Bioinspired Design of Honeycomb Structure Interfaces with

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Bioinspired Design of Honeycomb Structure Interfaces with Controllable Water Adhesion Liping Heng,*,⊥,† Xiangfu Meng,‡ Bin Wang,§ and Lei Jiang⊥,† ⊥

School of Chemistry and Environment, Beihang University, 100191 China Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Chemistry, Capital Normal University, Beijing 100048, China § School of Environment, Tsinghua University, Beijing 100084, China †

S Supporting Information *

ABSTRACT: Inspired by biological attachment systems, we fabricated the honeycomb structural films with different diameters by breath figure (BF) method, which were similar to the patterned octopus suckers. The experimental results showed, besides different van der Waals forces between the polystyrene (PS) surfaces and water, another important factor; that is, different negative pressures produced by different volumes of sealed air could be a crucial factor for the different adhesions. So the water adhesive forces of the as-prepared films can be effectively controlled from relative high to relative low adhesion by varying the pore diameters, which effectively adjusted the negative pressures produced by the pores. This unique adhesive phenomenon of honeycomb structure will be very useful for manipulating water droplet behaviors, as well as controlling liquid collection and transportation. These findings are interesting and helpful for us to further understand the biological attachment systems and to optimize the design of artificial analogues. engineering,12,13 low-dielectric constant material for microelectronic devices,14 photonic band gaps,15 membranes for separation and purification,16 solid supports for sensors and catalysts,17 etc., in the past 10 years most work has focused on changing polymer pieces and solvents to prepare ordered porous films by breath figure (BF) process.18−23 Recently, research in this field started to concentrate on fabricating new structures, such as patterned structures and three-dimensional structures,24,25 and building new functional honeycomb films with different properties, such as photoelectric conversion,26 photocatalysis,27 antireflection,28 hydrophobicity,29 high mechanical strength,30 and cell adhesion.31,32 However, their new applications, particularly in the controllable water adhesion aspects, are still in their infancy.33 The geometry of the honeycomb structure is similar to the sucker of the octopuses. Such structure is expected to display high adhesive behavior. Furthermore, it should be of great scientific interest to extend its applications to new fields, because the high adhesive behaviors of the honeycomb structure are more suitable for controlling liquid collection and transportation.34,35 To our best knowledge, the development of bioinspired honeycomb structures with high adhesion is still a challenge for modern science.

1. INTRODUCTION In nature, many species can walk on smooth vertical and even inverted surfaces, which has interested entomologists for a long time.1,2 This remarkable native ability of these animals is explained by quite a few adhesive mechanisms, such as van der Waals’ forces,3,4 wet adhesion,5,6 hook claw adsorption,7 vacuum adsorption,8,9 and so on. Researchers have investigated this special ability in detail. Limpets, cephalopods, starfish, frogs, and geckos cling to smooth surfaces using mechanisms ranging from suction in the Mollusca10,11 to intermolecular adhesion in geckos.3 Among these species, octopus is a special organism, and it uses remarkable suckers for a variety of tasks, including anchoring the body to the substratum, holding prey, locomotion, cleaning maneuvers, chemotactile recognition, behavioral displays, and manipulating, sampling, and collecting objects.10 The suckers are capable of adhering to a wide diversity of objects. These strong suctions are achieved by negative pressure generated by octopus suckers. This finding has inspired the creation of novel adhesives that work by mimicking the sucker mechanism. Natural honeycombs with hexagonal structure have the properties of large surface area, high structural stability, good permeability, etc. In recent years, there has been a growing research interest in the field of highly ordered porous polymeric films, which imitate the honeycombs’ structure. Because such films have manifold applications in the fields of tissue© XXXX American Chemical Society

Received: March 12, 2013

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The aim of the present research is to fabricate the bioinspired honeycomb-like sucker structures and achieve high adhesion. Below, we will describe the preparation of hexagonal honeycomb structure from chloroform solution of polystyrene by breath figure (BF) process. Characterization of the adhesion indicates that the porous honeycomb structure films can serve as high adhesive surface. Besides, the adhesion can be controlled by changing the size of the pore. The honeycomb structure with the pore diameter of 1.9 ± 0.1 μm has the highest adhesion, which can snap a water droplet from the original water. In contrast, smooth films and other honeycomb structure films cannot snap a water droplet or snap very little. The unique adhesive phenomenon of honeycomb structure will be useful for manipulating water droplet behaviors and suitable for the application of controlling liquid collection and transportation.

2. EXPERIMENTAL SECTION Preparation of the Honeycomb Structure. PS with a molecular weight of about 300 000 was purchased from Aldrich. PS solutions with different concentrations (1.0, 2.0, and 3.0 wt %) were prepared by dissolving appropriate amounts of polymer in chloroform and then magnetic stirring (1200 rpm) for 30−60 min until PS was totally dissolved. The films were fabricated on a slide glass, which were treated by acetone, ethanol, and deionized water step by step to ensure cleanness. Each film contained about 20 μL of solution and was fabricated under the temperature of about 20 °C and the relative humidity (R.H.) of ca. 85%. Figure S1 is the schematic diagram of experimental equipment. For comparison, smooth films were fabricated by casting the same amount of solution on substrates under ambient atmosphere (R.H. ≈ 30% at about 25 °C). Water was purified using a Milli-Q purification system (Millipore Corp., Bedford, MA) to give a resistivity of 18 MΩ cm. Characterization. The morphology observation was performed on a field emission scanning electron microscope (FE-SEM, JEOL JSM6700F) 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 contact angle average value of five measurements was performed at different positions on the same sample. The force required to remove the water drops from the substrate was measured by using a high-sensitivity microelectro-mechanical balance system (DCAT 11, Dataphysics). The adhesion values were the averages of 10 independent measurements. Each measurement was taken at one sample surface. A water droplet of about 6 μL was first suspended with a copper cap, which was washed by acetone, ethanol, and deionized water step by step to ensure cleanness, and then treated by the plasma to make it become superhydrophilic. The substrate was placed on the balance table. The substrate was moved upward at a constant speed of 0.01 mm/s until it contacted the water droplet, and then the substrate was moved down. Throughout the entire process, the water droplet first changed its shape until the force reached its maximum, then decreased gradually with the further increasing distance. At some position, the force decreased sharply when the substrate moved down, and the water droplet was broken or returned to original spherical. The water droplets of about 4 and 8 μL were also used to test the adhesive force.

Figure 1. SEM images of the honeycomb film with different pore sizes: (a) smooth film, (b) porous film 1 with pore diameter of 1.9 ± 0.1 μm, (c) porous film 2 with pore diameter of 5.6 ± 0.8 μm, and (d) porous film 3 with pore diameter of 18.3 ± 2.8 μm. Cross-sectional SEM images of the honeycomb film with different pore sizes: (e) porous film 1, (f) porous film 2, and (g) porous film 3.

solution concentration is 1 wt %, the holes of the film in Figure 1b are the most inerratic and uniform. The diameter is 1.9 ± 0.1 μm, and the distance between holes is about 0.9 μm. The density of these pores is about 1.53 × 105 pores per square millimeters. Such a film in Figure 1b is called porous film 1. In Figure 1c and d, the diameters of the pores become inhomogeneous, which are 5.6 ± 0.8 and 18.3 ± 2.8 μm, respectively. The distances between holes are about 6 and 22 μm, respectively. The densities of the pores are about 2.1 × 104 and 1.8 × 103 pores per square millimeters, respectively. These films are called porous film 2 and porous film 3, respectively. From these figures, it can be seen that the pores have become more disorderly and heterogeneous as the concentrations increased. The under-layer structure cannot be observed through the pores of the top layer. To further confirm the internal structure of the porous films, the cross-sectional images are measured as shown in Figure 1e−g. The cross-sectional images show that these porous films are made of single layer of empty spherical voids and the pores do not connect each other. The formation mechanism of the honeycomb structure has first been suggested by François et al.,36 and then revealed by Shimomura and co-workers in detail.37 After casting a droplet of chloroform solution on the substrate, the chloroform started

3. RESULTS AND DISCUSSION 3.1. Morphology of the Honeycomb Structure Films. Figure 1a−d showed typical field-emission scanning electron microscopy (FE-SEM) images of the smooth film and the asprepared PS honeycomb structure films. From Figure 1b to d, the porous films were fabricated by the PS solution with the concentrations of 1.0, 2.0, and 3.0 wt %, respectively. It is clear that the orderliness and the homogeneity of the pores become worse as the polymer concentration increases. When the PS B

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where f is the area fraction of the solid on the surface, θ is the contact angle of a liquid droplet on a smooth surface in air, and θ′ is the contact angle of a liquid droplet on a rough surface in air. Equation 1 indicates that the fractions of air in the honeycomb film surfaces are all around 48%. So these solid area fractions on the surface are approximate to each other, because the contact angles on the honeycomb films are approximate. 3.3. Tunable Water Adhesion Properties of Honeycomb Films. As reported above, the water droplets on these surfaces are ball lacks in shape, resulting from hydrophobicity. However, it cannot roll off when the surface is titled vertically, even if the surface is turned upside down. The shapes of the water droplet on the surfaces are shown in Figure 3. The

to evaporate. This led to the cooling of the solution and condensing of micrometer-sized water droplets onto the chloroform solution of PS. Because the surface tension between water and chloroform was reduced by polymer molecules, the water droplets were stabilized against fusion. The droplets were transported to the three-phase line and were hexagonally packed by convectional flow or capillary force. After the solvent and water were completely dry, repeated water condensation formed a three-dimensional (3D) porous structure. For comparison, the SEM image of smooth PS film prepared by dipping the same amount of PS solution under ambient conditions is shown in Figure 1a. 3.2. Wettability of the Smooth Film and the Honeycomb Structure Films. The contact angles (CAs) of the smooth film and the porous films are measured. The CA photos of films are shown in Figure 2a−d. The corresponding

Figure 3. The water droplets on the different surfaces remained attached even when the surfaces were tilted 180°: (a) smooth film, (b) porous film 1, (c) porous film 2, and (d) porous film 3. It is shown that the surfaces have a high adhesion for water.

Figure 2. Contact angle photos of the smooth and the honeycomb films with different pore sizes: (a) smooth film with CA of about 96°, (b) porous film 1 with CA of about 123°, (c) porous film 2 with CA of about 120°, and (d) porous film 3 with CA of about 124°.

volume of the water droplet on the surface is as high as 6 μL. From these photos, it can be seen that these four kinds of surfaces also have a high adhesion for water droplets. The adhesive force was defined as the force required to lift the water droplet off the substrate and can be assessed by a highly sensitive micromechanical balance system. An optical microscope lens and a charge-coupled-device (CCD) camera system were used to record images during the experiment. The smooth film optical photos of the whole process are shown in Figure 4a, which demonstrate that the film has low adhesion, because it cannot snap the water from the original water droplet. The whole curve of the force versus the distance between the solid surface and the water droplet in air is plotted in Figure 4b. Force−distance curves were recorded before and after the water droplet made contact with the smooth film. First, the smooth film was placed on the plate of the balance system, a 6 μL water droplet was suspended on a metal cap, and the force of this balance system was initialized to zero. Next, the film was brought into contact with the water droplet while maintaining the force balance at zero (process 1). The film was moved at a rate of 0.01 mm/s. When the film left the water droplet after contact, the balance force increased gradually and reached its maximum of 121 μN (Table 1) at the end of process 2. When the film kept leaving the water droplet, the balance force decreased gradually until the water droplet moved

CAs of the smooth film and the porous films are about 96°, 123°, 120°, and 124° (Table 1), respectively. As compared to Table 1. Average Contact Angles (CAs) and Average Adhesive Force Data of the Smooth Film and the Porous Films film style

CA (°C)

maximum force (μN)

final force (μN)

smooth film porous film 1 porous film 2 porous film 3

96 123 120 124

121 145 131 122

0 −54 −3 0

the smooth film, the CA increases of the porous films indicate that the hydrophobicity of porous films is increased as the pores are added. Yet the porous films with different pore sizes have shown similar CAs data, which indicate these three kinds of porous films have the same roughness. The reason for the increase of the hydrophobicity is mainly ascribed to the air trapped in the pores, which can prevent the intrusion of water into the pores, resulting in the large contact angle. The theoretical explanation can be expressed by eq 1, which was first proposed by Cassise.38 cos θ′ = f cos θ + f − 1

(1) C

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Figure 5. (a) Photographs of the water droplet shape taken at the corresponding stages during the measuring process. (b) Force− distance curves recorded before and after the water droplet makes contact with the porous film 1. Process 1, the film approaches the water droplet; process 2, the film leaves the water droplet after contact, the length of the water column is increasing, and the adhesive force is going up with the increase of the distance between the film and the suspender; process 3, the film continues leaving the water droplet, the diameter of the water column in the middle is becoming slim, and the adhesive force is going down with the increase of the distance between the film and the suspender; and process 4, the film breaks away from the water droplet. From this curve, we can see that the porous film 1 has relatively high adhesion, which can snap a large amount of water from the original water droplet, the average final adhesive force becomes negative. This result is consistent with the phenomenon observed in (a). (c) Corresponding schematic diagrams of the triplephase contact line movement process. The triple-phase contact line is always pinned, and the contact area holds the same line during the whole adhesion measurement process.

Figure 4. (a) Photographs of the water droplet shape taken at the corresponding stages during the measuring process. (b) Force− distance curves recorded before and after the water droplet makes contact with the smooth film. Process 1, the film approaches the water droplet; process 2, the film leaves the water droplet after contact; process 3, the film continues leaving the water droplet; and process 4, the water droplet moves away from the surface. From this curve, we can see that the smooth film has relative low adhesion, so it cannot snap water droplet from the original water droplet, and the average final adhesive force becomes zero. This result is consistent with the phenomenon observed in (a). (c) Corresponding schematic of the triple-phase contact line movement process. 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 water droplet breaks away from the surface.

away from the smooth film surface (process 3). The adhesive force is the balance force (121 μN) because the network of water molecules was not broken. Finally, in process 4, the balance force decreased immediately when the smooth film broke away from the water droplet to finish one cycle of the force measurement. From this curve, we can see that the smooth film has relatively low adhesion, so it cannot snap the water from the original water droplet. The final balance force becomes zero. This result is consistent with the phenomenon observed in Figure 4a. We also observed that the triple-phase contact line was moving and the contact area was decreasing with the increase of the distance between the film and the suspender in the whole process until the water droplet broke away from the surface (Figure 4c). The relative humidity was maintained at 80% during the whole measurement process, so the evaporation of the drop could be ignored. The same experiments were conducted to study the adhesion behavior of the porous films with different pore sizes. The porous film 1 photographs of the water droplet shapes taken at the corresponding stages during the measuring process are shown in Figure 5a. The whole curve of the force versus the distance between the porous film 1 and the water droplet is plotted in Figure 5b. From these photos, we can see that the porous film 1 displays different adhesive behavior and relatively high adhesive forces as compared to the smooth film, which can snap a large amount of water from the original water droplet. At first, the film was brought into contact with the water droplet (process 1) at a rate of 0.01 mm/s. When the film left the water droplet at the end of process 1, the balance force increased gradually and reached its maximum of 145 μN (Table 1) at the end of process 2. The three-phase contact line was pinned, and the contact area held the same line during this process. The length of the water column was increasing and the adhesive force was going up with the increase of the distance between the film and the suspender. With the film continuing to leave

the water droplet, the three-phase contact line was also pinned and the contact area also held the same line, so the diameter of the water column in the middle was becoming slim and the adhesive force was going down with the increase of the distance between the film and the suspender (process 3). The adhesive force should be larger than the maximum stretching force (145 μN) because the network of water molecules was broken. Finally, in process 4, the balance force decreased immediately when the film broke away from the water droplet to finish one cycle of the force measurement. The final balance force became negative (−54 μN), which indicated that much water was left on the film after it was transported by the metal cap. This result is consistent with the phenomenon observed in Figure 5a. During this adhesion measurement process, we observed that the triple-phase contact line was always pinned and the contact area held the same line until the water droplet broke away from the surface (Figure 5c). These results show that the porous film 1 has a relative high adhesion. Similarly, porous film 2 photographs of the water droplet shapes taken at the corresponding stages during the measuring process are shown in Figure 6a, and the whole curve of the force versus the distance is plotted in Figure 6b. From these photos, we can see that the porous film 2 displayed relatively high adhesive forces as compared to the smooth film, but relatively low adhesive forces as compared to the porous film 1. Porous film 2 can snap a small amount of water from the original water droplet. When the film left the water droplet at the end of process 1, the balance force increased gradually and reached its maximum of 131 μN (Table 1) at the end of process 2. The three-phase contact line was moving and the contact area was decreasing with the increase of the distance between the film and the suspender in process 2. In process 3, D

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Figure 6. (a) Photographs of the water droplet shape taken at the corresponding stages during the measuring process. (b) Force− distance curves recorded before and after the water droplet makes contact with the porous film 2. Process 1, the film approaches the water droplet; process 2, the film leaves the water droplet after contact, and the adhesive force is enhancing with the increase of the distance between the film and the suspender; process 3, the film continues leaving the water droplet; and process 4, the film breaks away from the water droplet. From this curve, we can see that the porous film 2 can also snap small amounts of water from the original water droplet, and the average final adhesive force becomes negative too. This result is consistent with the phenomenon observed in (a). (c) Corresponding schematic diagrams of the triple-phase contact line movement process. The triple-phase contact line is moving, and the contact area is decreased with the increase of the distance between the film and the suspender during the adhesion measurement process. Yet the surface has a little water residue.

Figure 7. (a) Photographs of the water droplet shape taken at the corresponding stages during the measuring process. (b) Force− distance curves recorded before and after the water droplet makes contact with the porous film 3. Process 1, the film approaches the water droplet; process 2, the film leaves the water droplet after contact, the adhesive force is enhancing too with the increase of the distance between the film and the suspender; process 3, the film continues leaving the water droplet; and process 4, the water droplet moves away from the surface. From this curve, we can see that the porous film 3 has relative low adhesion, it cannot snap the water from the original water droplet, and the average final adhesive force becomes zero. This result is consistent with the phenomenon observed in (a). (c) Corresponding schematic of the triple-phase contact line movement process. 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 water droplet breaks away from the surface.

the three-phase contact line pinned and the contact area held the same line with the film continuing to leave the water droplet, and so the diameter of the water column at the bottom was becoming slim and the adhesive force was going down with the increase of the distance between the film and the suspender. The adhesive force should be the maximum stretching force (131 μN) because the surface does not have a large amount of water residue. Finally, in process 4, the balance force decreased immediately when the film broke away from the water droplet to finish one cycle of the force measurement. The final balance force became negative (−3 μN), which indicated that there is a little water residue on the surface. This result is consistent with the phenomenon observed in Figure 6a. During this adhesion measurement process, the triple-phase contact line moved, and the contact area decreased with the increase of the distance between the film and the suspender until the water droplet broke away from the surface in the porous film 2 (Figure 6c). For porous film 3, optical photos of the whole process are shown in Figure 7a; the whole curve of the force versus the distance between the porous film 3 and the water droplet is plotted in Figure 7b. The porous film 3 displayed similar adhesive behavior and relatively low adhesive forces as compared to the smooth film, which makes the water droplet unable to snap on the surface. When the film left the water droplet at the end of process 1, the balance force increased gradually and reached its maximum of 122 μN (Table 1) at the end of process 2. The film continued to leave the water droplet, and the balance force decreased gradually until the water droplet moves away from the surface in the smooth film (process 3). The adhesive force should be the maximum stretching force (122 μN) because the network of water molecules was not broken. We observed that the triple-phase contact line was moving and the contact area was decreasing when the distance between the film and the suspender

increased in this process until the water droplet broke away from the surface (Figure 7c). Finally, the balance force decreased immediately when the film broke away from the water droplet in process 4 to finish one cycle of the force measurement. Before the film was about to leave the water drop, the shape of the oil drop first changed from spherical to elliptical, and then returned to spherical after the water drop was displaced from the surface (Figure 6a). These results indicate the porous film 3 also shows relatively low adhesion. Furthermore, we also used a water droplet with the volume of 8 μL to test the adhesive force. The results showed that all of the films (smooth film, porous film 1, porous film 2, and porous film 3) can snap a large amount of water from the original water droplet. The average maximum adhesive force is 146, 158, 152, and 145 μN (shown in Table S1), respectively. The average final balance force is −59, −83, −68, and −58 μN (shown in Table S1), respectively, which directly proves that the films can snap lots of water from the original water droplet. When a water droplet with the volume of 4 μL was used to test the adhesive force, the results showed that all of the films (smooth film, porous film 1, porous film 2, and porous film 3) cannot snap water from the original water droplet. The average maximum adhesive force is 117, 138, 128, and 119 μN (shown in Table S2), respectively. The average final balance force is 0, −2, −1, and 0 μN (shown in Table S2), respectively. This result indicated that the films cannot snap water from the original water droplet. From the above data, we can see that the adhesion force increases with increasing mass of water droplets on the same film. This is because the adhesion force of the water droplet is only dependent on the apparent contact area for the same film. The apparent contact area increases with increasing mass of water droplets on the same film.43 The adhesion force ariation of the 4 and 8 μL water droplet is the same as that of the 6 μL water droplet. Yet for different films, E

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only the 6 μL water droplets test can exhibit different triplephase contact line movement processes caused by different structures. So in this work, we used the 6 μL water droplet to test the different structural film adhesions in detail. 3.4. Mechanism Affecting the Adhesive Force. In general, the adhesion of the surface can be governed by both the chemical composition and the geometrical microstructures.39 In this work, all surfaces were prepared with the same hydrophobic material PS. Thus, it is easy to find that the different adhesions of these honeycomb surfaces are mainly caused by their different porous geometries (pore densities and pore diameters). As was previously reported, the adhesion between polar water and nonpolar PS surface can be explained mainly by the dispersive adhesion caused by van der Waals forces.40 The van der Waals forces are proportional to the contact areas.41 The above experimental results show that area fractions of the solid on the honeycomb structure film surface are approximate to each other. If only the van der Waals forces were taken into consideration, the adhesive force of the honeycomb structure films also should be similar too. However, the different maximum adhesive forces between water droplet and these surfaces were observed in the experiment. The conflict indicates that something else affects the adhesion. The only possible explanation is: when a water droplet is placed on the honeycomb surface, there are sealed pockets of air trapped in the pores (Figure 8). Because the capillary water existed in

air-expansion ratio (ΔV/V0), so ΔP would be rather large. That is, the force produced by this negative pressure is large. In this case, the volume of air sealed in a pore varies with its diameter. Larger pore would be expected to have lower air-expansion ratios (ΔV/V), and thus lower negative pressure.43,44 From the above analysis, we can see that a single honeycomb structural pore with small diameter would require a large pulling-off force, and the density is higher when the diameter is smaller. Therefore, the total surface adhesive force would be larger for the honeycomb structure film with smaller pore diameter. That is, F1 > F2 > F3 (see Table 1, here F1 is the porous film 1 adhesive force, F2 is the porous film 2 adhesive force, and F3 is the porous film 3 adhesive force). Noticeably, for the same water droplet, the whole water/PS contact area of the honeycomb film surface should be smaller than that for the smooth surface. Accordingly, the adhesive force produced by the van der Waals forces would be lower because the van der Waals forces are proportional to sizes of contact areas. Here, because the smooth film surface cannot form a closed system to generate the negative pressure, their contribution to the adhesion may be negligible. Although the surface with honeycomb structure has a smaller van der Waals force, the negative pressure would be large enough to endow the surface with higher adhesion. Therefore, the adhesive forces for these surfaces not only depend on the van der Waals forces but also relate to the air sealed between the surface and the water, indicating that the geometries of these surfaces are important for their adhesive properties. Furthermore, from the above results, one can find that the initial volume of the sealed air is crucial for the adhesion. Briefly, a larger volume of sealed air would be expected to have a lower air expansion ratio and lower negative pressure. Hence, the adhesive force can be controlled easily by changing the volume of the air sealed in the pores, and this can be an effective way to fabricate surfaces with different adhesions. It has proved that the water adhesive force of the honeycomb structure surfaces can be tuned by varying the diameters of the pores.

Figure 8. Schematic illustrations of the interfaces between water and a single honeycomb structural pore and the volume change of the sealed air in one PS pore upon the action of external force. Capillary adhesion arises when a water droplet sitting on the pore is gradually drawn upward because the convex air/liquid interface produces an inward pressure ΔP. W represents water droplet, and S represents solid film.

4. CONCLUSIONS In summary, inspired by the negative pressure principle generated by octopus suckers, we designed a high-adhesive honeycomb structure porous surface with controllable wateradhesive force by BF method. We found that different negative pressures produced by the sealed air can be the crucial factor for their different adhesions. Furthermore, the initial volume of the air sealed between water and the surface is important for the adhesion; that is, larger volume would result in the lower adhesion. The experiment results verified that the surface adhesive force of the honeycomb structure film surface can be effectively tuned by changing the fractions of air pockets in sealed systems, which depend on the pore diameters and densities. It should be of great scientific benefit to extend the related honeycomb research from existing applications to new applications. The unique adhesive phenomenon of honeycomb structure can be useful for manipulating liquid collection and transportation, as well as controlling the triple-phase contact line movement and the surface adhesive properties. These interesting findings can help us further understand the biological attachment systems and can also guide the design of new functional nanomaterials with custom-tailored surface adhesion.

the pore, the surface of the air/water is meniscus (Figure 8). To some extent, the water droplet is an elastic body. When we applied an external force to it, a deformation occurred along the direction of external force. We also observed the drop deformation along the direction of force during the adhesion measurement process. Thus, when the droplet is gradually retracted from the sample surface, the meniscus on each pore is changed from concave to convex. This can result in an increased volume of air sealed in each pore by the liquid/air interface. Assuming the sealed air is ideal gas, the negative pressure ΔP can be described as42 1 ΔP = −P0 V0/ΔV + 1 (2) where P0 is the initial pressure of the sealed air for a single pore, V0 is the initial volume of the sealed air for a single pore, and ΔV is the increased volume of sealed air under the action of external force. From eq 2, one can find that the relationship between ΔV and V0 is important for ΔP; that is, when ΔV ≪ V0, ΔP would be rather small, while when ΔV ≈ V0, ΔP would be very large.42,43 In brief, larger V0 would result in smaller ΔP. According to eq 2, one pore with smaller diameter has a higher F

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ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram of the experimental equipment, and the average adhesive force data of the smooth film and the porous films with 4 and 8 μL water droplet. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+86) 10 8262 1396. Fax: (+86) 10 8262 7566. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Research Fund for Fundamental Key Projects (2010CB934700, 2013CB834705, 2011CB935700) and by the National Natural Science Foundation of China (Grant no. 21003138)

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