Durable omniphobicity of oil-impregnated anodic aluminum oxide nanostructured surfaces

Journal of Colloid and Interface Science 553 (2019) 734–745

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Durable omniphobicity of oil-impregnated anodic aluminum oxide nanostructured surfaces Junghoon Lee a,b, Youhua Jiang a, Ferdi Hizal a, Ga-Hee Ban c, Soojin Jun c, Chang-Hwan Choi a,⇑ a

Department of Mechanical Engineering, Stevens Institute of Technology, Castle Point on Hudson, New Jersey 07030, USA Department of Metallurgical Engineering, Pukyong National University, Busan 48547, Republic of Korea c Department of Human Nutrition, Food and Animal Sciences, University of Hawaii at Manoa, HI 96822, USA b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 10 April 2019 Revised 18 June 2019 Accepted 19 June 2019 Available online 20 June 2019 Keywords: Anodic aluminum oxide Nanostructures Oil-impregnation Omniphobic surface Anti-bacterial adhesion

a b s t r a c t Recently, various types of porous surfaces have been demonstrated for lubricant (e.g., oil) impregnated omniphobic surfaces. However, the retention of the lubricating liquid within the porous layer and the omniphobic durability still remain challenges. Here, the omniphobic durability of the oil-impregnated surfaces of various types of anodic aluminum oxide (AAO) nanostructures is investigated. The oil impregnation into nanoporous AAO with high porosity enhances droplet mobility by eliminating the pinning site of a contact line on the solid surface, whereas that with low porosity allows the pinning site to result in less mobility. In the cases of nanopillared AAO layers, although the oil-impregnation enhances the repellency to liquids, oil is prone to be depleted by external force such as fluid flow due to the nature of the interconnected oil through the passages between pillars, which limits the omniphobic durability. Among the various types of nanostructured AAO surfaces, the AAO with isolated pore geometry with high porosity exhibits the most durable omniphobicity for a wide range of liquids including organic liquids with low surface tensions. Moreover, the nanoporous AAO surface shows great anti-bacterial adhesion property, reducing the adhesion of bacteria (Escherichia coli K-12) up to 99.2% compared to a bare aluminum surface. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (C.-H. Choi). https://doi.org/10.1016/j.jcis.2019.06.068 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Sustainable functional surfaces with self-cleaning capability have aroused significant attention in a wide range of applications,

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

including biomedical equipment, solar cell, marine vessels, food container, anti-icing, hydrodynamic drag reduction, and water distillation [1–12]. Hydrophobic, oleophobic, omniphobic, and lubricant-infused slippery properties created by modifications of surface structures and chemical states have been explored to repel or remove contaminations easily without any damages [13–15]. A solid-air-liquid composite interface, inspired by nature’s Lotus leaves, enhances the hydrophobicity by reducing the solid-liquid contact area for a droplet, referred to as superhydrophobicity [16,17]. However, complex reentrant or mushroom-shaped pillar structures which are further required for superhydrophobicitybased omniphobicity to repel even low-surface-tension liquids are mechanically fragile so that the durability of such structures for real applications still remains a challenge [18–21]. In particular, the depletion of air at the solid-air-liquid composite interface due to several thermodynamic and hydrodynamic effects is another practical concern for the application of superhydrophobic surfaces [22,23]. The geometry of surface porous structures used to enhance the hydrophobicity also plays a critical role in such a way that the air layer formed by interconnected pore structures is more easily depleted than that by isolated pore structures [24–26]. In contrast, slippery liquid-infused porous surfaces (so-called SLIPS), inspired by nature’s Nepenthes pitcher plant having a liquid-solid-liquid composite interface at the surface, have shown remarkable omniphobicity without such sophisticated and complex structural geometries [27]. Porous structural geometry is little vulnerable to the mechanical wear. Moreover, the immiscible lubricating fluids (e.g., perfluorinated oil) retained in porous solid structures with well-matched chemical affinity and roughness inhibit the contact of other liquids to the solid structures so that the multifunctional omniphobicity including anti-bacterial adhesion, self-cleaning, and anti-icing properties can effectively be realized on various materials [28–33]. The liquid-infused porous surfaces can sustain their versatile omniphobic properties as long as the porous structures retain the lubricating oil in their pores [1,27,32]. Therefore, controlling the morphology and dimensions of the surface porous structures is one of the key design factors to realize a robust omniphobic surface based on the oil-impregnation. While micropillar structures of silicon wafer fabricated by lithographic approaches have typically been used for theoretical studies [33–35], various techniques have been explored to realize porous structures on commercial materials for real applications, such as layer-by-layer deposition [36] and inverse conical layer [32] for glass, chemical etching for stainless steel [37], and anodizing for aluminum [1,38]. Especially, the anodizing (or anodic oxidation) is a scalable and viable technique to create nanoporous oxide layers on metallic substrates for the broad applications of the omniphobic surfaces based on the oil impregnation with enhanced corrosion resistance for the metals [1,39]. Despite the several advantages of the oil-impregnation-based omniphobicity over the superhydrophobicity-based omniphobicity, the degradation and failure of the omniphobicity by depletion of the oil in the porous structures have also been reported [40–42]. In particular, the capillary suction caused by external liquids ruins the stability and longevity of the oil impregnated in the surface surfaces [40]. Therefore, more systematic understanding of the effects of structural morphology and dimensions of the porous layer on the stability of oil and the robustness of omniphobicity is necessary for the better design of durable omniphobic surfaces based on oil-impregnation. In this study, we investigate the effects of structural morphology and dimension of nanostructures realized by the anodizing process for metal, especially aluminum. Although the advantages of the disconnected high-aspect-ratio dead-end nanopore structures for the retention of oil, compared to the interconnected and microscale pore structures, was reported in the previous study [1,39], the effects of the pore dimensions and the

735

modification of their morphology such as combination of pillar structures on top of the porous layer have not yet been reported on the durability of the omniphobicity. Previously, such effects were only studied for superhydrophobicity [43,44]. More specifically, in this work, we examine two-dimensional nanoporous as well as three-dimensional nanopillared surfaces of anodic aluminum oxide (AAO) with regulated porosities and morphologies for the study of omniphobic durability. For twodimensional nanoporous surfaces, conventional cylindrical nanoporous AAOs with two different (i.e., small and large) porosities are tested. For three-dimensional nanopillared surfaces, pillar-onpore hybrid AAOs [43,44] with two different pillar morphologies (i.e., single-pillared and bundle-pillared) realized by additional pore-widening process [6] are tested. We investigate the omniphobic properties (contact angles, roll-off angles, and droplet mobility) and durability of such surfaces for various types of liquids with different surface tensions including complex fluids (ketchup and olive oil) under different hydrodynamic conditions (continuous droplet impingement and jet flow). We also demonstrate the antibacterial adhesion property of the oil-impregnated AAO surface against bacteria (Escherichia coli) for multifunctional omniphobicity.

2. Martials and methods 2.1. Fabrication of nanostructures of anodic aluminum oxide Aluminum foil (99.99%, thickness: 0.2 mm, Alfa Aesar) was used as a substrate for the fabrication of AAO nanostructures. The substrate was degreased in acetone with ultrasonication for 5 min followed by electropolishing in perchloric acid and ethanol mixture (1:4 in volumetric ratio) under constant voltage of 20 V for 8 min at 15 °C. Uniform and well-ordered pre-patterns on the aluminum substrate were first fabricated by anodizing in 0.3 M oxalic acid at 40 V at 0 °C for 12 h followed by the removal of the sacrificial oxide layer in the mixture of 1.8 wt% CrO3 and 6 wt% H3PO4 at 65 °C for 6 h. The second anodizing process was applied at the same anodizing condition for 15 min to prepare a conventional AAO layer with a cylindrical pore (Fig. 1a). In order to modify the porosity and transform the porous morphology to pillared one of the original nanoporous AAO layer, the specimen was further immersed in 0.1 M phosphoric acid at 30 °C with modulating time, e.g., 60 min for larger pore diameter (Fig. 1b), 90 min for single-pillared morphology with individually-standing pillars (Fig. 1c), and 100 min for bundle-pillared morphology with the collapsing and aggregation of the individual pillars into a cone shape (Fig. 1d). To improve the affinity between the AAO nanostructures and the perfluorinated oil (Krytox GPL 100, DuPont) to be impregnated, a thin layer of Teflon (
2 nm) was spin-coated on the AAO surfaces [1]. Before the Teflon coating, the sample was baked on a hotplate at 300 °C for 10 min, followed by O2 plasma cleaning (Harrick plasma) for 15 min which was to remove organic residues on AAO surfaces. Teflon solution of 0.2 wt% AF1600 (DuPont) in perfluoro-compound FC-75 (Fisher Scientific) was then applied and spin-coated at 200 rpm for 30 s. Then, the specimen was baked on a hotplate at 110 °C for 10 min to evaporate the solvent and then baked again at 165 °C for 5 min and at 330 °C for 15 min to improve adhesion of the Teflon film to the AAO surfaces. The dimensions and morphologies of the AAO nanostructures both before and after the Teflon-coating were examined using fieldemission scanning electron microscopy (FE-SEM, Quanta FEG 450, FEI Inc.), which shows that the Teflon coating (
2 nm) does not alter the dimensions and morphologies of the as-prepared AAO nanostructures significantly. To check the affinity of the

736

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

Fig. 1. Schematics of the cross-sectional morphologies of the various types of anodic aluminum oxide (AAO) nanostructures, including (a) small-pored (S-Po), (b) large-pored (L-Po), (c) single-pillared (S-Pi), and (d) bundle-pillared (B-Pi) nanostructures with Teflon coating.

Teflon-coated AAO nanostructures with the lubricant oil (Krytox GPL 100, DuPont), the apparent contact angle of the lubricant oil on the Teflon-coated AAO surfaces was measured to be
0°. It indicates that the Teflon-coated AAO surfaces have well-matched affinity with the perfluorinated oil (Krytox GPL 100, DuPont) employed for the lubrication so that probe liquids would not easily penetrate into the nanoporous structures of the AAO surfaces [33,35]. 2.2. Oil impregnation via solvent exchange method To fully fill the hydrophobized AAO layers with perfluorinated oil (Krytox GPL 100, DuPont), a solvent exchange method was employed, whose details have recently been reported elsewhere [1]. In the solvent exchange method, the specimen was sequentially immersed in ethanol, Vertrel XF (DuPont), and Krytox GPL 100 for 10 min, respectively. The temperature of all liquids used in the solvent exchange process was maintained at 25 °C. The specimen was kept wet throughout the process. To help the impregnation and exchange of each liquid into the pores, ultrasonication (80 W, 40 kHz) was used. However, the ultrasonication was not used for the cases of single-pillared and bundle-pillared AAOs, which can be destroyed under severe vibration. Compressed air was used to blow off the excessive oil from the surfaces. The thickness of the over-filled oil layer was
10 lm, which was estimated with the weight change of the specimen and the density of the oil (Krytox GPL 100). 2.3. Characterization of wetting properties The wetting properties of the AAO surfaces were characterized by measuring apparent contact angles, contact angle hysteresis, and roll-off angle of a sessile droplet (
5 ll) using a goniometer (Model 500, Rame-hart). Three liquids (deionized water, ethylene glycol, and hexadecane) with different surface tensions (72, 47, and 28 mN/m, respectively, at room temperature) were tested for the measurement. The contact angle hysteresis was measured from a contact line movement induced by increasing (advancing contact angle) and decreasing (receding contact angle) the volume of a droplet. The roll-off angle was measured when a droplet showed the evident movement over the substrate fully (at both uphill and downhill sides) without any pinning. All measurements were averaged over at ten different locations on each specimen. The wetting property (i.e., droplet mobility) of organic complex liquids, including ketchup (Heinz Tomato Ketchup, H.J. Heinz Company) and olive oil (Extra Virgin Olive Oil, California Olive Ranch) was also examined on the surfaces pre-inclined at 10°. 2.4. Characterization of durability of oil retention and immobilization To investigate the durability of the oil retention and immobilization on the nanostructured AAO surfaces, water droplets (
5 ll) were continuously dropped on the surfaces which were pre-inclined at 5° by using a syringe at a distance of 8 mm. The

flow rate from the syringe was regulated by a controller to dispense a droplet at around every 5 s. Droplet movements on the pre-inclined surfaces were recorded using a digital camcorder integrated with the goniometer (Model 500, Rame-hart). The number of droplets rolling off along the surfaces was counted until the droplet began to be pinned on the surfaces. A continuous jet flow was also applied to investigate the omniphobic durability, where a jet flow of water (1 l/min) was applied onto the surfaces preinclined at
5°, using a tube with 6 mm in diameter. Wetting and slipperiness of a water droplet on the surfaces were checked after applying the jet flow of water for 10 min. 2.5. Characterization of Anti-Bacterial adhesion properties Frozen stock culture of Escherichia coli (E. coli) K-12 was used for the bacterial adhesion test. E. coli K-12 was grown in 10 ml of tryptic soy broth (TSB; Difco) at 37 °C for 24 h. Cells were collected by centrifugation, and suspended in phosphate-buffered saline (PBS), corresponding to the initial population of 107–108 colony-forming units (CFU)/ml. The sample was immersed into 10 ml of suspensions of E. coli K-12 in PBS (ca. 107–108 CFU/ml) and incubated at 4 °C for 4 h to facilitate the attachment of bacteria. Adhered bacteria were detached with sterile glass beads and suspended in PBS. The suspension was spread-plated onto MacConkey Agar (Oxoid) to enumerate the number of E. coli K-12, and the colonies were counted after 24 h of incubation at 37 °C. In addition, the surfaces with attached bacteria were also examined using FE-SEM (Hitachi S-4800). More detailed procedure of the bacterial adhesion test can be found in Supplementary Material. 3. Results and discussion 3.1. AAO nanostructures Fig. 2a–d show SEM images of the different dimensions and morphologies of the hydrophobized (i.e., Teflon-coated) AAO nanostructures prepared for this study, including small-pored (S-Po), large-pored (L-Po), single-pillared (S-Pi), and bundlepillared (B-Pi) nanostructures. Structural features (pore/pillar diameter, inter-pore/pillar distance, and porosity/air-fraction) for the pored/pillared nanostructures were summarized in Table 1. As a result of the two-step anodizing, highly-ordered AAO nanopore structures with a pore diameter of 20 nm (S-Po, Fig. 2a) and inter-pore distance of 100 nm were created on the aluminum substrate. Then, the pore-widening process dissolving the cell walls of the AAO pore array enlarged the pore diameter and increased the porosity of the nanopore structure with the increase in the duration of pore-widening. Pore-widening for 60 min resulted in a large-pored nanostructure (L-Po with 74 nm of pore diameter, Fig. 2b). The hexagonal cell walls of the nanopore structures were etched away with further pore-widening, resulting in the emergence of pillared nanostructures [43,44], such as single-pillared (S-Pi with
60 nm of inter-pillar distance, Fig. 2c) and bundle-

737

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

Fig. 2. SEM images of tilted (45°) and top (inset) views and schematics of cross sections for (a) small-pored (S-Po), (b) large-pored (L-Po), (c) single-pillared (S-Pi), and (d) bundle-pillared (B-Pi) nanostructures of anodic aluminum oxide (AAO) layer with Teflon coating. White and black scale bars in the SEM images indicate 0.5 and 1 lm, respectively. Schematic cross-sections of AAO structures shown in (a-d) can be found in Fig. 1.

Table 1 Structural morphology and dimensions of the Teflon-coated AAO nanostructures. For the pored nanostructures (S-Po and L-Po), the porosity (i.e., air fraction = 1 - solid fraction) was estimated from the pore diameter and inter-pore distance in the hexagonally packed pore array [43]. In the case of the pillared nanostructures (S-Pi and B-Pi), the air fraction was estimated as the ratio of the open area on the top plane to the projected (flat) area of the surface, which can be occupied by oil.

Flat S-Po L-Po S-Pi B-Pi

Structural morphology

Pore or pillar diameter (nm)

Interpore or interpillar distance (nm)

Porosity or air fraction (%)

None Disconnected pores Disconnected pores Single (disconnected) pillars Bundled pillars

– 20 ± 2 74 ± 3 13 ± 2 35 ± 14 (diameter of a clustered conical tip)

– 100 ± 3 100 ± 2 60 ± 5 340 ± 110 (distance between the clustered conical tips)

0 5±1 50 ± 4 81 ± 6 95 ± 1

pillared (B-Pi with
340 nm of distance between aggregated conical tips, Fig. 2d) nanostructures with the pore-widening for 90 and 100 min, respectively. Both S-Po and L-Po have cylindrical deadend pores, where each pore is isolated from neighboring pores by their own cell walls. In contrast, due to the excessive dissolution of cell walls during the pore-widening, the pores become interconnected, which eventually leaves the single-pillared nanostructures (S-Pi). The aspect ratio of the individual pillar nanostructure increases with the further elongated pore-widening. The slender pillar nanostructures become self-aggregated and form a clustered conical structure (B-Pi) during the evaporative drying process due to the capillary force [45]. The higher porosity or air fraction (Table 1) indicates that a greater amount of oil can be impregnated with a greater coverage of the surface. 3.2. Wetting properties Fig. 3 shows apparent contact angle (CA), contact angle hysteresis (CAH), and roll-off angle (RA) of a sessile droplet (
5 ll) of water (surface tension: 72 mN/m), ethylene glycol (surface tension: 47 mN/m), and hexadecane (surface tension: 28 mN/m) on the hydrophobized smooth (electropolished Al, Flat), S-Po, L-Po, S-Pi, and B-Pi surfaces without and with oil-impregnation [35]. The images used to obtain the raw data can be found in Supplementary Material (Figs. S1–S6). The raw data are also summarized in Supplementary Material (Tables S1 and S2). As shown in Figs. S1–S6, the impregnated surface oil forms an annular wetting ridge around the test liquid droplets, so that the actual contact angle between solid and droplet cannot be determined by the goniometer technique [35]. Nevertheless, since the apparent contact angles obtained from the spherical-cap droplet shape on the oil-impregnated surface provide useful information for the comparison of the wetting state and mobility of a droplet (i.e., contact angle hysteresis) to those on a dry surface, the results are presented for such discussion.

Without oil-impregnation, the static contact angle mostly increased with the increase in the porosity or air fraction of the nanostructured AAO surfaces, regardless of the surface tensions of the different liquids tested (Fig. 3a–c). It suggests the hydrophobized nanostructured AAO surfaces generally support the CassieBaxter wetting state [46], where air should be entrained within the pores or the open spaces between pillars underneath the liquid droplet. A hexadecane droplet on the B-Pi surface shows only the exception (Fig. 3c), where the contact angle is lower than S-Pi and even than L-Po whose porosities or air fractions are significantly lower than that of B-Pi. It indicates that the wetting state of the hexadecane droplet on B-Pi is no longer in the pure Cassie-Baxter state but in the intermediate state [47,48] between the pure Cassie-Baxter [46] and Wenzel [49] states, where the droplet is not only supported by the pillar top surfaces but also the sidewalls or valleys between the pillar structures. It is attributed to the positively tapered sidewall angle of the conical shape of the clustered nanostructures as well as the significantly low surface tension than the other liquids, which should allow the significant penetration of the hexadecane liquid into the voids between the clustered nanostructures. Meanwhile, the droplet contact angle of the liquid with lower surface tension is generally lower for the given same surface. After the impregnation of oil, the effect of the porosity or air fraction on the contact angle is not present any longer, regardless of the surface tensions of the liquids and the surface morphologies. The droplet contact angle of all the liquids on the oil-covered Flat surface is slightly lower than that on the hydrophobized Flat surface without any oil-impregnation and the value does not change despite the change in surface morphologies by nanostructures. It indicates that the oil fully covers the surfaces, regardless of the different surface morphologies. The contact angle values on the oil-impregnated surfaces only depend on the liquid type (i.e., surface tension). In other words, the initial contact of a droplet to the oil-impregnated surfaces is affected by the interfacial force balance among the oil, liquid, and air, instead of the

738

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

Fig. 3. Wetting properties of water, ethylene glycol, and hexadecane droplets on electropolished flat aluminum (Flat) and small-pored (S-Po), large-pored (L-Po), singlepillared (S-Pi), and bundle-pillared (B-Pi) AAOs, without and with oil-impregnation after the hydrophobization of Teflon. (a-c) Apparent contact angle (CA). (d-f) Advancing contact angle (hA), receding contact angle (hR), and contact angle hysteresis (CAH). (g-i) Roll-off angle (RA). ‘‘w/o” and ‘‘w/” in (a-i) indicate ‘‘without” and ‘‘with”, respectively.

solid, liquid, and air for the cases without oil-impregnation [33–35]. Therefore, the surface tension of liquid droplets becomes more important than the features of nanostructures in determining the apparent contact angle on the oil-impregnated surfaces. Without oil-impregnation, the advancing contact angle generally follows the same trend with the apparent contact angle of an as-deposited droplet, as shown in Fig. 3d–f, while the advancing contact angle is slightly greater than the apparent contact angle of an as-deposited droplet. It indicates that the apparent contact angle of an as-deposited droplet generally represents the advancing contact angle. Meanwhile, the receding contact angle shows quite distinct behaviors depending on the liquid type and surface morphology. As for the water having relatively high surface tension (Fig. 3d), the receding contact angle decreases with the increase in the porosity on the two-dimensional nanoporous surfaces (Flat ? S-Po ? L-Po), which results in the increase in the contact angle hysteresis as well. Experimental results with a similar trend on surfaces with pores were recently reported and explained in detail [50]. While the receding contact angles are lower with the lower surface tension of the liquid, the same trends are also shown for the ethylene glycol (Fig. 3e) and hexadecane (Fig. 3f). It is attributed to the higher density of the three-phase contact line along the droplet boundary (the ratio of the length of the actually three-phase contact line to the apparent boundary length) on the

pore surface with a higher porosity, resulting in the greater droplet pinning on a solid surface [51,52]. The significant increase in the droplet pinning leads to the complete pinning with no displacement even in the vertical inclination (inclination angle = 90°) on the two-dimensional nanoporous surfaces (both S-Po and L-Po), regardless of the liquid type, as shown in Fig. 3g–i, while the droplet on Flat rolls off at
40° in inclination. In the cases of threedimensional nanopillared surfaces, the behaviors of the receding contact angle, contact angle hysteresis, and roll-off angle are quite dependent on the liquid type and surface morphology. As for the water (Fig. 3d), the receding contact angle on S-Pi is lower than that on Flat and the contact angle hysteresis is greater than that on Flat. This is because the maximum contact line density (the ratio of pillar perimeter to center-to-center pitch) can be larger than that on a flat surface (unity) [51,52], although the effect is less significant than L-Po (the length of contact line is even larger as the contact line not only exists along pores but also on the surface in between pores due to the continuity of porous surface [50,53]) to the extent that it leads to the complete pinning even in the vertical inclination (Fig. 3g). However, the B-Pi having little solid fraction can significantly reduce the three-phase contact line density along the droplet boundary [51–54] so that the receding contact angle is close to the advancing contact angle, resulting in very low contact angle hysteresis (<5°, Fig. 3d) and roll-off angle (<8°, Fig. 3g). As for

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

the ethylene glycol, the receding contact angles of the nanopillared surfaces (S-Pi and B-Pi) are lower than the nanoporous surfaces (S-Po and L-Po, Fig. 3e), which results in the larger contact angle hysteresis (Fig. 3h) with the complete pinning even in the vertical inclination (Fig. 3h), while the droplet on Flat rolls off at
18° in inclination. It indicates that the nanopillared surface morphology cannot support the pure Cassie-Baxter wetting state for the ethylene glycol having lower surface tension than water, but the intermediate wetting state [47,48] with the partial penetration of the droplet into the voids between pillared structures. It results in a more significant increase in the three-phase contact line density along the droplet boundary on the nanopillared surfaces than the nanoporous surfaces and hence the larger contact angle hysteresis, which is more significant on B-Pi than S-Pi. It is also attributed to the positively tapered sidewall profile of the B-Pi in addition to the lower surface tension of ethylene glycol than that of water, which should allow the more significant penetration of the droplet into the structural voids. It could not clearly be revealed by the apparent contact angle of the as-deposited droplet (Fig. 3b), indicating that the measurement of advancing/receding contact angles can provide clearer understanding of the wetting states. As for the hexadecane, the receding contact angles show the similar behaviors to those of ethylene glycol (Fig. 3f). Although the contact angle hysteresis of B-Pi is lower than S-Pi, which is due to the lower advancing angle (i.e., more significant penetration and wetting of a droplet into the structural voids, close to the pure Wenzel state [49]), both show the complete pinning even in the vertical inclination (Fig. 3i), while the droplet on Flat rolls off at
18° in inclination. Such significant pinning and wetting of the nanostructured AAO surfaces against the liquids with relatively low surface tensions indicate that the nanostructured surfaces with the hydrophobization only on the basis of superhydrophobicity, i.e., reducing the droplet-surface contact area by entrapping air pockets underneath a droplet, are not effective for omniphobic surfaces (aiming for liquids with various surface tensions). With oil-impregnation, such pinning is significantly reduced so that the droplet shows the high mobility (i.e., low contact angle hysteresis and roll-off angle) [32,36], regardless of the liquid type. In most cases (i.e., except for the cases of high-surface-tension liquid on the Flat surface and the nanostructured surface with low air fraction such as water and ethylene glycol droplets on S-Po), the advancing and receding contact angles are not much different from the apparent contact angle of the as-deposited droplet and alike each other so that the contact angle hysteresis on the nanostructured AAO surfaces with oil impregnation is not more than 1° (Fig. 3d–f) and the roll-off angle not more than 2° (Fig. 3g–i), regardless of the liquid type and surface morphology. The roll-off angle for the hexadecane having the lowest surface tension among the tested liquids is even <1°. It indicates that the nanostructured surface with a higher air fraction (i.e., more room for oilimpregnation) can more effectively provide the elimination of the droplet pinning sites on the solid surface with the oilimpregnation for the liquid droplet with lower surface tension. The surface tension of liquid causes a pull-up of a wetting ridge around the droplet [33,35,55,56]. Moreover, the Laplace pressure of a probe droplet (DP), which is proportional to the interfacial tension between the probe liquid and lubricant oil and the inverse of the radius (R0) of a droplet curvature (i.e., DP / c=R0 ), exerts normal force to the liquid-oil interface so that the oil layer between the droplet and solid surface is thinner than the outside of droplet, causing the droplet to partially contact to the solid surface [35,57– 59]. Compared to the Flat surface, the capillary force provided by the nanostructure on the surface retards the redistribution of the oil layer under the droplet, leading to a more stable oil/solid interface to inhibit the direct contact of the applied liquid onto the solid surface [36]. However, when the porosity or the air fraction of the

739

nanostructured surface is too low (i.e., Flat and S-Po), such effects are relatively weak so that the droplet can create the pinning site of a solid surface more readily. Therefore, in the cases of Flat and S-Po surfaces for water and ethylene glycol droplets, the pinning on a solid surface is not completely eliminated even with the oilimpregnation so that the relatively high contact angle hysteresis and roll-off angle are shown on them compared to the other nanostructured surfaces (i.e., L-Po, S-Pi, and B-Pi) having the higher porosity or air fraction. In addition, the liquid with a lower surface tension and a larger droplet curvature has a lower Laplace pressure to expel the oil layer under the droplet [58], so that the oil/solid interface is more stable. Therefore, at the given nanostructured AAO surface with the oil-impregnation, the contact angle hysteresis and roll-off angle of an ethylene glycol or hexadecane droplet are lower than those of a water droplet. Especially, in the case of hexadecane having the lowest surface tension among the tested liquids, the nanostructural morphology of the oil-impregnated AAO surfaces makes no significant difference in the droplet mobility. It indicates that the oil layer underneath the hexadecane droplet is the most stable among the tested liquids due to the lowest surface tension and Laplace pressure so that the oil layer completely inhibits the hexadecane droplet from opening up and creating the pinning site of a solid surface [35,56]. Despite such dependency on the liquid type and surface morphology, the results clearly show that the nanostructured AAO surfaces with oilimpregnation can make the surface omniphobic to cover a broad range of the surface tension of liquid, as long as the oil is stably retained on the nanostructured surface. The results also show that the stability of the oil retention at the oil/solid interface, which determines the omniphobic durability of the oil-impregnated surface, is affected by the structural morphology and dimensions. 3.3. Durability The durable omniphobicity (i.e., the stability of oil retained on the nanostructured surfaces) was further examined by continuously dripping water droplets (
5 ll) on the same location of the oil-impregnated surfaces inclined at 5°. A liquid droplet moving on oil-impregnated surfaces also induces complex force interactions causing the wetting ridge and capillary suction so that the movement accompanies the dynamics and redistribution of oil along the droplet boundary [33,35,40,41,60]. As a result, the liquid oil within the nanostructured surfaces can be depleted by the movement of droplets on the surface. The number of water droplets which rolled off along the given surface without showing any significant retardation indicates the stability of the oil retention and the omniphobic durability. Fig. 4 shows the sequentially captured images of the water droplets dripping and moving on the oil-impregnated surfaces. The full videos of the experiments conducted on the five different surfaces (Flat, S-Po, L-Po, S-Pi and B-Pi) can also be found in Supplementary Videos S1-S5, where those without oil impregnation are also included for the comparison to those with oil-impregnation. For the hydrophobized surfaces without oil-impregnation, a water droplet strongly pins on the Flat, S-Po, L-Po and S-Pi, as soon as it impinges on the surfaces (see Supplementary Videos S1-S4). In the case of the hydrophobized B-Pi surface without oil-impregnation, the water droplet moves for a while right after impacting, but the movement is not continuous and the droplet gets pinned after a short period (
0.1 s) of movement (see Supplementary Video S5), because the inclination angle (5°) is less than the roll-off angle of a water droplet on the B-Pi surface. In the cases of oil-impregnated surfaces (Fig. 4), the movements of water droplets can be classified into three regimes in sequence; quick bouncing, rolling, and pinning. Initially, the excessive oil layer on the surfaces repels the dripping water droplet, totally

740

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

Fig. 4. Sequential images of the water droplets continuously dripped on the oil-impregnated surfaces of flat aluminum (Flat) and small-pored (S-Po), large-pored (L-Po), single-pillared (S-Pi), and bundle-pillared (B-Pi) AAO surfaces. White scale bars in (a)-(e) indicate 1 mm. See also Supplementary Videos S1-S5.

inhibiting their contact to the solid surfaces. As a result, the droplet bounces off from the surfaces and quickly rolls off. With an increase in the number of water droplets dripped onto the surfaces, the excessive oil layer gets thinner, and finally a water droplet sits on the surface with the formation of the wetting ridge. Then, the droplet begins to roll off at considerably low speeds, instead of quick bouncing. In the case of the Flat surface (Fig. 4a), the droplet showed quick bouncing until the 8th droplet, followed by rolling for the 9th droplet. The 10th droplet that initially rolled following the path of the 9th droplet became pinned on the surface after a short period of the rolling motion. The 10th droplet did not move, although it was merged with the 11th droplet to form a bigger droplet. It indicates that the rolling water droplets sheared off and depleted the oil layer on the Flat surface to such an extent that the pinning site of a solid surface was eventually exposed to the water droplet. It also indicates that the Flat surface is not effective to maintain the stable oil/solid interface under dynamic conditions such as the rolling water droplets redistributing the oil layer on the solid surface. The S-Po surface (Fig. 4b) showed a similar behavior to the Flat surface. After quick bouncing up to the 9th droplet, the rolling motion was observed from the 10th droplet and the pinning started from the 12th droplet around the impacting site. Even with the merging by the 13th droplet, the droplet did not move with the strong pinning. It also indicates that the rolling motion of the water droplets eventually sheared off and depleted the oil layer from the

surface even with the nanopore structures. It is attributed to the relatively low pore density or porosity of the S-Po surface. Due to the disconnected nanoscale dead-end pore geometry of the S-Po surface, the capillary force maintaining the oil inside the pores is much larger than the surface tension force pulling the wetting ridge up so that the oil impregnated within the pores of the S-Po surface is stably retained against the capillary suction force exerted by the rolling water droplets [61,62]. However, the lubricating oil layer on the relatively large flat area (i.e., the gap size of ~80 nm) between the small pores (i.e., the pore diameter of ~20 nm) of the S-Po surface is easily redistributed and opens up the pinning sites of a solid surface during the shearing motion of the rolling droplet [60,63]. In contrast, the L-Po surface having high pore density (Fig. 4c) showed almost unlimited slipperiness for the water droplets. The initial quick bouncing motion was shown up to the 20th droplets. Then, the following droplets showed the rolling motion with no significant pinning or delay in the rolling velocity up to the 176th droplet, which was the maximum number of droplets tested in this study. Compared to the S-Po surface, the flat area between the pores of the L-Po surface is smaller (i.e., the gap size of 26 nm and the pore diameter of 74 ± 3 nm). The oil layer on such a little flat area (i.e., high porosity) of the L-Po surface cannot easily be redistributed and opened up by the shearing droplet to reveal the solid pinning sites, compared to the case of the S-Po surface. In the case of nanopillared surfaces, both S-Pi (Fig. 4d)

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

and B-Pi (Fig. 4e) show the similar behaviors to those of Flat and S-Po surfaces, regardless of their air fractions. On S-Pi (Fig. 4d), the droplet shows quick bouncing until the 13th droplet and then rolling for the 14th and pinning from the 15th droplet. On B-Pi (Fig. 4e), although the initial quick bouncing was maintained until the 19 droplets, the droplet starts to show the pinning from the 21st droplet, right after the rolling for the 20th droplet. Despite the much larger air fraction (i.e., more room of oil-impregnation)

741

than porous AAO surfaces, the oil impregnated on the nanopillared AAO surfaces is not confined by the structures but continuously connected along the valleys so that it is relatively easier for the oil layer to move and redistribute by the capillary suction of the rolling motion of a water droplet [40,41]. As a result, the oil is relatively easy to be depleted and open the underlying solid surface to provide the pinning sites to the running droplets [42]. The results show that the three-dimensional nanopillared surface

Fig. 5. Durability test of the oil-impregnated surfaces of flat aluminum (Flat) and small-pored (S-Po), large-pored (L-Po), single-pillared (S-Pi), and bundle-pillared (B-Pi) AAOs under external shear flow of water jet. The surfaces are inclined at
5°. (i) Surfaces under the application of water jet. (ii) Surfaces after the application of water jet for the specified durations (10 min for Flat, S-Po, S-Pi, and B-Pi; 48 h for L-Po). (iii-iv) Wetting and mobility of water droplets on the surfaces after the application of water jet. Black scale bars in the images indicate 2 cm. See also Supplementary Videos S6-S10.

742

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

morphologies are not effective to provide the durable omniphobicity to the oil-impregnated surfaces. Instead, the synergetic effects of the disconnected nanoscale dead-end pore geometry with high porosity allow the durability in omniphobicity, stably retaining the oil/solid interface and effectively eliminating the pinning sites of a solid surface even to the droplet in dynamic motions. The durability was also examined under the continuous jet flow of water applied on the surfaces at the inclination of
5°, as shown in Fig. 5 (see also Supplementary Videos S6-S10). After the application of the water jet, the wetting and slipperiness of a water droplet on the surfaces were also checked, which should indicate the stability and retention of oil on the surfaces even after the application of jet flow. After the application of the water jet for 10 min, the oil-impregnated surfaces of Flat (Fig. 5a), S-Po (Fig. 5b), S-Pi (Fig. 5d), and B-Pi (Fig. 5e) morphologies show the pinning of a water droplet at the inclination of
5°, which indicates the depletion of oil and the loss of omniphobicity by the jet flow [42]. In contrast, the oil-impregnated surface of L-Po morphology (Fig. 5c) shows the consistent non-wetting and slipperiness for a water droplet even after the application of the water jet flow for 48 h. It indicates that the surface morphology of the L-Po surface (i.e., synergetic effects of the disconnected nanoscale dead-end pore geometry with high porosity) provides excellent stability and retention of oil within the nanostructures even under the continuous jet flow of water, in addition to the droplet applications. The results further corroborate that the surface morphology and dimension are critical to realize durable omniphobic surfaces based on oil-impregnation and that it should be more advantageous to use the disconnected dead-end pore morphology than pillared one, along with the pore dimension on a smaller scale with higher porosity for the durable omniphobicity. 3.4. Omniphobicity The omniphobicity of the L-Po surface that shows the superior durability has further been examined for complex fluids such as ketchup and olive oil. Fig. 6 shows the movements of ketchup and olive oil on the L-Po surfaces without and with oilimpregnation inclined at
10° (see also Videos S11 and S12 in Supplementary Material). On the hydrophobized L-Po surfaces before

w/o oil-impregnation

oil-impregnation (Fig. 6a for ketchup and Fig. 6c for olive oil), the complex liquids stick on the surfaces with no movement. In contrast, on the L-Po surfaces after oil-impregnation (Fig. 6b for ketchup and Fig. 6d for olive oil), they do not stick but move along the surfaces relatively well. The perfluorinated oil which was impregnated within the nanopore structures is immiscible with organic oils such as the olive oil so that the oil-impregnated L-Po surface even repels the organic oil [35]. The results further show that the oil-impregnated L-Po AAO surface has great omniphobicity, allowing exceptional anti-stickiness and contamination resistance even for complex fluids. As for the anti-stickiness and contamination resistance of the omniphobic surface of the oil-impregnated L-Po AAO surface, we have also examined the anti-bacterial adhesion property of the L-Po surface. As shown in Fig. 7, the analysis of SEM images and agar plating assay revealed the different adhesion responses of E. coli K-12 to the Flat, hydrophilic L-Po, hydrophobic (i.e., with Teflon-coating) L-Po, and oil-impregnated L-Po surfaces. The initial level of E. coli K-12 in PBS was 7.9 log CFU/coupon. A statistically significant reduction (p < 0.05) in the number of E. coli K12 adhered to the surface of hydrophobic L-Po was observed compared to the hydrophilic Flat and L-Po surfaces. It is attributed that the air trapped within the nanopores and the weaklypolarizable Teflon layer with a low surface energy on the hydrophobic L-Po surface minimize the van der Waals interactions between the bacteria and the solid surface [64–67]. The oil-impregnation into the hydrophobized L-Po resulted in an additional 0.7 log reduction in the bacterial adhesion compared to the hydrophobic L-Po. Compared to the electropolished Al, the attachment of E. coli K-12 was reduced by 95.9 and 99.2% on the hydrophobic L-Po and oil-impregnated L-Po surfaces, respectively. The most pronounced reduction of E. coli K-12 adhesion is shown on the oil-impregnated hydrophobic L-Po surface. Such significant reduction of bacterial adhesion is attributed to the stably immobilized oil layer on the L-Po surface which inhibits the direct contact of E. coli as well as the extracellular polymeric substance of the E. coli in the liquid media to the solid surface of L-Po [68]. The robust omniphobicity to prevent the bacterial adhesion can potentially lead to the prevention of biofilm formation [8,69].

w/ oil-impregnation

Fig. 6. Sequential images of the wetting and droplet mobility of ketchup (a-b) and olive oil (c-d) on the large-pored (L-Po) AAO surfaces without (a and c) and with (b and d) oil impregnation. Each sample is inclined at
10°. Black scale bars in the images indicate 2 cm. See also Supplementary Videos S11 and S12.

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

743

Fig. 7. Characterizations of bacteria (E. coli K-12) adhesion. (a-d) SEM and schematic (inset) images of the bacteria adhesion on electropolished Al (Flat), hydrophilic largepored (L-Po) AAO, hydrophobic (i.e., with Teflon-coating) L-Po, and oil-impregnated L-Po, respectively. White scale bars in (a-d) indicate 5 lm. (e) Bacteria population (colony forming unit, CFU) measured with the four different surfaces. Asterisks in (e) indicate statistical significance (p < 0.05) between indicated groups.

4. Conclusions Although a thin layer of Teflon-coating on nanostructured AAO enhances the chemical affinity of the perfluorinated oil for the impregnation to the nanostructures, the oil-impregnation into the less porous AAO (i.e., as-anodized AAO without pore widening) failed to completely eliminate the pinning sites of a solid surface to a liquid droplet, especially for the liquid with high surface tension. When the oil layer on the top surface is further sheared off by the dynamic motion of a droplet or continuous jet flow, the oil layer is more prone to be depleted away to lose the initial omniphobicity. In contrast, the more porous AAO surface (e.g., made with a porewidening process) provides the stable retention of the lubricant oil on the surface even against such dynamic effects so that the durable omniphobicity is attained regardless of the surface tension of the applied liquids. Although the nanopillared AAO surfaces (made with a more elongated pore-widening process) also initially exhibit omniphobicity with the oil-impregnation, the pillared surface morphology is not effective to stably retain the lubricant oil on the surface under the dynamics effects and to show the durable omniphobicity, regardless of their dimensions, due to the open and interconnected nature of the surface morphology for the relatively easy redistribution of the oil. Previously, various techniques have been reported to realize oilimpregnated omniphobic surfaces on commercial materials [32,37,39], and their multifunctional properties were demonstrated [8,38,70,71]. However, this work reveals that the combined effects of the disconnected dead-end pore morphology with the high porosity on a nanoscale are essential to attain the stable oil/solid interface for oil retention and hence the durable omniphobicity even under dynamics effects. Moreover, the oil-impregnated nanoporous surface also has damage tolerance with self-healing capability [1,32,37]. Such enhanced omniphobic durability of the oil-impregnation-based omniphobic surface is of great significance in real applications such as anti-bacterial adhesion [8,68,72], anti-corrosion [1,73], condensation [74], and anti-icing [5]. In particular, the anodizing processes employed in this study for the engineering of nanostructures on aluminum substrate is readilyscalable industrial processes for various metallic materials, so that the realization of the durable omniphobic surfaces based on the

nanoporous surfaces with oil-impregnation can also be realized readily for many civil and military systems and applications using metallic materials. Acknowledgement This work was mainly supported by the US Office of Naval Research (ONR) Award N00014-14-1-0502. It was also partly supported by the Agriculture and Food Research Initiative Grant No. 2015-67017-23083 from the USDA National Institute of Food and Agriculture, Improving Food Safety, A1331. J. Lee acknowledges the support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT, Ministry of Science and ICT) (No. 2018R1C1B6006156) and the local industry promotion business linked with public institutions (Gyeongnam) (P0004798) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.06.068. References [1] J. Lee, S. Shin, Y. Jiang, C. Jeong, H.A. Stone, C.-H. Choi, Oil-Impregnated nanoporous oxide layer for corrosion protection with self-healing, Adv. Funct. Mater. 27 (2017) 1606040. [2] Z. Lu, P. Wang, D. Zhang, Super-hydrophobic film fabricated on aluminium surface as a barrier to atmospheric corrosion in a marine environment, Corros. Sci. 91 (2015) 287–296. [3] E. Aljallis, M.A. Sarshar, R. Datla, V. Sikka, A. Jones, C.-H. Choi, Experimental study of skin friction drag reduction on superhydrophobic flat plates in high reynolds number boundary layer flow, Phys. Fluids 25 (2) (2013) 025103. [4] M.A. Sarshar, C. Swarctz, S. Hunter, J. Simpson, C.-H. Choi, Effects of contact angle hysteresis on ice adhesion and growth on superhydrophobic surfaces under dynamic flow conditions, Colloid Polym. Sci. 291 (2) (2013) 427–435. [5] P. Kim, T.-S. Wong, J. Alvarenga, M.J. Kreder, W.E. Adorno-Martinez, J. Aizenberg, Liquid-infused nanostructured surfaces with extreme anti-ice and anti-frost performance, ACS Nano 6 (8) (2012) 6569–6577. [6] C. Jeong, J. Lee, K. Sheppard, C.-H. Choi, Air-impregnated nanoporous anodic aluminum oxide layers for enhancing the corrosion resistance of aluminum, Langmuir 31 (40) (2015) 11040–11050.

744

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745

[7] Y.-B. Park, H. Im, M. Im, Y.-K. Choi, Self-cleaning effect of highly waterrepellent microshell structures for solar cell applications, J. Mater. Chem. 21 (3) (2011) 633–636. [8] A.K. Epstein, T.S. Wong, R.A. Belisle, E.M. Boggs, J. Aizenberg, Liquid-infused structured surfaces with exceptional anti-biofouling performance, Proc. Natl. Acad. Sci. U. S. A. 109 (33) (2012) 13182–13187. [9] C. Lee, C.-J. Kim, Underwater restoration and retention of gases on superhydrophobic surfaces for drag reduction, Phys. Rev. Lett. 106 (1) (2011) 014502. [10] N. Subramanian, A. Qamar, A. Alsaadi, A. Gallo Jr, M.G. Ridwan, J.-G. Lee, S. Pillai, S. Arunachalam, D. Anjum, F. Sharipov, Evaluating the potential of superhydrophobic nanoporous alumina membranes for direct contact membrane distillation, J. Coll. Interf. Sci. 533 (2019) 723–732. [11] W. Lei, D.R. McKenzie, Nanoscale capillary flows in alumina: testing the limits of classical theory, J. Phys. Chem. Lett. 7 (14) (2016) 2647–2652. [12] Z. Hendren, J. Brant, M. Wiesner, Surface modification of nanostructured ceramic membranes for direct contact membrane distillation, J. Membr. Sci. 331 (1–2) (2009) 1–10. [13] D.F. Cheng, C. Urata, M. Yagihashi, A. Hozumi, A Statically oleophilic but dynamically oleophobic smooth nonperfluorinated surface, Angew. Chem. 124 (12) (2012) 3010–3013. [14] C. Aulin, S.H. Yun, L. Wågberg, T. Lindström, Design of highly oleophobic cellulose surfaces from structured silicon templates, ACS Appl. Mater. Interf. 1 (11) (2009) 2443–2452. [15] Z. Cheng, H. Lai, Y. Du, R. Hou, C. Li, N. Zhang, K. Sun, Assembling mixed carboxylic acid molecules on hierarchical structured aluminum substrates for the fabrication of superoleophobic surfaces with controlled oil adhesion, Chem. Plus. Chem. 80 (1) (2015) 151–157. [16] K. Koch, B. Bhushan, Y.C. Jung, W. Barthlott, Fabrication of artificial lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion, Soft Mat. 5 (7) (2009) 1386–1393. [17] T. Koishi, K. Yasuoka, S. Fujikawa, T. Ebisuzaki, X.C. Zeng, Coexistence and transition between cassie and wenzel state on pillared hydrophobic surface, Proc. Natl. Acad. Sci. U. S. A. 106 (21) (2009) 8435–8440. [18] T. Liu, C. Kim, Turning a surface superrepellent even to completely wetting liquids, Science 346 (6213) (2014). [19] R. Dufour, P. Brunet, M. Harnois, R. Boukherroub, V. Thomy, V. Senez, Zipping effect on omniphobic surfaces for controlled deposition of minute amounts of fluid or colloids, Small 8 (8) (2012) 1229–1236. [20] A. Susarrey-Arce, A.G. Marin, H. Nair, L. Lefferts, J.G.E. Gardeniers, D. Lohse, A. van Houselt, Absence of an evaporation-driven wetting transition on omniphobic surfaces, Soft Mat. 8 (38) (2012) 9765–9770. [21] A. Tuteja, W. Choi, J.M. Mabry, G.H. McKinley, R.E. Cohen, Robust omniphobic surfaces, Proc. Natl. Acad. Sci. U. S. A. 105 (47) (2008) 18200–18205. [22] A. Lafuma, D. Quéré, Superhydrophobic states, Nat. Mater. 2 (2003) 457. [23] Y.C. Jung, B. Bhushan, Wetting behaviour during evaporation and condensation of water microdroplets on superhydrophobic patterned surfaces, J. Microsc. 229 (1) (2008) 127–140. [24] E.M. Domingues, S. Arunachalam, H. Mishra, Doubly reentrant cavities prevent catastrophic wetting transitions on intrinsically wetting surfaces, ACS Appl. Mater. Interf. 9 (25) (2017) 21532–21538. [25] E.M. Domingues, S. Arunachalam, J. Nauruzbayeva, H. Mishra, Biomimetic coating-free surfaces for long-term entrapment of air under wetting liquids, Nat. Commun. 9 (1) (2018) 3606. [26] S. Arunachalam, R. Das, J. Nauruzbayeva, E.M. Domingues, H. Mishra, Assessing omniphobicity by immersion, J. Coll. Interf. Sci. 534 (2019) 156–162. [27] T.-S. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal, J. Aizenberg, Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity, Nature 477 (7365) (2011) 443–447. [28] P. Wang, D. Zhang, Z. Lu, Slippery liquid-infused porous surface bio-inspired by pitcher plant for marine anti-biofouling application, Coll. Surf B 136 (2015) 240–247. [29] C. Shillingford, N. MacCallum, T.-S. Wong, P. Kim, J. Aizenberg, Fabrics coated with lubricated nanostructures display robust omniphobicity, Nanotechnology 25 (1) (2014) 014019. [30] D.C. Leslie, A. Waterhouse, J.B. Berthet, T.M. Valentin, A.L. Watters, A. Jain, P. Kim, B.D. Hatton, A. Nedder, K. Donovan, E.H. Super, C. Howell, C.P. Johnson, T. L. Vu, D.E. Bolgen, S. Rifai, A.R. Hansen, M. Aizenberg, M. Super, J. Aizenberg, D. E. Ingber, A bioinspired omniphobic surface coating on medical devices prevents thrombosis and biofouling, Nat. Biotechnol. 32 (11) (2014) 1134– 1140. [31] A. Grinthal, J. Aizenberg, Mobile interfaces: liquids as a perfect structural material for multifunctional, antifouling surfaces, Chem. Mater. 26 (1) (2014) 698–708. [32] N. Vogel, R.A. Belisle, B. Hatton, T.S. Wong, J. Aizenberg, Transparency and damage tolerance of patternable omniphobic lubricated surfaces based on inverse colloidal monolayers, Nat. Commun. 4 (2013) 10. [33] J.D. Smith, R. Dhiman, S. Anand, E. Reza-Garduno, R.E. Cohen, G.H. McKinley, K. K. Varanasi, Droplet mobility on lubricant-impregnated surfaces, Soft Matter 9 (6) (2013) 1772–1780. [34] K. Rykaczewski, T. Landin, M.L. Walker, J.H.J. Scott, K.K. Varanasi, Direct imaging of complex nano- to microscale interfaces involving solidliquid, and gas phases, ACS Nano 6 (10) (2012) 9326–9334. [35] F. Schellenberger, J. Xie, N. Encinas, A. Hardy, M. Klapper, P. Papadopoulos, H.-J. Butt, D. Vollmer, Direct Observation of drops on slippery lubricant-infused surfaces, Soft Matter 11 (2015) 7617–7626.

[36] S. Sunny, N. Vogel, C. Howell, T.L. Vu, J. Aizenberg, Lubricant-infused nanoparticulate coatings assembled by layer-by-layer deposition, Adv. Funct. Mater. 24 (42) (2014) 6658–6667. [37] A.B. Tesler, P. Kim, S. Kolle, C. Howell, O. Ahanotu, J. Aizenberg, Extremely durable biofouling-resistant metallic surfaces based on electrodeposited nanoporous tungstite films on steel, Nat. Commun. 6 (2015) 8649. [38] P. Wang, D. Zhang, Z. Lu, S. Sun, Fabrication of slippery lubricant-infused porous surface for inhibition of microbially influenced corrosion, ACS Appl. Mater. Interf. 8 (2) (2016) 1120–1127. [39] P. Wang, Z. Lu, D. Zhang, Slippery liquid-infused porous surfaces fabricated on aluminum as a barrier to corrosion induced by sulfate reducing bacteria, Corros. Sci. 93 (2015) 159–166. [40] J.S. Wexler, I. Jacobi, H.A. Stone, Shear-driven failure of liquid-infused surfaces, Phys. Rev. Lett. 114 (16) (2015) 168301. [41] I. Jacobi, J. Wexler, H. Stone, Overflow cascades in liquid-infused substrates, Phys. Fluids 27 (8) (2015) 082101. [42] J. Zhang, L. Wu, B. Li, L. Li, S. Seeger, A. Wang, Evaporation-induced transition from nepenthes pitcher-inspired slippery surfaces to lotus leaf-inspired superoleophobic surfaces, Langmuir 30 (47) (2014) 14292–14299. [43] C. Jeong, C.-H. Choi, Single-step direct fabrication of pillar-on-pore hybrid nanostructures in anodizing aluminum for superior superhydrophobic efficiency, ACS Appl. Mater. Interf. 4 (2) (2012) 842–848. [44] W. Lee, B.G. Park, D.H. Kim, D.J. Ahn, Y. Park, S.H. Lee, K.B. Lee, Nanostructuredependent water-droplet adhesiveness change in superhydrophobic anodic aluminum oxide surfaces: from highly adhesive to self-cleanable, Langmuir 26 (3) (2010) 1412–1415. [45] J.-G. Fan, D. Dyer, G. Zhang, Y.-P. Zhao, Nanocarpet effect: pattern formation during the wetting of vertically aligned nanorod arrays, Nano Lett. 4 (11) (2004) 2133–2138. [46] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [47] X.J. Feng, L. Jiang, Design and creation of superwetting/antiwetting surfaces, Adv. Mater. 18 (23) (2006) 3063–3078. [48] P. Roach, N.J. Shirtcliffe, M.I. Newton, Progess in superhydrophobic surface development, Soft Matter 4 (2) (2008) 224–240. [49] R.N. Wenzel, Resistance of soild surfaces to wetting by water, Ind. Eng. Chem. 28 (8) (1936) 988–994. [50] Y. Jiang, W. Xu, M.A. Sarshar, C.-H. Choi, Generalized models for advancing and receding contact angles of fakir droplets on pillared and pored surfaces, J. Coll. Interf. Sci. (2019). [51] A.T. Paxson, K.K. Varanasi, Self-similarity of contact line depinning from textured surfaces, Nat. Commun. 4 (2013) 1492. [52] W. Xu, C.-H. Choi, From sticky to slippery droplets: dynamics of contact line depinning on superhydrophobic surfaces, Phys. Rev. Lett. 109 (2) (2012) 024504. [53] Y. Jiang, Y. Sun, J.W. Drelich, C.-H. Choi, Spontaneous spreading of a droplet: the role of solid continuity and advancing contact angle, Langmuir 34 (17) (2018) 4945–4951. [54] R. Raj, R. Enright, Y. Zhu, S. Adera, E.N. Wang, Unified model for contact angle hysteresis on heterogeneous and superhydrophobic surfaces, Langmuir 28 (45) (2012) 15777–15788. [55] A. Carré, J.-C. Gastel, M.E.R. Shanahan, Viscoelastic effects in the spreading of liquids, Nature 379 (1996) 432. [56] M. Sokuler, G.K. Auernhammer, M. Roth, C. Liu, E. Bonacurrso, H.-J. Butt, The softer the better: fast condensation on soft surfaces, Langmuir 26 (3) (2010) 1544–1547. [57] S.J. Park, B.M. Weon, J. San Lee, J. Lee, J. Kim, J.H. Je, Visualization of asymmetric wetting ridges on soft solids with X-Ray microscopy, Nat. Commun. 5 (2014). [58] L. Chen, E. Bonaccurso, T. Gambaryan-Roisman, V. Starov, N. Koursari, Y. Zhao, Static and dynamic wetting of soft substrates, Curr. Opin. Coll. Interf. Sci. 36 (2018) 46–57. [59] R. Pericet-Cámara, A. Best, H.-J. Butt, E. Bonaccurso, Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: an experimental investigation, Langmuir 24 (19) (2008) 10565– 10568. [60] M.J. Kreder, D. Daniel, A. Tetreault, Z. Cao, B. Lemaire, J.V. Timonen, J. Aizenberg, Film dynamics and lubricant depletion by droplets moving on lubricated surfaces, Phys. Rev. X 8 (3) (2018) 031053. [61] T. Gambaryan-Roisman, Liquids on porous layers: wetting, imbibition and transport processes, Curr. Opin. Coll. Interf. Sci. 19 (4) (2014) 320–335. [62] B. Zhmud, F. Tiberg, K. Hallstensson, Dynamics of capillary rise, J. Coll. Interf. Sci. 228 (2) (2000) 263–269. [63] D. Daniel, J.V. Timonen, R. Li, S.J. Velling, J. Aizenberg, Oleoplaning droplets on lubricated surfaces, Nat. Phys. 13 (10) (2017) 1020. [64] F. Hizal, N. Rungraeng, J. Lee, S. Jun, H.J. Busscher, H.C. Van der Mei, C.-H. Choi, Nanoengineered superhydrophobic surfaces of aluminum with extremely low bacterial adhesivity, ACS Appl. Mater. Interf. 9 (13) (2017) 12118–12129. [65] J.A. Callow, M.E. Callow, Trends in the development of environmentally friendly fouling-resistant marine coatings, Nat. Commun. 2 (2011) 244. [66] S. Jiang, Z. Cao, Ultralow-fouling, functionalizable, and hydrolyzable zwitterionic materials and their derivatives for biological applications, Adv. Mater. 22 (9) (2010) 920–932. [67] K. Autumn, M. Sitti, Y.A. Liang, A.M. Peattie, W.R. Hansen, S. Sponberg, T.W. Kenny, R. Fearing, J.N. Israelachvili, R.J. Full, Evidence for van der Waals adhesion in Gecko Setae, Proc. Natl. Acad. Sci. U. S. A. 99 (19) (2002) 12252– 12256.

J. Lee et al. / Journal of Colloid and Interface Science 553 (2019) 734–745 [68] Y. Kovalenko, I. Sotiri, J.V.I. Timonen, J.C. Overton, G. Holmes, J. Aizenberg, C. Howell, Bacterial interactions with immobilized liquid layers, Adv. Healthc. Mater. 6 (15) (2017) 1600948. [69] J. Yin, M.L. Mei, Q. Li, R. Xia, Z. Zhang, C.H. Chu, Self-Cleaning and antibiofouling enamel surface by slippery liquid-infused technique, Sci. Rep. 6 (2016) 25924. [70] B.R. Solomon, K.S. Khalil, K.K. Varanasi, Drag reduction using lubricantimpregnated surfaces in viscous laminar flow, Langmuir 30 (36) (2014) 10970–10976. [71] K. Rykaczewski, S. Anand, S.B. Subramanyam, K.K. Varanasi, Mechanism of frost formation on lubricant-impregnated surfaces, Langmuir 29 (17) (2013) 5230–5238.

745

[72] J. Bruchmann, I. Pini, T.S. Gill, T. Schwartz, P.A. Levkin, Patterned SLIPS for the formation of arrays of biofilm microclusters with defined geometries, Adv. Healthc. Mater. 6 (1) (2017) 1601082. [73] R. Qiu, Q. Zhang, P. Wang, L. Jiang, J. Hou, W. Guo, H. Zhang, Fabrication of slippery liquid-infused porous surface based on carbon fiber with enhanced corrosion inhibition property, Coll. Surf. A 453 (2014) 132–141. [74] S. Anand, A.T. Paxson, R. Dhiman, J.D. Smith, K.K. Varanasi, Enhanced condensation on lubricant-impregnated nanotextured surfaces, ACS Nano 6 (11) (2012) 10122–10129.