Evaporation-Induced Transition from Nepenthes Pitcher-Inspired

Nov 6, 2014 - Martial Rey , Menglin Yang , Linda Lee , Ye Zhang , Joey G. Sheff , Christoph W. Sensen , Hynek Mrazek , Petr Halada , Petr Man , Justin...
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Evaporation-Induced Transition from Nepenthes Pitcher-Inspired Slippery Surfaces to Lotus Leaf-Inspired Superoleophobic Surfaces Junping Zhang,*,† Lei Wu,†,‡ Bucheng Li,† Lingxiao Li,†,§ Stefan Seeger,∥ and Aiqin Wang† †

Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, PR China ‡ Graduate University of the Chinese Academy of Sciences, Beijing, PR China § College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, PR China ∥ Department of Chemistry, University of Zurich, Zurich, Switzerland S Supporting Information *

ABSTRACT: The newly developed Nepenthes pitcher (NP)-inspired slippery surfaces, formed by immobilizing fluoroliquids on lotus leaf (LL)-inspired superoleophobic surfaces, are of great general interest, whereas there are many interesting phenomena and fundamental scientific issues remaining to be unveiled. Here we present our findings of the effects of evaporation of the fluoroliquid, an inevitable process in most cases, -induced transition from NP-inspired to LLinspired surfaces on the wettability, transparency, and self-cleaning property of the surfaces. The transition is controlled by regulating the evaporation temperature of the model fluoroliquid, Krytox100. The evaporation of Krytox100 has great a influence on the wettability, transparency, and self-cleaning property. An intermediate “sticky” state is observed in the transition process. We believe that our findings in the transition process are helpful in understanding the similarities and differences between the NP-inspired and LL-inspired surfaces and in designing new bioinspired antiwetting surfaces and exploring their potential applications.

1. INTRODUCTION Biomimetic antiwetting surfaces inspired by plants and animals, e.g., lotus leaf (LL), fish scales, and Nepenthes pitcher (NP), in the natural world are of great general interest owing to their unique properties such as low sliding angles (SAs) for various liquids and self-cleaning behavior.1−6 The LL-inspired superhydrophobic surfaces develop very quickly;7−9 however, it is still very challenging to create superoleophobic surfaces with low SAs (< 10°) for nonpolar liquids because of their low surface tension, e.g., 23.8 mN/m for n-decane, compared to that of water (72.8 mN/m).10−12 Nonpolar liquids often have high contact angles (CA ≈ 150°) but adhere to most of the reported superoleophobic surfaces and cannot roll down. This means that these surfaces have no self-cleaning property once they encounter nonpolar liquids. In fact, both materials of very low surface tension and special microstructure are very important for nonpolar liquids to roll down a superoleophobic surface.13 Until now, only a few studies have reported superoleophobic surfaces with high CAs and low SAs for nonpolar liquids by using fluoro-POSS with very low surface tension or by introducing some special patterns such as an overhang structure, re-entrant surface curvature, candle soot, and silicone nanofilaments (SNFs).14−19 Alternatively, Jiang et al. developed novel underwater low-adhesive superoleophobic surfaces inspired by fish scales.20−22 However, these superoleophobic surfaces still have common problems such as low © XXXX American Chemical Society

mechanical stability and failure under pressure, which restrict their potential applications. To address these problems, a new group of NP-inspired slippery surfaces were developed in 2011 by immobilizing fluoroliquids on LL-inspired superoleophobic surfaces.23,24 The NP-inspired surfaces showed overwhelming properties over their natural counterparts and state-of-the-art LL-inspired superoleophobic surfaces in many aspects, e.g., repelling various liquids, quick self-healing after physical damage, and high transparency. Instead of fluoroliquids, Hozumi et al. reported an ionic liquid-infused NP-inspired surface that exhibited excellent stability even under high temperature and vacuum conditions.25 The NP-inspired surfaces have promising applications in many fields, including anti-icing, antibiofouling, antiscaling, and smart surfaces.26−30 Pioneering work on newly developed NP-inspired surfaces is encouraging, but the information we have about them is just the tip of the iceberg compared to the developed LL-inspired superhydrophobic surfaces. There are a lot of interesting phenomena and fundamental scientific issues remaining to be unveiled. Also, comprehensive experimental investigations of the similarities and differences between the NP-inspired and Received: August 18, 2014 Revised: October 22, 2014

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400 mL of toluene with a water concentration of 120 ppm in the custom-made chamber. Subsequently, 250 μL of TCMS was injected into the chamber at 25 °C to trigger the growth of SNFs. Twenty-four hours later, the SNFs were successfully grown on the glass slides. The coated samples were rinsed with 10.0 mL of toluene, 10.0 mL of ethanol, and 10.0 mL of a 50% v/v deionized water/ethanol solution successively and then dried under a flow of nitrogen. To prepare the branched SNFs, the SNFs-coated glass slides were repeatedly treated with O2 plasma at an O2 flow rate of 10 sccm and a power of 100 W for 5 min (Femto, Germany) and then coated twice with TCMS according to the above procedure. 2.3. Preparation of Fluoro-SNFs. Fluoro-SNF-coated glass slides were prepared according to our previously reported method.19 The O2-plasma-activated sample (10 sccm and 100 W for 5 min) was immersed in 75.0 mL of dry toluene, and then 5.0 mL of toluene containing 40.0 μL of 1H,1H,2H,2H-perfluorodecyltrichlorosilane was added. The sample was kept in the above solution for 24 h at room temperature to ensure complete modification of the SNFs. The fluoroSNF-coated glass slide was washed with 10.0 mL of dry toluene and dried under a flow of nitrogen. 2.4. Preparation of Fluoro-SNFs/Krytox100 Slippery Surfaces. Krytox100 (30 μL) was dropped using a microsyringe (50 μL) onto the horizontal fluoro-SNF-coated glass slide to form a flat Krytox100 layer with a thickness of 20 μm. The thickness of the overcoated Krytox100 layer can be controlled by the fluid volume given a known surface area of the sample.23 The fluid spreads quickly and spontaneously over the whole substrate via the capillary effect. The sample was kept at room temperature for 4 h to ensure uniform wetting of the surface by Krytox100. 2.5. Evaporation of the Krytox100-Induced Transition. The fluoro-SNFs/Krytox100 surfaces were placed horizontally in an oven at 65, 70, and 75 °C. The samples were taken out of the oven for measurement at predetermined time intervals. 2.6. Characterization. Measurements of the CA, SA, liquid drop bouncing, and sliding speed were performed at 25 °C using liquid drops of 10 μL on a Contact Angle System OCA 20 (Dataphysics, Germany) equipped with a tilting table. For the CA measurements, the syringe was positioned in a way in which the droplet of liquid could contact the sample surface before leaving the needle. The tilting angle of the table is adjustable (0−70°) and allows for the subsequent measurement of SA at the same position on the sample. For the bounce tests, liquid drops were released from a height of 10 mm, and the bounce times and bounce behavior were recorded using the highspeed camera of the contact angle system at 400 fps. The sliding speed was measured using a timekeeper while moving a liquid drop on the 10°-tilted sample. At least three readings were recorded for each test, and the average values were presented. The micrographs of the samples were taken using a field emission scanning electron microscope (SEM, JSM-6701F, JEOL). Before SEM observation, all samples were fixed on aluminum stubs and coated with gold (∼7 nm). The transmittance of the samples at 300−800 nm was recorded using a UV−vis spectrophotometer (Specord 200, Analytik Jena AG).

the LL-inspired surfaces in wettability and the self-cleaning property are rare.6,31 Recently, we have prepared NP-inspired surfaces by locking Krytox liquids, a kind of perfluoropolyether liquid, on the fluoro-SNFs superoleophobic surfaces, and we have reported that the sliding speed of liquid drops on the NPinspired surface is obviously slower than on the LL-inspired superoleophobic surface.32 In addition, the CAs of liquids on NP-inspired surfaces is lower than on LL-inspired surfaces, which influences the motion of drops and the self-cleaning property of the surfaces. Moreover, we found that evaporation of the fluoroliquids from the NP-inspired surfaces is inevitable in most cases, especially at a high temperature, which should have an influence on the wettability, transparency, and selfcleaning property of the surfaces. Here we report the evaporation of the Krytox100-induced transition from the NP-inspired fluoro-SNFs/Krytox100 slippery surfaces to the LL-inspired fluoro-SNFs superoleophobic surfaces on the basis of our work on SNFs, superoleophobic surfaces, and slippery surfaces (Figure 1a).19,32−34 The present work aims to advance the fundamental

Figure 1. (a) Schematic showing the evaporation of the Krytox100induced transition from the NP-inspired to LL-inspired surface. (b) Image of NP, SEM image of the NP-inspired fluoro-SNFs/Krytox100 surface, and representative water and DIM drops on the surface. (c) Image of LL, SEM image of the LL-inspired fluoro-SNFs surface, and representative water and DIM drops on the surface.

understanding of (1) the transition process from the NPinspired to the LL-inspired surface and (2) the similarities and differences between them. The interesting phenomena and scientific issues in the controlled transition process are disclosed. The evaporation of Krytox100 has a great influence on the wettability, transparency, and self-cleaning property of the surfaces.

3. RESULTS AND DISCUSSION 3.1. Evaporation-Induced Transition from NP-Inspired Slippery Surfaces to LL-Inspired Superoleophobic Surfaces. The NP-inspired fluoro-SNFs/Krytox100 surfaces were prepared by immobilizing Krytox100 on fluoro-SNFcoated glass slides.32 First, SNFs were grown on glass slides by the hydrolysis of TCMS in toluene (Figure S1). The SNFs are 50−90 nm in diameter, several micrometers in length, and about 9 μm in thickness. The SNFs-coated glass slide is superhydrophobic (CAwater = 170°, SAwater = 2°) and can be easily wetted by Krytox100 but cannot hold it in place owing to the large difference in the surface tension. Therefore, the SNFs were subsequently treated with O2 plasma to convert their −CH3 groups to −OH groups and then modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane to decrease the

2. EXPERIMENTAL SECTION 2.1. Materials. Glass slides (Menzel, Braunschweig, Germany, 25 mm × 60 mm, no. 1) were used as the substrates. Trichloromethylsilane (TCMS), (97%, Gelest) and 1H,1H,2H,2H-perfluorodecyltrichlorosilane (97%, Gelest) were handled under water-free conditions and used without further purification. Toluene (99.85%, extra dry over molecular sieves) was purchased from Acros Organics. Krytox100 was kindly supplied by Dupont. Diiodomethane (DIM), methylene blue (MB), and all other reagents were purchased from China National Medicines Corporation Ltd. 2.2. Growth of SNFs. First, the glass slides were ultrasonicated for 30 min in a 10% v/v solution of Deconex 11 Universal (Borer Chemie AG) at 50 °C, rinsed with deionized water, and dried under a flow of nitrogen. Eight pieces of the activated glass slides were immersed in B

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interface once the liquids are dropped onto the fluoro-SNFs/ Krytox100 surface (Figure 2c). More and more wetted fluoroSNFs by Krytox100 are exposed to the vapor phase with the gradual evaporation of Krytox100, and the surface roughness increases. The fluoro-SNFs exposed to the vapor phase still should be covered with a thin layer of Krytox100 owing to the capillary effect. Meanwhile, the homogeneous liquid/liquid interface sequentially changes to the heterogeneous liquid/ liquid/solid liquid/liquid/solid/vapor and liquid/solid/vapor interfaces once the liquids are dropped on the surface (Figure 2d−f). The increased surface roughness and the continuous change in the interface are responsible for the increase in CA with the evaporation of Krytox100 according to the Wenzel and Cassie−Baxter equations. The introduction of proper surface roughness will make a flat oleophobic surface more oleophobic or even superoleophobic.36 The trapped air beneath the drop could evidently decrease the liquid/solid contact area, which also contributes to the increase in CA.37 3.2.2. Changes in SAs and Maximum Static Friction Forces. The mobility of drops on surfaces is important in many biological and industrial processes.38 SA provides a direct measurement of the adhesion of liquid drops parallel to a tilted surface. In spite of the large difference in CAs between the NPinspired and the LL-inspired surfaces, water and DIM drops could easily roll off both slightly tilted surfaces (SA ≤ 4°) (Figure 1b,c). However, big changes in SAs were recorded in the transition process from the NP-inspired to the LL-inspired surface (Figure 3a,b). SAwater is 4° and SADIM is 2° on the

surface tension in order to hold Krytox100 firmly. In addition, the SNFs were branched by repeating the TCMS coating and O2-plasma steps three times in order to hold Krytox100 stably (Figure 1c and Figure S2). The LL-inspired fluoro-SNFs surface is superoleophobic with high CAs (CAwater = 172.5°, CADIM = 169°) and low SAs (SAwater = 3°, SADIM = 3°) for the water and diiodomethane (DIM) drops. Water and DIM drops are in the Cassie−Baxter state on the fluoro-SNFs surface. Finally, Krytox100 was spread out on the fluoro-SNFs-coated glass slide via the capillary effect. Thus, the LL-inspired fluoroSNFs superoleophobic surface was converted to the NPinspired fluoro-SNFs/Krytox100 slippery surface (Figure 1b). Consequently, the CAs decrease evidently (CAwater = 115.4°, CADIM = 74.5°), whereas the SAs remain almost unchanged. Instead, the evaporation of Krytox100 could lead to a transition from the NP-inspired fluoro-SNFs/Krytox100 slippery surfaces to the LL-inspired fluoro-SNFs superoleophobic surfaces. The transition could be accelerated by heating the horizontal samples in an oven because the evaporation of Krytox100 is slow at room temperature (Table S1). The transition could be carried out in a controlled way by simply regulating the evaporation temperature. The interesting changes in CA, SA, sliding speed and bounce behavior of liquid drops as well as the transparency and self-cleaning property of the bioinspired antiwetting surfaces in the transition process were studied by using water (surface tension 72.8 mN/m) and DIM (surface tension 50.8 mN/m) as the probe liquids. 3.2. Effect of Evaporation on Wettability. 3.2.1. Changes in CAs. Static CA is an important index of wettability of a surface. CAwater is 115.4° and CADIM is 74.5° on the fluoro-SNFs/Krytox100 surface, which are among the highest on flat surfaces.35 The evaporation of Krytox100 leads to gradual increases in CAwater and CADIM (Figure 2a,b). With

Figure 2. Variation of (a) CAwater and (b) CADIM with representative images of water and DIM drops in the transition process with heating time. (c−f) Schematics showing changes in the interface in the transition process.

Figure 3. Variation of (a) SAwater and (b) SADIM with representative images of water and DIM drops in the transition process with heating time. (c) Relationship between SAwater and the interface in the transition process at 65 °C. (d) Forces loaded on the liquid drop on the tilted surface.

increasing time to 240 min at 65 °C, CAwater increases to 172.2° and CADIM increases to 161.2°, which are very close to those on the fluoro-SNFs surface, indicating that the NP-inspired surface is successfully converted to the LL-inspired surface. A higher temperature makes the transition quicker. For example, both CAwater and CADIM are higher than 164° after being heated to 75 °C for only 60 min. For the fluoro-SNFs/Krytox100 surface, the fluoro-SNFs are completely wetted and covered by the flat Krytox100 film. The interface is a homogeneous liquid/liquid

fluoro-SNFs/Krytox100 surface. SAwater and SADIM first evidently increase to the maximum with increasing heating time and then decrease gradually with further evaporation of Krytox100. For example, SAwater increases to 33° and SADIM increases to 23° after being held at 65 °C for 90 min and then decreases to 2 and 3°, respectively, with further increases in the time to 660 min. In fact, the fluoro-SNFs/Krytox100 surface was completely converted to the fluoro-SNFs surface after being held at 65 °C for 660 min (Figure 2f). Similar changes in C

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Figure 4. Variation of bounce times of (a) water and (b) DIM drops in the transition process with heating time. The liquid drops were released from a height of 10 mm. (c) Variation of bounce behavior of water and DIM drops on the horizontal surfaces with heating time at 70 °C.

SAs were observed at 70 and 75 °C. With the gradual evaporation of Krytox100, the homogeneous liquid/liquid interface changes to the heterogeneous liquid/liquid/solid interface as mentioned above. Therefore, the contact area between the surface and the drops increases, and the exposed wetted fluoro-SNFs could interact directly with the drops (Figure 2d). Consequently, the surface becomes sticky and it is difficult for the drops to roll down the surface. The interface continuously changes to the heterogeneous liquid/liquid/solid/ vapor and liquid/solid/vapor interfaces with further evaporation of Krytox100 (Figure 2e,f). The liquid drops are in the Cassie−Baxter state on the surface.37 More and more area beneath the liquid drops is occupied by the vapor phase with the continuous evaporation of Krytox100, which results in a gradual decrease in the SAs. The relationship between SAwater and changes in the interface during the transition from the NPinspired surface to the LL-inspired surface is summarized in Figure 3c. Macroscopically, the mobility of a liquid drop on a surface could be affected by two forces parallel to the substrate: the gravitational force of the liquid drop, F1, and the maximum static friction force, f, between the liquid drop and the surface (Figure 3d).32,39 The liquid drop moves on the surface when F1 > f. Thus, f could be estimated by measuring F1 at θSA using the following equation f = −ρVg sin θSA

between drops and the surface along the surface. In addition, the large contrast in f could facilitate precise control of the motion of drops on the target surface and the trapping of drops.40 Moreover, the large fluctuation in f should have a great influence on the dynamic wettability and self-cleaning property of the surface. 3.2.3. Changes in Bounce Behaviors and Adhesion Forces. The outcome of the drop impact depends on many factors, including surface wettability, drop size, properties of the liquid, and impact velocity.41 The kinetic energy, Ek, of a drop, transformed from its gravitational potential energy, Ep, can be partially conserved by the deformation of the liquid drop and partially dissipated by the work of adhesion, Wa, during the impact against a surface. A drop impacting a surface deforms to store its kinetic energy and can bounce back when Ek > Wa. The outcome of drop impact in the transition process from the NP-inspired surface to the LL-inspired surface could reflect the change in the adhesion force perpendicular to the surface because all of the tests were carried out under the same conditions.42,43 The bounce times and the representative impact process of the 10 μL water and DIM drops released from a height of 10 mm on the surfaces were plotted as a function of heating time (Figure 4 and Movie S1). The outcome of the drop impact depends largely on the surface wettability in the transition process because all of the other factors are the same. The drops released from a height of 10 mm have a velocity of 0.44 m/s upon impacting the surface. No fragmentation of drops was observed upon impacting. As can be seen from Figure 4, the evaporation of Krytox100 has a great influence on the bounce times and the shape of the deformed drop in the impact process. The Ek of a 10 μL water drop released from a height of 10 mm is 9.8 × 10−7 J when the drop arrives at the surface. No bounce of water and DIM drops was observed on the NP-inspired fluoro-SNFs/Krytox100 surface (Figure 4c, first row for water and DIM), indicating that the perpendicular adhesion force is high and Ek is completely dissipated by Wa. The first water bounce appeared after the fluoro-SNFs/Krytox100 surface was held at 70 °C for 75 min (Figure 4c, second row). Then, the water bounce times increase

(1)

where ρ is the liquid density, V is the droplet volume, and g is the gravitational acceleration constant. The maximum static friction forces loaded onto the 10 μL water and DIM drops were plotted as a function of heating time (Figure S3). f water substantially increases from 6.8 to 53.4 μN with the evaporation of Krytox100 at 65 °C for 90 min; meanwhile, f DIM increases from 11.4 to 127.1 μN. f increases by approximately 7−10 times upon increasing the time to 90 min at 65 °C. f water and f DIM gradually decrease to 3.4 and 17.0 μN, respectively, with further increases in the time to 660 min at 65 °C. The change in f is more rapid at 75 °C. The great fluctuation in f in the transition process means a significant change in the interaction D

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gradually with heating time. Water drops could bounce about 11 times after being held at 70 °C for 150 min on the surface, which in fact is already the LL-inspired fluoro-SNFs surface. This means that the adhesion force is pretty low and about 9.8 × 10−8 J of Ek on average is dissipated by Wa after each bounce on the fluoro-SNF surface. Evaporation at a higher temperature makes the first bounce appear earlier (Figure 4a,b). Also, evaporation at 75 °C makes the water and DIM drops bounce more times, indicating that a trace amount of Krytox100 remains on the surfaces held at 65 and 70 °C even for a very long time. A longer heating time is needed for the DIM drops to bounce on the surface in comparison to the water drops. The first DIM bounce appeared after the fluoro-SNFs/Krytox100 surface was held at 70 °C for 150 min. Also, the DIM drops can bounce only one to two times in the transition process, which is obviously less than that of the water drops. This means that the adhesion force between the DIM drop and the surface is higher than that of the water drop owing to the lower surface tension of DIM. The Ek (3.3 × 10−6 J) of a 10 μL DIM drop released from a height of 10 mm is completely dissipated by Wa in one to two bounces. The bounce times of water and DIM drops were also plotted as a function of CA and SA (Figures S4 and S5) in order to establish their relationship in the transition process. The first water bounce appeared at CAwater = 133.1, 154.0, and 146.9° for the surface held at 65 °C for 90 min, 70 °C for 75 min, and 75 °C for 20 min, respectively, at which the SAwater increased to the maximum (Figure S5a). Then, the water bounce times increase gradually to the maximum and remain constant with increasing CAwater. The first bounce of the DIM drop appears at CADIM = 143−158.2°, which is comparable to that of water drops. The adhesion force of a drop on a surface should relate to the surface tension of the drop, its contact area with the surface (e.g., CA and volume), and the properties of the surface (e.g., chemical composition and roughness).44−46 So for a given drop, its adhesion force and bounce times on a surface should closely relate to the CA. A higher CAwater means a smaller contact area of water drops on the surface, which results in more bounce times. The bounce of water drops starts when SAwater increases to the maximum, and the bounce times increase with further decreases in SAwater (Figure S5a). Different from water drops, the bounce of DIM drops starts after the maximum SADIM and an even lower SADIM is necessary because of the low surface tension of DIM. 3.2.4. Changes in Sliding Speed. Liquid drops need a few seconds or even a few minutes to slide down a wafer scale, e.g., 40 mm, NP-inspired surface (Movie S2).32 A similar phenomenon was also observed by Hozumi et al. on the statically oleophilic but dynamically oleophobic smooth surface.47 However, as is well known, liquid drops could roll off a tilted LL-inspired superhydrophobic or superoleophobic surface in the twinkling of an eye. Figure 5 shows the variation in the sliding speed of water and DIM drops on the 10° tilted surfaces in the transition process. The water sliding speed is 2.2 mm/s on the fresh sample with a Kyrtox100 layer thickness of 20 μm. The water sliding speed gradually decreases from 2.2 mm/s to zero upon increasing the heating time to 60 min at 65 °C owing to the evaporation of Krytox100, which results in an increase in f. Water drops stay pinned on the 10° tilted surface with increasing heating time to 195 min and then suddenly could roll off the surface in 1 s with further increases in the heating time to 210 min. A shorter time is needed to change water drops from sliding at low speed to pinning on the surface

Figure 5. Variation of sliding speed of (a) water and (b) DIM drops on the 10° tilted surfaces in the transition process with heating time. A zero sliding speed indicated with green rectangles means that the drops are pinned on the 10° tilted surface. A sliding speed of 40 mm/s indicated with yellow rectangles means the drops roll down the 10° tilted surface in 1 s.

and from pinning to fast rolling at 70 and 75 °C. This is owing to the quicker evaporation of Krytox100 at a higher temperature. The sliding speed of DIM drops (36.4 mm/s) is obviously greater than that of water drops on the 10° tilted NPinspired fluoro-SNFs/Krytox100 surface because of its higher density (3.32 g/mL), which means a greater F1. The F1 of a 10 μL DIM drop is 3.32 times that of water. As a consequence, a longer heating time is needed for the sliding speed of DIM drops to decrease to zero in comparison to water drops, whereas a similar sudden rolling off of DIM drops was observed with further increases in the heating time. We have tried many times to catch the detailed change from the pinning to the rolling off of drops but failed, which means that the change is very fast. The sliding speed of a drop on the NP-inspired surface depends on many physical parameters of the surface and the drop, e.g., viscosity of the fluoro-liquid, density, and surface tension of the drop.32 The slow sliding speed on the NP-inspired surface indicates a stronger interaction between the drop and the surface, originating from the friction and adhesion forces between them. 3.3. Effect of Evaporation on Transparency. The effect of the evaporation of Krytox100 in the transition process could also be seen from changes in the transparency of the surfaces (Figure 6a,b and Figure S6). The fluoro-SNFs/Krytox100coated glass slide is transparent, and the transmittance is 77.6% at 600 nm. The evaporation of Krytox100 at 70 °C for 45 min

Figure 6. Variation of (a) transmittance, (b) appearance, and (c) selfcleaning property of the 10° tilted surfaces in the transition process with heating time at 70 °C. Water and DIM drops in (b) were colored with MB. E

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cannot (Figure 6c, fourth row). After being held for 480 min at 70 °C, the surface is in fact already converted to the LLinspired fluoro-SNFs superoleophobic surface. Liquid drops move in the mode of rolling on the fluoro-SNFs surface, which is more efficient in cleaning a surface than the sliding motion.49 Consequently, both water and DIM can remove the APT and Sudan I microparticles on the surface (Figure 6c, fifth row). In comparison to the NP-inspired surface, the rolling motion and the high rolling speed endow the LL-inspired surface with a higher self-cleaning efficiency, but a larger volume of liquid is needed to clean the same area of dirt owing to the high CA of liquid drops (Figure S7b). It should be noted that a higher tilting angle than the SA is necessary in the transition process in order to keep the self-cleaning property. Otherwise, liquid drops will pin on the surface and cannot remove any dirt on the surface. This is one of the unfortunate defects of the NPinspired surface when used for self-cleaning owing to the evaporation of the fluoro-liquid.

has no obvious influence on the transmittance, although evident increases in SAwater and SADIM were observed and water drops were pinned on the 10° tilted surface. The evaporation of Krytox100 in the first 45 min results in the loss of Krytox100 above the fluoro-SNFs layer, which does not alter the liquid/ vapor interface and thus has no obvious influence on the transmittance. The further evaporation of Krytox100 with increasing time to 90 min at 70 °C results in a sharp decline in the transmittance. Meanwhile, CAwater increases to 159.4° and can bounce four times. The fluoro-SNFs under the Krytox100 layer are gradually exposed to the vapor phase with the continuous evaporation of Krytox100, which increases light scattering at the solid/vapor interface and then results in a sharp decline in the transmittance. The transmittance remains almost constant with the continuous increase in heating time from 90 to 360 min, whereas evident changes in CA, SA, sliding speed, and bounce times were observed in this period of time owing to the evaporation of the residual Krytox100 adhered to the fluoro-SNFs. 3.4. Effect of Evaporation on Self-Cleaning Property. One of the most important motives for researchers to design such bioinspired antiwetting surfaces is to use their self-cleaning property. The self-cleaning property of a surface is closely related to (1) the interaction of the surface with the liquid drop and the dirt and (2) the mode of motion of liquid drops on the surface. The liquid drops tend to slide on the slightly tilted fluoro-SNFs/Krytox100 surface. No rolling motion was observed for water and DIM drops on the fluoro-SNFs/ Krytox100 surface when the tilting angle is not more than 10°, whereas liquid drops tend to roll on the slightly tilted fluoroSNFs surface.32 Considering the evident changes in CA, SA, bounce behavior, and sliding speed and the obviously different mode of motion, the self-cleaning property of the surface in the transition process from the NP-inspired slippery surface to the LL-inspired superoleophobic surface was studied (Figure 6c and Movie S3). Attapulgite (APT), a kind of hydrophilic silicate clay mineral, and Sudan I, a hydrophobic compound, were used as the model dirts for evaluating the self-cleaning property. The self-cleaning properties of the NP-inspired surface and the LL-inspired surface are obviously different from each other. Also, evident changes in the self-cleaning property were observed in the transition process. For the fluoro-SNFs/ Krytox100 surface, the self-cleaning property is closely related to the liquid used and the property of dirt. Water can easily remove the hydrophilic APT microparticles from the 10° tilted fluoro-SNFs/Krytox100 surface, but it is difficult to remove Sudan I owing to its oleophilic property, whereas DIM is effective at cleaning both APT and Sudan I (Figure 6c, first row). In addition, the contact area of the same liquid drop on the fluoro-SNFs/Krytox100 surface is much larger than that on the fluoro-SNFs surface owing to the lower CA in comparison to that of the fluoro-SNFs surface.48 Thus, every liquid drop of the same volume can remove a larger area of dirt on the NPinspired surface (Figure S7a), but with a lower efficiency owing to the lower sliding speed of liquid drops. Only DIM can remove the dirt on the surface after being held at 70 °C for 45 min owing to the fact that water drops are pinned on the surface (SA = 15°, Figure 6c, second row). Both water and DIM perform badly in cleaning APT and Sudan I on the surface after being heated for 75 min because of their high SAs (SAwater = 26° and SADIM = 28°, Figure 6c, third row). Interestingly, water can remove both APT and Sudan I microparticles with further increases in the heating time to 180 min, whereas DIM

4. CONCLUSIONS We have studied the evaporation of the Krytox100-induced transition process from the NP-inspired fluoro-SNFs/Krytox100 slippery surface to the LL-inspired fluoro-SNFs superoleophobic surface. The evaporation of the fluoro-fluid on the NP-inspired surface results in evident changes in the wettability, transmittance, and self-cleaning property of the surface. The evaporation of Krytox100 also results in evident changes in the interaction forces, parallel and perpendicular to the surface, between the surfaces and the liquid drops in the transition process. We believe that our basic findings have shed new light on the fundamental understanding of the transition process from the NP-inspired surface to the LL-inspired surface, and the similarities and differences between them. This study can have serious implications on the design of new bioinspired antiwetting surfaces and their potential applications, a topic of great general interest.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of SNFs, physical properties of Krytox100, variation of friction force and transmittance with evaporation time, relationship of water bounce time with CA and SA, schematic illustration of the self-cleaning property, and videos. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support of the “Hundred Talents Program” of the Chinese Academy of Sciences. F

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dx.doi.org/10.1021/la503300k | Langmuir XXXX, XXX, XXX−XXX