Dual High Adhesion Surface for Water in Air and for Oil Underwater

ARTICLE pubs.acs.org/Langmuir

Dual High Adhesion Surface for Water in Air and for Oil Underwater Liping Heng,†,‡ Junxin Su,† Jin Zhai,*,† Qinglin Yang,*,† and Lei Jiang†,‡ †

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University, 100191 China ‡ Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

bS Supporting Information ABSTRACT: A new type of dual high surface adhesion both in an oil/water/solid system and in a water/air/solid system is reported. A walnutlike cuprous iodide (CuI) microcrystal surface, which is composed of numerous CuI nanocrystals, shows an amphiphobic, highly adhesive surface for water in air and for oil underwater. The maximum adhesive force is about 120.3 ( 1.6 μN in the air for a water droplet and about 23.8 ( 2.1 μN underwater for an oil droplet. These findings will help us to design novel high adhesive materials in two-phase or multiphase mediums.

’ INTRODUCTION Superhydrophobic surfaces have attracted considerable attention because of their great advantages in both fundamental research and practical applications,1
4 especially those with high liquid
solid adhesion.5,6 Inspired by naturally superhydrophobicity and high adhesion solid surfaces, such as the gecko’s attachment system on its feet and rose petals,7,8 many studies have focused on preparing artificial surfaces with special adhesion.9
13 For instance, to mimic a gecko’s feet, vertical aligned multiwalled carbon nanotubes with strong shear binding and easy normal lift off were designed.14 Also, an aligned superhydrophobic polystyrene (PS) nanotube film with a high adhesion to water was fabricated.15,16 At the same time, the rose petal surface was duplicated to obtain an artificial biomimic polymer film with superhydrophobicity and high adhesion.17,18 On such surfaces, a water droplet shows a static contact angle (CA) that is larger than 150°, but the droplet is pinned on the surfaces at any tilted angle, which could be used as a “mechanical hand” for the reversible no-loss transport of microdroplets.19 However, all of these bioinspired superhydrophobic and high adhesion surfaces were achieved only in air for water droplets (at the water/air/ solid interface). In addition, the wetting and adhesive behavior of oil droplets on a solid surface in an aqueous medium has recently become a new research focus.20
23 Similarly, superoleophobic surfaces underwater indicate that the surface can exhibit oil contact angles that are larger than 150°. In contrast to the achievement of high adhesion in the water/air/solid system, the superoleophobicity and high adhesion solid surfaces for oil have not been reported either in air or underwater. Moreover, the surface with dual high adhesive behavior for one sample both in the air (water/air/solid system) and underwater (oil/water/solid system), which can achieve no loss of handling trace liquids in a multiphase system, has not yet drawn much attention. r 2011 American Chemical Society

In this article, we will introduce dual high surface adhesion in both an oil/water/solid system and a water/air/solid system. A walnutlike cuprous iodide (CuI) microcrystal surface composed of numerous CuI nanocrystals shows an amphiphobic, high adhesive surface for water in air and for oil underwater. Interestingly, the walnutlike CuI crystal film can hold water in air and in oil underwater through strong adhesive forces, even when the film is turned upside down. The maximum adhesive force is about 120.3 ( 1.6 μN in air for a water droplet and about 23.8 ( 2.1 μN underwater for an oil droplet, which is assessed by a highsensitivity micromechanical balance system. These findings will help us to design novel high adhesive materials in two-phase or multiphase mediums. These materials can be used for the construction of next-generation microdevices in the areas of liquid transportation, biochemical separation, in situ detection, oil
water separation, and microfluid systems.

’ EXPERIMENTAL SECTION Preparation of the Walnutlike CuI Structure. The walnutlike CuI structure was prepared through a facile etching method in a DMF solution via the reaction of copper with I2 based on the following equation: etching

I2 þ 2Cu s f 2CuI The surface layer of the copper substrate was removed by polishing with 2000 grit sandpaper and then was cleaned in hydrochloric acid solution (concentrated hydrochloric acid/water, 1:1, v/v) for 30 s, rinsed with distilled water and acetone, and finally dried with dry nitrogen before use. Water was purified through a Milli-Q purification Received: July 14, 2011 Revised: August 28, 2011 Published: August 31, 2011 12466

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Langmuir system (Millipore Corp., Bedford, MA) with a resistivity of 18 MΩ cm. After being dried in the N2 gas, the copper substrate was inserted into a DMF (N,N-dimthylformamide) (SCRS, ACR) solution containing a calculated amount of I2 (0.3175 g of I2 dissolved in 50 mL of DMF), and the reaction lasted for 5 min under ice bath conditions. Control experiments with different reaction times were also undertaken in the same solution and in the CS2 solution for the same reaction time containing the same amount of I2. The analogous porous structure of the CuI crystal was fabricated in the CS2 solution, which contains the same amount of I2. The SEM image is shown in Figure S1. Characterization. The crystal structure samples were examined by X-ray diffraction (XRD, D/MAX 2200PC) using Cu Kα (λ = 0.154 nm) at 40 kV and 40 mA over the 2θ range of 10
80° at a scan rate of 6° min
1. The morphology observation was performed on a field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) at an accelerating voltage of 3 kV. CA was measured on a CA system (OCA20, Dataphysics) at ambient temperature. The water droplets (about 2 μL) were dropped onto the surface, and the average value of five measurements performed at different positions on the same sample was adopted as the contact angle. The force required to remove the water drops from the substrate was measured by using a high-sensitivity microelectromechanical balance system (DCAT 11, Dataphysics). A water droplet of about 5 μL was first suspended with a copper ring, and then the substrate was placed on the balance table. The substrate was moved upward at a constant speed of 0.005 mm/s until it contacted the water droplet, and then the substrate was moved down. Throughout the entire process, the water droplet changed its shape from spherical to elliptical until reaching its maximum with the force increasing gradually. Then the force decreased sharply when the substrate moved down, and the shape of the water droplet returned to spherical. Oil contact angles were measured on an OCA20 machine (Data Physics, Germany) at ambient temperature. The oil droplets (1,2dichloroethane (DCE), about 2 μL) were dropped carefully onto the materials, which were immersed in water. The average value of five measurements performed at different positions on the same sample was adopted as the contact angle. The force required to take the oil droplet away from the substrate was measured using a high-sensitivity microelectromechanical balance system (Data Physics DCAT 11, Germany) in a water environment. An oil drop (about 5 μL) was first suspended with a metal ring, and then the substrate was placed on the balance table. The substrate was moved upward at a constant speed of 0.005 mm/s until the substrate contacted the oil droplet. Then the substrate was moved down. The force increased, and the shape of the oil droplet changed from spherical to elliptical. When the oil droplet was about to leave the substrate, the contact force sharply decreased and the shape of the droplet returned to spherical.

’ RESULTS AND DISCUSSION Figure 1a is a scanning electron microscope (SEM) image of the synthesized walnutlike CuI crystal. It can be seen that the walnutlike particles have a diameter of 7
10 μm and are dispersed on the copper surface freely. Each walnutlike CuI crystal is composed of numerous nanoparticles with a size of 20 nm, which can be observed from the magnified view (Figure 1b). The walnutlike CuI crystal films have a mico-/ nanocomposite structure that leads to their superhydrophobic behavior with a water contact angles of about 150° (Figure 1c). Besides, they are superoleophilic for oil (e.g., 1,2-dichloroethane) in air. However, when we put the walnutlike CuI crystal films underwater, they showed superoleophobic properties with an oil contact angle of about 156° (e.g., 1,2-dichloroethane) (Figure 1d). By comparing their superoleophobic property underwater with their superoleophilic property in air, we believe

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Figure 1. SEM images of the as-prepared walnutlike CuI crystal film: (a) a top view of the surface with CuI crystal particles; (b) a magnified image of a, where the inset is a single walnutlike CuI crystal; (c) the shape of a water droplet on the walnutlike CuI crystal surface with a contact angle of about 150°; and (d) the shape of a oil droplet on the walnutlike CuI crystal surface underwater, with a contact angle of about 156°.

Figure 2. XRD patterns of a copper substrate with CuI grown on it: (a) the disordered sheets of the CuI crystal; (b) the walnutlike CuI crystal; and (c) the pure copper substrate. Compared to the pattern of the pure copper substrate, the diffraction peaks of the walnutlike CuI were at 26, 29, 43, and 50° referring to crystal faces (111), (220), (400), and (311), respectively, that can be obviously seen, which matches the standard pattern of γ-CuI well. Therefore, the as-grown crystal is the γ-CuI phase.

that the water surrounding the walnutlike CuI crystal films leads to a reversion in wettability. As is well known, superhydrophobic surfaces in air originate from the formation of new composite surfaces. According to the theory developed by Cassie et al.,24 air trapped in the rough area contributes greatly to the increase in hydrophobicity. Similarly, when the walnutlike CuI crystal films come in contact with the oil droplets underwater, water molecules can be trapped in the micro/nanostructures, forming an oil/water/solid interface. This new composite interface shows the superoleophobic property. To understand better the effect of the structure on the wetting behavior, we attempted to fabricate three kinds of designed surfaces that were selected as controlling surfaces. A high-density 12467

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Figure 3. (a) The water droplet on the superhydrophobic surface remained attached even when the surface was tilted 180°. (b) The oil droplet on the superoleophobic surface remained attached even when the surface was tilted 180° underwater. It is shown that the surface has a high adhesion for water in air and in oil underwater. The oil used is 1,2dichloroethane with a specific gravity of about 1.26, which is higher than that of water.

walnutlike particle film is obtained when the walnutlike particles are interconnected into a crystal film after a long reaction time (Figure S1a). The water contact angle on this surface is about 132° (Figure S1d), and the oil contact angle on this surface is about 138° (Figure S1e). The decrease in the contact angle is caused by a reduction in the roughness. Furthermore, a comparison was also undertaken by fabricating the copper substrate with disordered CuI crystal sheets, which is shown in Figure S1b, with a water CA of about 125° (Figure S1f) and an oil CA of about 140° (Figure S1g). The disordered CuI crystal sheets are obtained from the control experiments in CS2 solution. These scattered crystal sheets are arranged in an interwoven form, composing an analogous honeycomb structure. The rough copper substrate surface (Figure S1c) is hydrophobic, with a water CA of about 101° shown in Figure S1h and an oil CA of about 91° shown in Figure S1i. It is evidently seen that the growth of the CuI crystal can improve the hydrophobicity in the air and the oleophobicity of the surface underwater, but the hydrophobicity and the oleophobicity can decrease when the density of the walnutlike CuI crystal decreases. Moreover, the walnutlike CuI crystal morphology brings about superhydrophobicity and superoleophobicity because of the hierarchical micro/ nanostructure. The X-ray diffraction (XRD) spectra of both walnutlike CuI grown on the copper substrate and the pure copper substrate are shown in Figure 2. Compared to the pattern of the pure copper substrate, the diffraction peaks of the walnutlike CuI were at 26, 29, 43, and and 50°, referring to crystal faces (111), (220), (400), and (311) that can be obviously seen, which matches the standard pattern of γ-CuI (JCPDS no. 06-0246) well. Therefore, the as-grown crystal is the γ-CuI phase (space group F4/mm; a = 0.6051 nm), which is consistent with what is reported in the literature25,26 that the γ phase of CuI is stable at low temperature (below 350 °C). Other strong peaks are attributed to the characteristic diffraction of copper metal. As reported above, the water droplet on such a surface is spherical in shape, resulting from superhydrophobicity. However, it cannot roll off when the surface is titled vertically, even if the surface is turned upside down. The shape of the water droplet on the surface is shown in Figure 3a. The volume of the water droplet on the surface is as high as 5 μL. Meanwhile, the oil droplet on such a surface underwater is also spherical in shape as a result of the superoleophobicity, but it cannot roll off, similarly

Figure 4. (a) Force
distance curves recorded before and after the water droplet makes contact with the as-prepared walnutlike CuI crystal film. Process 1: the walnutlike CuI crystal film approaches the water droplet. Process 2: the walnutlike CuI crystal film leaves the water droplet after contact, the triple-phase contact line is moving, the contact area is decreased with increasing distance between the film and the suspender, and the adhesive force is also enhanced. Process 3: the walnutlike CuI crystal film continues to leave the water droplet, but the triple-phase contact line is pinned and the contact area holds the same line, where the diameter of the water column in the middle is becoming slim and the stretching force decreases with the increase in the distance between the film and the suspender. Process 4: the walnutlike CuI crystal film breaks away from the water droplet. (Insets) Photographs of the shapes of the water droplets taken at the corresponding stages during the measuring process. (b) Corresponding schematic diagrams of the triplephase contact line movement process. A water droplet on a micro/ nanostructured substrate in air, in which G represents air and W represents water. This is the metastable state between the Wenzel and Cassie states at the contacting area edge. The water droplet partially wets the roughness features that remain to trap air on composite surface. However, it is the Wenzel state at the contacting area center. A droplet penetrates the surface features and is pinned the three-phase contact line during the adhesion measurement process. The triple-phase contact line is moving, and the contact area is decreased with the increasing distance between the film and the suspender. Then the triple-phase contact line is pinned, and the contact area holds the same line.

showing a high adhesion for oil droplets (Figure 3b). The volume of the oil droplet on the surface is as high as 5 μL. Here, the oil used is 1,2-dichloroethane with a specific gravity of about 1.26, which is higher than that of water. Therefore, the downward force on the oil droplet is likely not nearly as high as in the case of a water droplet in air. The adhesive force was defined as the force required to lift the water droplet off of the substrate and can be assessed by a 12468

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Figure 5. (a) Force
distance curves recorded before and after the oil droplet is in contact with the as-prepared walnutlike CuI surface underwater. Process 1: the surface approaches the oil droplet. Process 2: the surface moves downward after being in contact with the oil droplet, and the adhesive force is increased with the increasing distance between the film and the suspender. Process 3: the oil droplet moves away from the surface. (Insets) Photographs of the oil droplet shapes underwater taken at the corresponding stages during the measuring process. (b) Corresponding schematic diagrams of the triple-phase contact line movement process. An oil droplet on a micro/nanostructured substrate in water, in which O represents oil and W represents water. It is the metastable state between the Wenzel and Cassie states. The oil droplet partially wets the roughness features that remain to trap water on the composite surface. The triple-phase contact line is moving and the contact area is decreasing when the distance between the film and the suspender increases in this process until the oil droplet breaks away from the surface.

high-sensitivity micromechanical balance system. An optical microscope lens and a charge-coupled-device (CCD) camera system were used to record images during the experiments. The whole curve of the force versus the distance between the solid surface and the water droplet in air is plotted in Figure 4a. Force
distance curves are recorded before and after the water droplet makes contact with the as-prepared walnutlike CuI crystal film. First, the walnutlike CuI crystal film was placed on the plate of the balance system, a 5 μL water droplet was suspended on a metal ring, and the force of this balance system was initialized to zero. Then, the walnutlike CuI crystal film was brought into contact with the water droplet while maintaining the force balance at zero (process 1). The walnutlike CuI crystal film was moved at a rate of 0.01 mm/s. When the walnutlike CuI crystal film left the water droplet after contact, the balance force increased gradually and reached its maximum at the end of

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process 2. We observed that the triple- phase contact lines were moving and that the contact area was decreasing while the distance between the film and the suspender increased. When the movement of the triple-phase contact lines stopped, the force reached its maximum of 120 μN (Figure 4b). Then the water droplet was stretched until ‘‘broken’’ (process 3). The adhesive force should be larger than the maximum stretching force (120 μN) because the network of water molecules was broken. Finally, in process 4, the balance force decreased immediately when the walnutlike CuI crystal film broke away from the water droplet to finish one cycle of the force measurement. The relative humidity was maintained at 80% during the whole measurement process, so the evaporation of the drop could be ignored. The force that the water droplet was subjected to can be considered to be the adhesive force between the film and the water. The final balance force became negative, which indicated that there was some water left on the film after it was transported by the metal ring. These results show that in air, this composite surface has a relative high adhesion that can snap some water away from the droplet. The same experiments were conducted underwater to study the adhesion behavior of the walnutlike CuI crystal film with oil. The whole curve of the force versus the distance between the solid surface and the oil droplet is plotted in Figure 5a. The oil droplet could adhere to the surfaces. However, this surface displayed different adhesive behavior and relatively low adhesive forces compared to the adhesion of water in air, which makes the oil droplet not able to snap on the surface. In our experiment, the adhesive force was defined as the force required remove the oil drop from the substrate and can be assessed using a highsensitivity micromechanical balance system.19 At first, the walnutlike CuI crystal film was placed on the plate of the balance system underwater, and a 5 μL oil droplet was suspended on a metal ring. Then, the walnutlike CuI crystal film was brought into contact with the oil droplet (process 1). The walnutlike CuI crystal film was moved at a rate of 0.01 mm/s. When the walnutlike CuI crystal film left the oil droplet after contact, the balance force increased gradually and reached its maximum of 24 N at the end of process 2. We observed that the triple-phase contact line is moving and the contact area is decreasing when the distance between the film and the suspender increased in this process until the oil droplet broke away from the surface (Figure 5b). Finally, the balance force decreased immediately when the walnutlike CuI crystal film broke away from the oil droplet in process 3 to finish one cycle of the force measurement. Before the substrate was about to leave the oil drop, the shape of the oil drop first changed from spherical to elliptical and then the shape changed back to spherical after the oil drop was displacement from the surface (Figure 5). These results show that in oil/ water/solid systems a walnutlike CuI crystal surface with micro/ nanostructures also shows relatively high adhesion. Therefore, the dual adhesive surface in air and underwater is obtained, which can be used because there is no loss of handling trace liquids in multiphase systems. It should be noted that even if superhydrophobic surfaces exhibit comparable apparent CAs, their adhesion to liquid may be quite different.27,28 Whether a droplet could be pinned on a superhydrophobic surface is ascribed to the distinct contact modes27,29 (the Wenzel state or the Cassie state) and the triple-phase liquid/air/solid contact line (TCL).30,31 In the Wenzel state,32 a water droplet fully penetrates the valleys of a textured surface (in wet contact mode), and the TCL is 12469

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Figure 6. (a) Effect of surface structure on the wetting behavior of solid substrates in water/air/solid triple-phase systems. A water droplet is on a micro/nanostructured substrate in air. It is the gecko state between the Wenzel and the Cassie states at the beginning of the contact time. The water droplet partially wets the roughness features that remain to trap air on the composite surface. Then, it can become the Wenzel state with increasing contact time because of the opening structures. The air can be crowded out and the droplet can penetrate the surface features, which are pinned to the triple-phase contact line during the measuring process of adhesion. Because of the coexistence of these two states, the adhesive force between the water and solid is very high. This explains why this surface has a relatively high adhesion that can snap some water from the droplet. (b) Effect of surface structure on the wetting behavior of solid substrates in oil/water/solid triple-phase systems underwater. It is the gecko state between the Wenzel and Cassie states. The oil droplet partially wets the roughness features that remain to trap water on the composite surface. It cannot become the Wenzel state with increasing contact time because the water is liquid. Therefore, this surface can not snap any oil from the droplet.

continuous and stable. Thus, the surface generates relatively high adhesion to pin the droplet. In contrast, in the Cassie state,24 the water droplet is suspended by the vapor pockets trapped on the surface (in composite contact mode), and TCL is discontinuous. Thus, the adhesion of the surface is relatively decreased, and the droplet easily rolls off the surface. However, in most cases, a water droplet may partially wet a superhydrophobic textured surface because of air partially trapped in the valleys, which is an intermediate state between the Wenzel and Cassie states. Such an intermediate state of the solid/liquid contact is referred to as a gecko state.27,33 According to the above basic theories of liquid
solid adhesion on superhydrophobic surfaces, a mechanism scheme was proposed to explain the high adhesion property on such surfaces, as shown in Figure 6. The superhydrophobicity and the high adhesion of such a surface in air can be attributed to its hierarchical micro/nanostructure. The micro/nanostructure made of walnutlike CuI crystal particles allows the water film to impregnate the texture. It is the gecko state33,34 between the Wenzel and Cassie states at the contacting area edge (Figure 6a). The water droplet may partially wet the superhydrophobic rough features that remain to trap air on a composite surface at the beginning of the contact time. The surface in the gecko states can prevent the spontaneous wetting-state transition because of an

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energy barrier.35,36 The triple-phase contact lines in this state can move in process 2 of the measuring process of the adhesion, and the contact area is becoming smaller (Figure 4b). Then, it can finally transfer to the Wenzel state (Figure 6a) at the contacting area center with increasing contact time because of the opening structures. The air can be crowded out, and the liquid completely fills the grooves of the rough surfaces where they are in contact. In this type of behavior, the TCL is continuous and the liquid
solid contact area is enlarged. Therefore, the TCL is pinned during measuring process 3 of the adhesion (Figure 4b). Because there is a transformation between the gecko state and the Wenzel state, the adhesive force between the water and the solid is very high. This can explain why this surface can snap some water from the droplet. Underwater, the adhesive property remains consistent with that of a water/air/solid interface. Water molecules can be trapped in the micro/nanostructures to form a water-trapped composite structure instead of air. These trapped water molecules will greatly decrease the adhesive force between the oil droplet and the solid surface. In an oil/water/solid system, however, where the rough surface is composed of solid and water (Figure 6b), the Cassie model makes some amendments to adapt to new situations. The gecko state between the Wenzel and Cassie states exists in this situation. The oil droplet partially wets the rough features that remain to trap water on the composite surface. The TCL can move during process 2 of the measuring process of adhesion, and the contact area becomes smaller (Figure 5b), which is consistent with process 2 of the water adhesion measurement. It cannot become the Wenzel state with increasing contact time because in the liquid medium it is impossible to let the water out of the grooves via oil, which is different in air. Therefore, this surface cannot snap oil from the droplet.

’ CONCLUSIONS We fabricated a walnutlike CuI crystal particle film with hierarchical micro/nanostructure. The substrate with such a structure exhibits a dual high adhesive surface for the water in the air and oil underwater. These findings will help us to design novel high adhesive materials in two-phase or multiphase systems, which will be used for the construction of the future generation of microdevices and have potential applications in liquid transportation, biochemical separation, in situ detection, microfluid systems, and other areas. This study also gives us a more profound understanding of the wettability of solid surfaces, which has a bright future in inorganic bionic applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We are grateful for financial support by the National Research Fund for Fundamental Key Projects (2011CB935700, 2010CB934700, 2009CB930404, and 2007CB936403), the National Natural Science Foundation (20920102036, 20974113, 12470

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and 21003138) and the Innovation Foundation of the Chinese Academy of Sciences (CX-201013) for continuing financial support. The Chinese Academy of Sciences is gratefully acknowledged.

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