which the droplet started to slide was considered the SA of the liquid. More than three pristine samples for each roughening condition were tested to determine the statistical error of the CA and SA data. Results and discussion Developing micro, nano, and hierarchical-roughness Al alloy surfaces The surface morphology of Al alloy surfaces immersed in boiling DI water for various durations are shown in Fig. 2b f. The increase in time from 1 to 120 min of immersion caused a slight increase in the nanograss structural thickness starting from less than 10 nm for 1 min and reaching to *20 nm after 120 min of immersion. However, a significant increase in density of the nanostructures can be noticed. As the boiling time is increased, a denser, more interconnected network structure forms and layer porosity of the nanograss surface decreases. Structures of nanograss obtained at 1 min of immersion (Fig. 2b) are like nanopores with thin layer boundaries with a thicker outer layer at the top of pores. After 5 min of treatment with boiling DI water, separate overlapping nanograss structures are developed. After 120 min of immersion (Fig. 2f), the film density significantly increased compared to 1 min of immersion (Fig. 2b) with a consequent increase in the overlapping of nanostructure that

7 Fig. 2 Top-view and associated side-view SEM images of the aluminum alloy surfaces for a polished flat, and treated with boiling DI water for different periods of b 1, c 5, d 20, e 60, and f 120 min causes a decrease in film porosity. The porosity of 60 % for the first minute of immersion reduces significantly to approximately 10 % after 40 min of immersion time [30, 32]. Thus, the duration of the immersion time produces a significant variation in the morphology of the nanograss surfaces. The detailed mechanism of the formation of this interesting nanoscale roughness is still not clear. It is believed

8 that, after immersing Al or Al-alloy in warm DI water, the intrinsic oxide layer on the surface reacts with water, becomes hydrolyzed, and dissolves to be later precipitated on the Al surface [29]. The film undergoes a morphological change with very rapid growth for the first few minutes followed by a slow growth to form nanograss roughness. The film has been identified as a low-crystal quality boehmite (a-al 2 O 3 H 2 O) for DI water temperatures above 75 C and bayerite (b-al 2 O 3 3H 2 O) when the DI water temperature is below 75 C [36]. The physical property of this hydrous alumina film has been extensively studied by both Vedder et al. [29] and Alwitt [30]. It was shown that the film thickness increases with the increasing immersion time, but the trend of increase is not linear. Its thickness increases by only 20 % after 40 min of immersion compared to its initial thickness after the first min of immersion. The side-view SEM images associated with each topview SEM image depicted in Fig. 2 show that the thickness of the nanograss layer is approximately 200 nm for all boiling times, which indicates that rapid vertical growth occurs within the first few minutes of immersion in boiling DI water. The reported thicknesses in the literature using indirect techniques have been estimated differently ranging from 200 nm to more than a micrometer. Rider et al. [32] investigated the treatment of the Al alloy used in this study with boiling water for time periods ranging from 0.5 to 240 min. In their study, the film thickness of *200 nm for the sample treated with boiling DI water for 15 min was estimated using a sputter-depth profile method. In addition, our SEM images also show that lateral dimensions of the individual nanostructured features continue to increase even at later stages of growth. Figure 3 shows SEM images of hierarchical samples having both micro and nanoroughness. Microroughness topographies were obtained either by random-directional sanding (Fig. 3a f) or by one-directional sanding (Fig. 3g l) followed by immersion in boiling DI water for 1 min. Figure 3a e shows hierarchical structures introduced by first random-directional sanding with various sandpaper grit sizes followed by immersion in boiling DI water for 1 min to form the nanograss layer. As can be seen, the surfaces microstructures involve valleys and elongated hills, which are randomly oriented at large scales but can have a slight anisotropy at smaller scales. Typical sizes of the imparted microstructures decrease from several tens of micrometers down to few micrometers as the sandpaper grit size is increased from 36 to This is expected due to the decrease in average particle size from *700 to *10 lm on the surface of sandpaper as the grit size increases from 36 to 1000 (Fig. 4a). However, all these microscale surface topographies incorporate a nanograss layer, which can be seen in higher-resolution SEM images. A typical example of higher magnification of the Al alloy surface sanded with 36 grit size and post-treated with boiling DI water for 1 min, depicted in Fig. 3f, shows microstructures conformally covered with nanograss structures and exhibits a hierarchical roughness. On the other hand, as shown in Fig. 3g l, one-directional sanding of the untreated Al alloy surfaces with different grit sizes leads to the development of semiregular microgrooves having lateral widths similar to those of the randomly sanded ones (Fig. 3a f). However, grooves become deeper and longer in the one-directional-sanded surfaces. In addition, the surface is highly anisotropic, and the grooves seem to acquire more distinguished approximate width values compared to the more random morphology of randomly sanded samples. Based on the analysis of the SEM images in Fig. 3g k, microgrooves are approximately 20 30, 15 20, 10 15, 5 10, and \5 lm wide for grit sizes of 36, 60, 120, 400, and 1000, respectively. All of these samples also impart a nanograss layer, which is obtained after immersion in boiling DI water for 1 min. For example, the higher-resolution SEM image shown in Fig. 3l, which was taken from the surface of a one-directional-sanded sample using a 36 grit size, demonstrates the uniform nanograss layer covering the surface of microgrooves. Having deeper microgrooves covered with nanograss can potentially introduce a re-entrant surface property due to the morphology s approaching a more three-dimensional shape (e.g., horizontally lying nanograss and pores among them on the side walls of grooves). This can lead to the development of SOP surfaces as investigated in the following sections. The sandpaper grit size has a significant effect on the dimensions of microfeatures, while the sanding direction can further modify the shapes, isotropy, and sizes of microstructures. Compared to microstructures obtained by random sanding, it can be noticed that the microgrooves obtained by one-directional sanding introduced unidirectionally elongated, deeper, well-distinguished microgrooves. This could be because, while randomly sanding, as any structure is created on the surface, there is a possibility of its being partially removed, while the sanding is in progress. Furthermore, the material removed from one location and brought to another in the random-directional sanding process could hinder the overall material removal process. This may also explain the fact that, in industry, random sanding is usually used for smoothening the rough surfaces by continuing the process for prolonged periods as well as using a combination of small and large sandpaper grit sizes. In the case of one-directional sanding, the imparted microgrooves are less likely to be interrupted or partially removed during sanding. In other words, the side walls of these microgrooves are less likely to be broken off, which also leads to deeper grooves compared to the irregular ones produced on randomly sanded surfaces.

9 Fig. 3 SEM images of hierarchical aluminum alloy surfaces roughened via both random-directional sanding (a f) and one-directional sanding (g l) using various sandpaper grit sizes and post immersion in boiling DI water for 1 min. Morphology incorporates combination of random-microstructures and nanograss for sandpaper of grit sizes of a 36, b 60, c 120, d 400, and e 1000, and microgrooves and nanograss To the best of our knowledge, the effect of the sanding direction along with different grit sizes on the rough structures of metallic surfaces has not been addressed in the literature previously. The tuning of the microstructures of for sandpaper of grit sizes of g 36, h 60, i 120, j 400, and k SEM images of (f) and (l) are higher magnification images of randomly sanded and one-directional sanded samples both with grit sizes of 36 (a) and (h) respectively., which demonstrate that the microstructures are uniformly covered with the nanograss layer metallic surfaces in terms of both size and design by the usage of such a simple technique could be very advantageous for several industries. For example, all of these differences in surface morphology in micron-scales as a

10 Fig. 4 a Average particle diameter of sandpapers used in this study as a function of their grit size. Liquid contact angle (CA) profiles on PFDTS-treated aluminum alloy samples with different microroughness against various grit sizes using b random-directional sanding, function of grit size and sanding direction can strongly affect wetting properties, as discussed below. Surface energy reduction of rough Al alloy surfaces Surface energy (i.e., tension) reduction is the second prerequisite for developing super-repellent surfaces. The lowest surface energy materials known so far in a decreasing order are CH 2 [ CH 3 [ CF 2 [ CF 2 H [ CF 3 [37]. Studies have shown that a solid s surface homogenously covered with CF 3 groups possesses the lowest surface energy (approximately 6 mn/m) [38]. On the other hand, Hoque et al. [39] have shown that the longer chain of low surface energy agents causes better orientation, and consequently, more densely packed agents due to an increase in the Van der Waals attractive force, which is a function of chain length. A solid s surface densely packed with low surface energy agents provides fewer sites for oxidized agents to be able to coexist and c one-directional sanding. d Liquid CAs for PFDTS-treated nanograss aluminum alloy surfaces (without sanding, i.e., microroughness) versus boiling DI water treatment time between the low surface agent materials and consequent bonding to the surface of the coated solids. A study of chain length of perfluorocarboxylic acid on SAP properties of different metallic surfaces was conducted by Meng et al. [20]. It was shown that, from within the range of carbon chain length between 6 and 14 carbons, perfluorocarboxylic acid with ten carbons demonstrated the highest water and oil CAs. PFDTS, the low surface energy material used in this study to lower the surface energy of the prepared rough samples, has a chemical formula of (C 13 H 13 F 19 O 3 Si). The molecule is a combination of both methoxy or functional groups of (OCH 3 ) 3 that react with hydroxyl groups of the Al alloy surface and hydrophobic groups of (CH 2 ) 2, (CF 2 ) 7, and a (CF 3 ). Thus, the hydrophobic attachment of PFDTS that is responsible for repelling liquids is also a 10 carbon chain-length terminating with the lowest surface energy group of CF 3. According to the above mentioned studies, a PFDTS agent with the hydrophobic part of 10 carbon chain-length and

11 ending with the lowest surface energy group of CF 3 would provide the lowest possible surface energy between Teflon (18.4 mn/m) and only CF 3 groups (6 mn/m). Lakshmi et al. [40] also showed that the sol gel nanocomposite coatings were SHP with water CA of 155 but superoleophilic with CA of \10 for lubricant oil in the absence of fluoropolymer. The surfaces showed enhanced oleophobic property only after a significant incorporation of more than 8 wt% of fluoropolymer into the coatings. For the reasons mentioned earlier, PFDTS was chosen as a low surface energy material to chemically modify the Al alloy rough surfaces prepared in this study. However, in spite of its low surface energy, a polished flat Al alloy coated with PFDTS showed water and ethylene glycol CA of greater than 90 of *110 and 91, respectively, but a CA of significantly less than 90 of *60 for peanut oil. Any liquid, depending on its surface tension (c l ), can maintain a CA greater than 90 on a flat solid s surface if the surface energy of the solid (c s ) is less than ( 1 4 c l)[13]. Therefore, for the case of water, which has a surface tension of around 72 mn/m, any solid s surface having a surface energy less than *18 mn/m should be sufficient to introduce a hydrophobic property and might not require the use of such low surface energy coatings as in the case of CF 3 treatment. However, the challenge comes from the low surface tension of organic liquids, such as oils, which are typically \35 mn/m. Therefore, one might need to have solid s surface energies of only a few mn/m that have not been found, either naturally or artificially, to obtain both oleophobic (and therefore amphiphobic) properties. For this reason, a combination of low surface energy treatments with re-entrant structures was proposed as an alternative to the need for extremely low surface energies of only a few mn/m in addition to surface roughness for developing SAP surfaces [41, 42]. Wetting analysis of chemically modified rough Al alloy surfaces Figure 4 shows the CA values of three different types of liquids (water, ethylene glycol, and peanut oil) on randomly sanded (Fig. 4b), one-directionally sanded (Fig. 4c), and boiling-water-treated polished-flat (Fig. 4c) Al alloy samples after treating their surfaces with PFDTS. Therefore, these samples either had microroughness (Fig. 4b, c) or nanoroughness only (Fig. 4c) (see Fig. 2 for some of the SEM images of nanograss samples not having any hierarchical structure). In addition, one-directionally sanded samples showed a slight anisotropy depending on whether we measured CAs along or perpendicular to the microgrooves. The results reported here were obtained along the microgrooves. A comparison of Fig. 4b, c shows that the CAs for all of the liquids used increased by about when one-directional sanding was used as opposed to random-directional sanding. Overall, randomly sanded surfaces are slightly hydrophobic (CA water * , which are [90 ), while they still have oleophilic property (CA oil * 60 85, and \90 ). On the other hand, onedirectional sanding significantly increased CAs such that the surfaces became highly hydrophobic (CA water * ) and slightly oleophobic (CA oil * for smaller grit sizes). Nevertheless, the one-directional-sanded surfaces could not bring the CA high enough ([150 ) to characterize them as SHP or SOP. In general, for both one-directional and randomly sanded surfaces, grit size did not have a noticeable effect on the CA of water and ethylene glycol. However, the oil CA decreased slightly with the increase in grit size. For randomly sanded surfaces, this decrease was accompanied by fluctuations in oil CAs, the origins of which are yet not clear. Moreover, liquid droplets on one-directionally sanded samples showed anisotropy as noted above. This is believed to be because the droplet encounters more roughness when spread in the direction perpendicular to microgrooves compared to spreading along the groove. The detailed effects of microgrooves introduced by one-directional sanding on anisotropic wetting properties are currently under investigation. Ehtylene glycol with its moderate surface tension of *47 mn/m compared to both water (*72 mn/m) and peanut oil (*34 mn/m) always demonstrates CAs lower than water but greater than oil. The values of its wetting properties are incorporated into the data obtained in this study only to showcase the trend of variation in the liquid s CAs against the variation in their surface tensions. Therefore, the explanation of wetting properties of the surfaces will primarily be given in the context of the two extremes of the surface tensions of both water (high surface tension) and oil (low surface tension). Unlike sanded samples of microroughness, the nanograss structures obtained by immersing polished-flat Al alloy in boiling DI water showed highly amphiphobic properties with water CA ranging from CA water * and oil CA ranging from CA oil * (Fig. 4d). The highest CAs were obtained for short immersion periods of 1 and 5 min in particular. At these short boiling times, nanograss structures super-repelled water with CAs larger than 150 and low sliding angle values of less than 10, but showed only oleophobic properties with no sliding in the case of the peanut oil droplet. The decrease in CAs with immersion time is believed to be due to the decrease in nanograss porosity as shown above (Fig. 2). Zhou et al. [34] have studied the effect of treating Al with heated DI water at 90 C, and it was shown that the surfaces immersed in hot water for different times between 30 and 240 min and post-salinization with PFDTS agents exhibited SHP properties. In

12 Fig. 5 CAs and sliding angles (SAs) of hierarchical micro and nanorough aluminum alloy surfaces coated with PFDTS for (a, c) random-directional sanded (microroughness) with various grit sizes followed by treatment with boiling DI water (nanograss structures) for another study by Jafari and Farzaneh [33], the effect of immersing Al alloy 6061 in boiling DI water for time periods between 1 and 30 min was reported. After sputtercoating the surfaces with Teflon, SHP surfaces were obtained. In both above mentioned studies, the water CA decreased with the increasing immersion time, but there was an increase in water droplet SA. Therefore, the results obtained in this study for the case of water are consistent with the above mentioned studies. For low surface tension liquids, such as oil, the nanograss surfaces showed highly oleophobic properties, with CAs approaching for short immersion periods of 1 and 5 min and was still oelophobic even after 60 min of immersion with a CA of *120. The oil droplet showed high adhesion to the surfaces with no sliding or detachment even after turning the substrate upside down. Yang et al. [24] have reported oleophilic properties for Al surfaces treated with boiling water for 20 min with an oil CA of less than 50. This could be the result of their using nanoscale structures and surface energy reduction agents, both being different 1 min, and (b, d) one-directional-sanded (microgrooves) with various grit sizes followed by treatment with boiling DI water (nanograss structures) for 1 min compared to our study. Al alloy 2024 treated with boiling water promotes more re-entrant-like structures compared to the elemental Al used in their study. On the other hand, Yang has used perfluorooctanoic acid for surface energy reduction, while, in our case, PFDTS molecules were used for that purpose. Hozumi et al. [35] have also reported SHP and oleophobic properties for an Al surface treated with boiling water for 30 min. The results obtained in our study for nanograss surfaces obtained with the immersion of Al alloy in boiling DI water for less than 30 min are in good agreement with their results. For hierarchical micro and nanorough Al alloy surfaces, CA and SA measurements are presented in Fig. 5. Samples prepared by random-directional sanding (microroughness) followed by treatment with boiling DI water for 1 min (hierarchical roughness with a nanograss layer; see Fig. 3 for SEM images) and by PFDTS coating showed CAs that are significantly higher compared to samples with microroughness (Fig. 4b, c) or nanoroughness (Fig. 4d) only. For randomly sanded hierarchical samples (Fig. 5a), the CAs

13 were generally in the range of * , * , and* , for water, ethylene glycol, and peanut oil, respectively. The corresponding values were in the ranges of *35 40, *45 50, and*50 75 higher compared to similar samples without nanograss (Fig. 4b) and *5 10, 0 5, *15 20 higher compared to unsanded samples with nanograss only (Fig. 4d), for water, ethylene glycol, and peanut oil, respectively. Hence, hierarchical samples of random-sanding were SHP and strongly oleophobic, but failed to become SOP and therefore SAP. In addition, samples were super-repellent toward ethylene glycol only at the 120 and 400 grit sizes. Moreover, CA values were further improved for hierarchical samples of one-directional sanding (microgrooves with a nanograss layer; see Fig. 3 for the SEM images) that were treated by PFDTS. These hierarchical microgroove/ nanograss samples showed CAs roughly in the range * , * , * for water, ethylene glycol, and peanut oil, respectively, which revealed their SHP, SOP, and therefore the SAP property. These CA values were in the ranges of *20 25, *30 35, and *55 60 higher compared to the one-directionally sanded samples without nanograss (Fig. 4c), and *5 10, *5 10, and *15 20 higher compared to unsanded samples with nanograss only (Fig. 4d), for water, ethylene glycol, and peanut oil, respectively. The peak CA values reached *158, 156, and 154, for water, ethylene glycol, and peanut oil, respectively, at smaller grit sizes in the range of These results show the importance of introducing an optimal surface morphology to obtain surfaces that can maintain oil CA of more than 150. Although one-directional and randomly sanded samples had similar microscale roughness and the same nanoroughness, only the onedirectionally sanded ones achieved a super-repelling CA of more than 150 for low surface tension liquids, such as peanut oil. In addition, the adhesion stickiness of the three tested liquids to the hierarchical surfaces was investigated by determining the substrate tilting angle liquid sliding angle at which the droplet begins to slide. The liquid droplet s SAs over hierarchical samples with a combination of both microroughness by random-directional sanding and nanograss structures are plotted in Fig. 5c. SAs in the ranges of 1 4, 5 25, and 24 35, for water, ethylene glycol, and peanut oil, respectively, were observed. Thus, the surfaces demonstrated SHP property for all grit sizes and super-repellency toward ethylene glycol for 120 and 400 grit sizes, but failed to super-repel peanut oil an indication of no SOP property. In the case of the three liquids, the lowest values of SAs were observed when sandpapers with moderate grit sizes of 120 and 400 were used. For hierarchical samples of microgrooves obtained by one-directional sanding with the nanograss layer, the three liquids SAs are plotted in Fig. 5d. The surfaces showed a significant decrease in all liquid SAs, especially for peanut oil, of more than 15 compared to randomdirectionally sanded surfaces covered with a nanograss layer. The lowest SAs were obtained for sandpaper with 60 grit sizes of 1, 3, and 8 for DI water, ethylene glycol, and peanut oil, respectively. Compared to all other rough surfaces produced in this study, the microgroove/nanograss hierarchical rough surface was the only candidate to super repel the three tested liquids showing SAP property. The high CA of more than 150 and the low SA of less than 10 for the tested liquids is the manifestation of extremely low adhesion or stickiness of the liquids to the surfaces. Such SAP surfaces could have a wide range of applications as self-cleaning surfaces against dirt, dust, glare, and also against microorganisms [5, 26, 43]. They could furthermore prove useful in the oil and microfluidic industries for transportation. Analysis of observed CA by fundamental wetting models In order to explain the results obtained in this study in terms of fundamental wetting models, a typical example of the prepared surfaces coated with PFDTS (flat, micro, nano, and hierarchical Al alloy surfaces), along with the three tested liquid droplet-photographs of water (blue), ethylene glycol (green), and peanut oil (red), is presented in Fig. 6. For the case of flat-polished Al alloy coated with PFDTS (Fig. 6a), the lower intrinsic CA values of 91 and 58 for ethylene glycol and peanut oil, respectively, compared to 110 of water is consistent with the Young s model (Eq. 1). According to the Young s model, a liquid s intrinsic CA on a flat-solid s surface is the function of all involved interfacial surface tensions in the interaction. It can be noticed that the decrease in the surface tension (c lv ) of the liquid results in smaller associated intrinsic CA for the same flat-solid s surface. The decrease in a liquid s surface tension is the manifestation of weaker cohesion forces among its molecules that can be dominated by the adhesion forces involved at the solid liquid interface, consequently causing more spreading of the liquid. For microscale rough Al alloy surfaces, in both cases of random-directional sanding and one-directional sanding, the overall increase in water and ethylene glycol CA is in good agreement with the Wenzel model (Eq. 2), where the liquid CA increases with an increase in surface roughness (h * [ h) ifitish [ 90, and decreases (h * \ h) ifitis h \ 90. However, the peanut oil CA also increased in both cases of random sanding as well as directional sanding for most of the sandpaper grit sizes (Fig. 4b, c) in spite of its

14 Fig. 6 The photographs for the three tested liquid droplets of water (blue), ethylene glycol (green), and peanut oil (red) on the PFDTScoated aluminum alloy surfaces of a polished-flat, b microgroove roughness, c nanograss roughness, and d hierarchical roughness of microgroove and nanograss structures oleophilic property (h \ 90 ) on the corresponding flat surface. The increase in peanut oil CA on microscale rough surfaces from *60 to *100 cannot be explained in terms of the Wenzel model. This scenario of wetting is described by the metastable Cassie state; the re-entrant design of geometries promotes a localized CA (h l ) on the top-side of the surface structures with values of h l B h, facilitating composite solid vapor liquid interface (Cassie state) instead of only liquid solid interface (Wenzel state). A relatively poor and discontinuous re-entrant property of microstructures and microgrooves can be seen in SEM images of the surfaces (Fig. 3), where the top of the structures at some locations are slightly bent toward the gaps. Nonetheless, the values of h * for the three liquids on the microrough surfaces are still much less than 150 as an indication of low aspect ratios and the poor re-entrant property of the geometries. A typical example for microscale roughness for Al alloy that was one-directionally sanded with 60 grit size is shown in Fig. 6b. In this figure, the anisotropy of the three liquids can also be observed, but the anisotropy is more significant for the peanut oil droplet due to its lower surface tension. Detailed analysis of anisotropic behavior due to microgroove structures is currently under investigation. Figure 6c shows the three liquid droplet-photographs on a nanograss Al alloy obtained after immersion of the polished-flat surface in boiling DI water for 1 min followed by its surface chemistry modification with PFDTS. A significant increase in their h * values can be noticed. Water showed SHP property with high h * * 152 and SA of slightly less than 10, which is a clear sign of adopting the Cassie state of wetting. Ethylene glycol also exhibited a high h * [ 150, but the SA was still higher than 10 which indicates the pinning of the liquid contact line into the structure partially wetting the texture. Interestingly, the nanograss roughness has also shown a relatively high oleophobic property with h * * 135 despite its low h * 58, which is significantly less than 90. This is a clear sign of the re-entrant property conveyed by the nanograss rough structures providing a composite liquid vapor-solid interface with h l B h metastable Cassie state. However, the peanut oil droplet contact line was also pinned into the nanograss structures causing extreme adhesion such that the droplet did not detach even after the surface was turned upside down. Studies have shown that the overlapped spherical and cylindrical structures can also provide re-entrant property [1, 41, 44, 45]. The values of h * for the three liquids decreased with an increase in immersion time of the surfaces in boiling DI water due to the decrease in porosity of the surfaces with time. This decline in the surface CAs is consistent with the Cassie model (Eq. 3), where a decrease in porosity will result in an increase in solid s area fraction (f) in contact with the liquids. In the case of hierarchical structures, the directionally sanded surface with 60 grit size sandpaper that was then immersed in boiling DI water for 1 min followed by PFDTS coating (Fig. 6d) demonstrated the highest values of h 158 ; 156 ; and 152 and lowest SAs of 1 ; 3 ; and 8 for water, ethylene glycol, and peanut oil, respectively. The high CA (h * [ 150 ) and nonadhesive property (SA \ 10 ) of the hierarchical surface toward both water (high surface tension) and peanut oil (low surface tension) are due to the SAP property of the surface, providing a strong Cassie state of wetting ðh [ 90 and h [ 150 Þ for water and robust metastable Cassie state ðh\90 and h [ 150 Þ for peanut oil. According to the Cassie model represented in Eq. (3), empirical calculations for the values of f based on observed CAs showed that water, ethylene glycol, and peanut oil are in touch with ratios of only 10 %; 12 %; and 15%, respectively, of solid s area at the composite solid vapor liquid interface. This indicates that the three liquid droplets were floating over more than 85 % of

15 vapor trapped in between the hierarchical rough structures providing extremely low adhesion to the surface. For hierarchical surfaces produced with random sanding (120 and 400 grit sizes) and then treated with boiling DI water for 1 min followed by their surface energy reduction using PFDTS agent, the surfaces also showed superrepelling properties (h * [ 150 and SA \ 10 ) toward both water and ethylene glycol but failed to super-repel low surface tension peanut oil (Fig. 5a, c). This could be due to the lack of robust metastable Cassie state that can sustain perturbations in Laplace pressure at the liquid vapor interface promoting partial wetting of the surface texture. Tuteja et al. [42] have introduced two nondimensional robustness parameters of robustness height and robustness angle to quantify the robustness of rough surface geometries in facilitating a robust metastable Cassie state of wetting (preventing the Cassie Wenzel transition) for a particular liquid. The parameter of robustness height is the function of texture geometries (aspect ratio), as well as the liquid s intrinsic CA and its capillary length, while the second parameter robustness angle was a function of the local geometric angle of the texture in addition to the size of geometries and the liquid s intrinsic CA, as well as its capillary length. It was shown that, in the absence of any external pressure, Laplace pressure within the droplet itself can also lead to the sagging of the liquid vapor interface causing the interface to reach the bottom of the structures and eventual penetration of the liquid into the texture [42, 46]. Regarding the effect of the first robustness parameter embedded in the aspect ratio of our hierarchical structures, it can be clearly noticed from the SEM images depicted in Fig. (3) that the hierarchical microgrooves/nanograss (Fig. 3g l) structures possess significantly higher aspect ratios compared to the hierarchical microstructures/nanograss combination (Fig. 3a f). Considering the second robustness parameter provided by the geometrical angle of the texture as the measure for the re-entrant property, the presence of conformal nanograss and particularly on the side walls of microgrooves, as shown in magnified SEM images of Fig. 3f, l, can provide the re-entrant property with h l \ h for peanut oil. In addition, the presence of the nanograss along the side walls all the way down to the bottom of the microgrooves helps the liquid contact line to stabilize and provide robust re-entrant property [46]. Moreover, the semi-closed boundaries of the microgrooves could have provided more support to the trapped vapor micropockets in containing perturbations occurring in Laplace pressure at the composite liquid vapor interface of low surface tension oil. Studies have shown that closed-cell geometries (such as bricks or honey combs) provide significantly higher pressure stability at the composite liquid vapor interface (preventing the Cassie Wenzel transition) compared to open structures (such as posts) [47, 48]. The lotus leaf, despite its SHP property which is the result of its hierarchical structure of microbumps covered with nanohairy wax, still becomes wet with low surface tension liquids exhibiting oleophilic properties [2]. In the case of low surface energy hierarchical structures, two scenarios of superhydrophobicity (CA [ 150 ) of low adhesion (lotus effect) [49, 50] and high adhesion (petal effect) [51 53] are possible. The hierarchical surfaces of microgrooves/nanograss obtained with sandpaper grit sizes of 60 and 120 and post-treatment with boiling DI water for 1 min followed by their surface modification with PFDTS showed lotus effect property for both water and oils. This can be attributed to the structure s optimal aspect ratio, re-entrant property, and semi-closed boundaries of microgrooves. The collective threshold of the three factors has provided optimal conditions that can result in an extremely robust Cassie state of wetting for both water and ethylene glycol and a robust metastable Cassie state of wetting for low surface tension peanut oil. Conclusions In this study, it was demonstrated that SAP property can be imparted to Al alloy 2024 surfaces using simple, low cost, and environmentally friendly techniques of mechanical sanding (for microscale roughness) and treatment with boiling water (for nanoscale roughness). From the analysis of a series of rough Al alloy surfaces of micro, nano, and two kinds of hierarchical surfaces of microstructures/ nanograss and microgrooves/nanograss, only the combination of microgrooves/nanograss showed SAP property. This was due to the optimal conditions of high aspect ratios, re-entrance, and semi-closed boundary geometries provided by the hierarchical microgrooves/nanograss. The combination of the optimal three factors facilitated the adoption of a robust metastable Cassie state of wetting promoting high CA of more than 150 and SA of less than 10 for all water, ethylene glycol, and peanut oil. In our future studies, we plan to investigate our hierarchically rough Al-alloy samples for their wetting response toward other types of organic oils with lower surface tensions (e.g., mn/m). Aluminum and its alloy are widely used for many industrial applications due to their light weight and mechanical robustness. They have been extensively used in aircraft industry, marine hulls, building construction, and many other applications such as packaging and power lines. Imparting SAP properties onto their surfaces will widen and enhance their usage in the applications that demand surface self-cleaning, anticorrosion, antifouling, and anti-icing properties. Al pipelines can also benefit from