Article pubs.acs.org/Langmuir
Bio-Inspired Low Frictional Surfaces Having Micro-Dimple Arrays Prepared with Honeycomb Patterned Porous Films as Wet Etching Masks Y. Saito† and H. Yabu*,‡ †
Graduate School of Engineering and ‡Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, Miyagi, Japan S Supporting Information *
ABSTRACT: Some kinds of snakes have micro-dimple arrays on their skins and show low frictional properties. Cost-effective and simple preparation methods of surfaces having micro-dimple arrays without burrs have been required. In this study, micro-dimple arrays were successfully prepared on aluminum plates and pipes by using honeycomb patterned porous films as wet etching masks. Resulting surfaces having 5 and 8 μm dimple diameters show low frictional coefficients compared with polished surfaces at a fluid lubrication regime.
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INTRODUCTION Friction reduction among material surfaces is one of the important challenges for realization of a future sustainable society. Many efforts have been done to achieve low friction between two artificial sliding surfaces including increasing area of frictional surfaces, optimization of lubricant, hardening friction surfaces, and so on.1,2 On the other hand, it has recently found that animals, such as bush crickets,3 sandfishes,4 and snakes,5 control their surface frictions with surface microstructures. Some kinds of snake have dimple arrays on their ventral scales, and they show low friction coefficients under wet conditions.5 To reduce frictions, surfaces having micro-dimple arrays (MDAs) have been studied, which mimic the lateral structures of snake skins,6−8 and these studies revealed that reduction of frictions by texturing of MDAs is originated by several reasons including hydrodynamic pressure caused by dimples, trapping debris in dimple holes to avoid wear, MDAs acting as reservoir of lubricants, and so on.9−12 For applying MDAs to practical frictional surfaces in the artificial devices, such as engine cylinders and slide bearings, a simple and cost-effective fabrication method for creating MDAs on the frictional surfaces of these devices, which usually consists of inorganic materials having curved surfaces, is required. To prepare MDAs, there are two ways: one is a direct processing process without templates, and the other is a pattern transferring from templates. The former one, physical fabrication methods, such as sand blast13,14 and laser ablation,11 are a simple and low-cost method for patterning surfaces. However, by using these techniques, resolution of patterning is low, and burrs were formed around dimples. Burrs contact the © 2014 American Chemical Society
other surface during sliding, and they make friction force higher. In the method of using templates, chemical etching,9,12,15 which is a one of the chemical fabrication methods, can prepare nano to micron surface patterns on metal surfaces by using patterned masks. The mask is prepared on a solid substrate, and then the substrate is exposed to an etching solution or etching gases to degrade the unmasked material’s surfaces. Although dry etching can form patterns with high accuracy, since dry etching required reactive gases and controlled vacuum systems, and they makes process complicate. Wet etching is a simple and low-cost method for patterning various inorganic substrates. The resolution and transfer patterns are determined by the quality of templates. Photolithography has been used to prepare etching masks on inorganic material’s surfaces. However, photolithography requires multistep processes, and it is also difficult to be applied patterning curved surfaces since it requires optical systems for planar surfaces. Honeycomb-patterned porous polymer films are formed by casting polymer solutions containing amphiphilic copolymers on a solid substrate under applying humid air.16−18 Water droplets condense on the solution surface, and the amphiphilic copolymers stabilize the water droplets and prevent them from fusing. When the polymer solvent evaporates, the water droplets are uniformly packed by the capillary force, and the Received: July 31, 2014 Revised: December 23, 2014 Published: December 29, 2014 959
DOI: 10.1021/la503883m Langmuir 2015, 31, 959−963
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and the measurement distance was 2 cm. Caster oil (0.68 Pa·s) was used as a lubricant; the temperature of the oil was kept at 27 °C. Friction measurements were carried out five times for each of the three samples at each sliding speed.
porous honeycomb structure is formed by using water droplets as templates. Finally, microporous films with regular arrangements of uniformly sized pores can be prepared from many polymer materials by this simple casting method. The diameter of the pores can be controlled from scales of tens of micrometers to submicrometers. The top layers of the honeycomb film can be peeled off easily with an adhesive tape to form pincushion-structure films. Recently, we have found top layer of honeycomb films can be used as wet etching masks of inorganic materials.19 In this report, we prepared MDAs on an aluminum plates and aluminum tube surface by using honeycomb films as etching masks, and the friction coefficients of their patterned surfaces are discussed.
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RESULTS AND DISCUSSION Figure 2a shows a typical SEM image of a honeycomb film. The film had hexagonally arranged pores, and top and bottom layers were connected with pillars from the cross-sectional images (inset of Figure 2a). An optical microscope image and a photograph of as-prepared aluminum plate are shown in Figure 2b. The surface of the plate was reflective, and there are some scratching traces originated by polished process. From an optical microscope image after fixing a top layer of honeycomb film, the top layer of the honeycomb film was observed on the aluminum plate (Figure 2c). The masked aluminum plate scatters light owing to micron-sized structures on its surface (inset of Figure 2c). An optical microscope image and an AFM image of etched aluminum plates are shown in Figure 2d. Regularly arranged MDAs were observed on its surface. The etched plate also scatters light, which also supported the formation of MDAs (inset of Figure 2d). From AFM results, average depth of MDAs was ca. 500 nm in the case of soaking into etching solution for 1 h. To investigate the effect of KOH concentration and soaking time on etching speed, masked aluminum plates were etched by soaking into various concentrations of KOH with changing soaking times. Under 2.5 wt % KOH, aluminum surfaces did not etch because the KOH concentration was lower than the threshold of etching. On the other hand, when KOH concentration was higher than 15 wt %, aluminum surfaces reacted vigorously and produced bubbles, which resulted in removal of masks and prevented from MDAs formation. Dimple depths etched in 5 or 10 wt % KOH condition are shown in Figure 3a. The dimple depth was successfully controlled by changing a soaking time, and the maximum depth was ca. 1800 nm at etching condition of 10 wt % KOH for 180 min. Figure 3 shows that optical microscope (Figure 3b,d,f) and SEM (Figure 3c,e,g) images of etched aluminum plates with masks with 5, 8, and 15 μm pore diameters by soaking into etching solutions containing 6.5 wt % KOH for 1 h. MDAs having similar dimple diameters to pore sizes of template honeycomb films were observed. These results suggested that dimple diameters were controlled by changing pore diameters of honeycomb films. The method can be also applied to curved surfaces because honeycomb films are flexible. Honeycomb films were fixed on aluminum pipes with 5 cm diameter by the method same as shown above. The masked aluminum pipe was etched by soaking into etching solution containing 5 wt % KOH for 1 h. From an optical microscope image, MDAs were observed on the surface of etched pipe, and scattering of light was also observed (Figure 4). Friction measurements were carried out with caster oil as a lubricant. Figure 5 shows friction coefficients of patterned surfaces having MDAs, whose diameters are 5, 8, and 15 μm (surface 1, 2, and 3, respectively) and depths were ca. 500 nm (482, 550, and 476 nm), compared with polished surfaces. This dimple depth was chosen based on the results of friction measurements with changing dimple depths from 330 to 939 nm (Supporting Information Figure S1). Friction measurement results show typical Stribeck curves, which are usually characterized by sliding speeds and friction coefficients. The
EXPERIMENTAL SECTION
Amphiphilic copolymer having a carboxyl group was synthesized from N-dodecylacrylamide and a N-acrylamide hexanoic acid according to the literature.17 The chemical structure of the amphiphilic copolymer (polymer 1) is shown in Figure 1a. The copolymerization ratio of N-
Figure 1. Chemical structures of polymer 1 (a), PS (b), and PVA (c). dodecylacrylamide and carboxyl monomer was 4 to 1. Honeycomb films were prepared on glass substrate. The honeycomb films were prepared by casting a 5 g/L chloroform solution of polystyrene (PSt, Figure 1b) and polymer 1 solutions (5 mL) under highly humid air supplied at a flow rate of 3.5 L/min. The weight ratio of PSt and polymer 1 was 10 to 1. The preparation method of MDAs is shown in Figure 2. Honeycomb films were treated by ultraviolet ozone for 5 min to make honeycomb film surfaces hydrophilic. 1 g/L of poly(vinyl alcohol) (PVA, Figure 1c) aqueous solution was spin-coated on the honeycomb film at 1000 rpm, and then honeycomb films were fixed on a polished aluminum plate (surface roughness; Rq = 32 nm, 3 × 3 cm2). A bottom layer of honeycomb films on the aluminum plate was peeled off with an adhesive tape, and then the aluminum plate with the top layer of a honeycomb film was washed by water. The masked aluminum plate was heated at 200 °C for 3 h. The masked aluminum plate was etched by soaking in ethanol (EtOH) solution containing potassium hydroxide (KOH) at 20 °C with stirring. Masked aluminum plates were etched in ethanol solution because PVA dissolved in water, which results in removing masks. After wet etching, the aluminum plate was washed by water and then ultrasonicated in water for 5 min. Finally, the aluminum plate was wiped with a paper towel impregnated with chloroform. The surface structure was observed by using an optical microscope and a scanning electron microscope (SEM). Depth of dimples was measured by using atomic force microscopy (AFM). Friction coefficients were measured by using a friction tester (HHS2000, SHINTO Scientific Co., Ltd.). Polystyrene curved surfaces whose root-mean-square surface roughness was 88 nm and curvature was 88 cm were faced to the aluminum plate. The load was 0.49 N, 960
DOI: 10.1021/la503883m Langmuir 2015, 31, 959−963
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Figure 2. Schematic illustration for patterning of micron-sized dimple arrays on aluminum plates. SEM images of honeycomb films are shown (a). A photograph and an optical microscope image of polished aluminum plate (b) and masked aluminum plate (c) and an AFM image, an optical microscope image, and a photograph of etched aluminum plate are shown (d).
Figure 3. (a) Average dimple depth plotted as a function of soaking time. Optical microscope images of honeycomb films having 5, 8, and 15 μm pore diameter (b, d, f) and SEM images of etched aluminum plate by using them as masks (c, e, g), respectively. Scale bars: 20 μm.
curves are divided into two regimes: a fluid lubrication and a mixed lubrication regime. When the sliding speed was faster than 8 mm/s, friction coefficients increase linearly with increase of sliding speed, which means that the region is the fluid lubrication regime and two surfaces do not contact each other. Slower sliding speed than 8 mm/s means the mixed lubrication regime, in which two surfaces contact partially each other and friction coefficients decrease and then increase with decreasing sliding speed. Since two friction surfaces collapsed each other in the mixed lubrication regime, the fluid lubrication regime is suitable for evaluating relation between lubricant and microdimple arrays. Figure 5b shows friction coefficients normalized by friction coefficients of polished surfaces at sliding speeds
Figure 4. Optical microscope images of original aluminum pipe surface (left) and etched aluminum pipe surface (right). Inset images show photographs of aluminum pipe.
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Figure 5. (a) Friction coefficients of patterned surfaces and polished surfaces at each sliding speeds. (b) Normalized friction coefficient of patterned surfaces by polished surfaces at each sliding speed.
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faster than 8 mm/s, which is considered as the fluid lubrication regimes. In the measurement range, friction coefficients of surface 1 and 2 were lower than that of polished surfaces owing to hydrodynamic pressure caused by dimples, which kept the two frictional surfaces apart and decreased viscosity resistance.9,20 In contrast, surface 3 had a higher friction coefficient than polished surfaces because MDAs acted as surface roughness, which enlarged the mixed lubrication regime, rather than origins of hydrodynamic pressure. Figure S1b shows friction coefficients of surfaces having 15 μm dimple diameter of various dimple depth. From the result, the surface having 15 μm diameter and 739 nm depth shows the lowest friction force of surfaces having 15 μm dimple diameter. It may suggest that there is the optimum aspect ratio of dimple diameter and dimple depth. These results indicate that bio-inspired patterned surfaces can realize low frictional surfaces in this system.
ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Challenging Exploratory Research (No. 26620170).
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(1) Aoki, S.; Yamada, Y.; Fukada, D.; Suzuki, A.; Masuko, M. Verification of the advantages in friction-reducing performance of organic polymers having multiple adsorption sites. Tribol. Int. 2013, 59, 57−66. (2) Grill, A. Review of tribology of diamond-like carbon. Wear 1993, 168, 143−153. (3) Varenberg, M.; Gorb, S. N. Hexagonal surface micropattern for dry and wet friction. Adv. Mater. 2009, 21, 483−486. (4) Baumgartner, W.; Saxe, F.; Weth, A.; Hajas, D.; Sigumonrong, D.; Emmerich, J.; Singheiser, M.; Böhme, W.; Schneider, J. M. The sandfish’s skin: Morphology, chemistry and reconstruction. J. Bionic Eng. 2007, 4, 1−9. (5) Berthe, R. A.; Westhoff, G.; Bleckmann, H.; Gorb, S. N. Surface structure and frictional properties of the skin of the Amazon tree boa Corallus hortulanus (Squamata, Boidae). J. Comp. Physiol., A 2009, 195, 311−318. (6) Baum, M. J.; Kovalev, A. E.; Michels, J.; Gorb, S. N. Anisotropic friction of the ventral scales in the snake Lampropeltis getula californiae. Tribol. Lett. 2014, 54, 139−150. (7) Baum, M. J.; Heepe, L.; Gorb, S. N. Friction behavior of a microstructured polymer surface inspired by snake skin. Belistein J. Nanotechnol. 2014, 5, 83−97. (8) Abdel-Aal, H. A.; Vargiolu, R.; Zahouani, H.; Mansori, M. E. Preliminary investigation of the frictional response of reptilian shed skin. Wear 2012, 290-291, 51−60. (9) Wang, X.; Adachi, K.; Otsuka, K.; Kato, K. Optimization of the surface texture for silicon carbide sliding in water. Appl. Surf. Sci. 2006, 253, 1282−1286. (10) Wang, X.; Kato, K.; Adachi, K.; Aizawa, K. Loads carrying capacity map for the surface texture design of SiC thrust bearing sliding in water. Tribol. Int. 2003, 36, 189−197. (11) Kovalchenko, A.; Ajayi, O.; Erdemir, A.; Fenske, G.; Etsion, I. The effect of laser texturing of steel surfaces and speed-load parameters on the transition of lubrication regime from boundary to hydrodynamic. Tribol. Trans. 2004, 47, 299−307. (12) Yan, D.; Qu, N.; Li, H.; Wang, X. Significance of dimple parameters on the friction of sliding surfaces investigated by orthogonal experiments. Tribol. Trans. 2010, 53, 703−712. (13) Zhao, G.; Raines, A. L.; Wieland, M.; Schwartz, Z.; Boyan, B. D. Requirement for both micron- and submicron scale structure for
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CONCLUSION In this report, we successfully adhered top layers of honeycomb films on metal surfaces and prepared MDAs on an aluminum plate and pipe by etching with using top layer of honeycomb films as etching masks. The dimple diameter was similar to the size of diameter of the honeycomb film. The dimple depth can be controlled by changing the time of soaking into the etching solution, and maximum depth was ca. 1800 nm. We also achieved preparation of low frictional surfaces. Friction coefficients of surfaces having MDAs of which diameters were 5 and 8 μm were lower than that of polished surfaces in this system. These results open the new way to realize noble lowfriction materials.
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ASSOCIATED CONTENT
S Supporting Information *
Friction coefficients of surfaces having various dimple depths. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (H.Y.). Notes
The authors declare no competing financial interest. 962
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