高深宽比微纳层次结构仿壁虎脚毛制作工艺研究
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摘要
自然界中壁虎具有非常优秀的吸附能力,可以自由行走在光滑的墙壁上,甚至是天花板上。壁虎脚毛的分级层次结构,尤其是高深宽比的微米级刚毛与纳米级绒毛结构,是保证其产生巨大粘附力且能适应不同表面形貌的关键。壁虎脚毛的这些特性在MEMS器件中以及一些爬壁机器人等领域有着极为广阔的应用前景。为此,本学位论文围绕仿壁虎脚毛的纤维阵列制作工艺开展了如下研究。
采用厚胶工艺,选用SU-8光刻胶作为纤维阵列材料进行试验,制备出了深宽比高达7的纤维阵列。由于较高的深宽比,纤维之间存在纠结现象,难以通过微模塑成型工艺进一步复制其结构以进行批量化生产。
采用半导体工艺中的硅刻蚀工艺,通过ICP刻蚀制作出了多种尺寸的硅孔阵列模具,最高深宽比可达到7以上。利用该硅模具成功复制出聚二甲基硅氧烷(PDMS)微米级纤维阵列。直径小于8μm的纤维阵列,仍然存在纠结现象。无论纤维是否纠结在一起,纤维阵列表面均展现出较好的疏水性,从平面PDMS的水接触角102.7°提升到131.7°以上。
提出了一种全新的简单,低成本的双级层次纤维阵列结构制作工艺。该工艺基于以上硅模具,在硅模具表面涂覆一层SU-8光刻胶,利用厚胶光刻工艺制作出微米孔阵列,与硅模具组成两级孔阵列模具。利用该复合模具成功复制出两级PDMS纤维阵列。由于模具可重复使用,因此可以实现批量化生产。经实验发现双级层次PDMS纤维阵列结构的疏水性相比单级PDMS纤维阵列有少量提升,达到145.9°。
In nature, geckos have developed complex adhesion structures capable of smart adhesion, the ability to maneuver on different smooth and rough surfaces, even ceilings. The multi-scale hierarchical structure of the gecko foot-hair, especially the high-aspect-ratio structure of its micro-scale seta and nano-scale spatulae is the critical factor of the gecko’s ability to cling to any different surface with strong adhesion force. The incredible characteristics of gecko foot-hair have a very broad prospect of applications in the deployment and disassembly of MEMS devices, wall-climbing robots, etc. Accordingly, the fabrication of high-aspect-ratio micro/nano hierarchical structures mimicking gecko foot-hair has been studied in this degree thesis including the following contents:
Thick film photolithography based on SU-8 photoresist was utilized to fabricate fiber arrays. The microfiber array with multiple parameters was obtained. The highest aspect ratio of SU-8 microfiber is 7. Due to high aspect ratio, the SU-8 microfibers tend to be bunched together, making it difficult to creating a mold to relicate the fibers with other polymer for batch process.
Utilizing inductively coupled plasma (ICP) etching process, which is very common in the semiconductor technology, the silicon mold with deep-hole arrays with multiple parameters was fabricated. The polydimethylsiloxane (PDMS) microfiber arrays were successfully replicated from the silicon mold. As for microfiber arrays with less than 8μm in diameter, the fibers tend to be bunched together. The hydrophobic property of the synthetic PDMS microfiber arrays was observed no matter the microfibers bunched together or not. The contact angle of water droplets on PDMS microfiber arrays shows increased hydrophobic property of PDMS microfiber arrays than non-patterned PDMS surface, from 102.7°to more than 131.7°.
A novel and simple method with low cost for fabricating two-level two-level hierarchical microfiber array was proposed. Utilizing the silicon mold with deep-hole arrays as a substrate, a SU-8 layer with microhole arrays was fabricated on it using thick film photolithography, and form a double layer mold. The two-level hierarchical PDMS microfiber arrays were replicated from the double layer mold. The double layer mold is reusable. Therefore this method is suitable for batch processes. The water contact angle of the two-level hierarchical PDMS microfiber array surface was observed to be 145.9°, which is a little bit higher than the one of single level PDMS microfiber array surface.
采用厚胶工艺,选用SU-8光刻胶作为纤维阵列材料进行试验,制备出了深宽比高达7的纤维阵列。由于较高的深宽比,纤维之间存在纠结现象,难以通过微模塑成型工艺进一步复制其结构以进行批量化生产。
采用半导体工艺中的硅刻蚀工艺,通过ICP刻蚀制作出了多种尺寸的硅孔阵列模具,最高深宽比可达到7以上。利用该硅模具成功复制出聚二甲基硅氧烷(PDMS)微米级纤维阵列。直径小于8μm的纤维阵列,仍然存在纠结现象。无论纤维是否纠结在一起,纤维阵列表面均展现出较好的疏水性,从平面PDMS的水接触角102.7°提升到131.7°以上。
提出了一种全新的简单,低成本的双级层次纤维阵列结构制作工艺。该工艺基于以上硅模具,在硅模具表面涂覆一层SU-8光刻胶,利用厚胶光刻工艺制作出微米孔阵列,与硅模具组成两级孔阵列模具。利用该复合模具成功复制出两级PDMS纤维阵列。由于模具可重复使用,因此可以实现批量化生产。经实验发现双级层次PDMS纤维阵列结构的疏水性相比单级PDMS纤维阵列有少量提升,达到145.9°。
In nature, geckos have developed complex adhesion structures capable of smart adhesion, the ability to maneuver on different smooth and rough surfaces, even ceilings. The multi-scale hierarchical structure of the gecko foot-hair, especially the high-aspect-ratio structure of its micro-scale seta and nano-scale spatulae is the critical factor of the gecko’s ability to cling to any different surface with strong adhesion force. The incredible characteristics of gecko foot-hair have a very broad prospect of applications in the deployment and disassembly of MEMS devices, wall-climbing robots, etc. Accordingly, the fabrication of high-aspect-ratio micro/nano hierarchical structures mimicking gecko foot-hair has been studied in this degree thesis including the following contents:
Thick film photolithography based on SU-8 photoresist was utilized to fabricate fiber arrays. The microfiber array with multiple parameters was obtained. The highest aspect ratio of SU-8 microfiber is 7. Due to high aspect ratio, the SU-8 microfibers tend to be bunched together, making it difficult to creating a mold to relicate the fibers with other polymer for batch process.
Utilizing inductively coupled plasma (ICP) etching process, which is very common in the semiconductor technology, the silicon mold with deep-hole arrays with multiple parameters was fabricated. The polydimethylsiloxane (PDMS) microfiber arrays were successfully replicated from the silicon mold. As for microfiber arrays with less than 8μm in diameter, the fibers tend to be bunched together. The hydrophobic property of the synthetic PDMS microfiber arrays was observed no matter the microfibers bunched together or not. The contact angle of water droplets on PDMS microfiber arrays shows increased hydrophobic property of PDMS microfiber arrays than non-patterned PDMS surface, from 102.7°to more than 131.7°.
A novel and simple method with low cost for fabricating two-level two-level hierarchical microfiber array was proposed. Utilizing the silicon mold with deep-hole arrays as a substrate, a SU-8 layer with microhole arrays was fabricated on it using thick film photolithography, and form a double layer mold. The two-level hierarchical PDMS microfiber arrays were replicated from the double layer mold. The double layer mold is reusable. Therefore this method is suitable for batch processes. The water contact angle of the two-level hierarchical PDMS microfiber array surface was observed to be 145.9°, which is a little bit higher than the one of single level PDMS microfiber array surface.
引文
[1] Autumn K, Liang Y A, Hsieh S T, et al. Adhesive force of a single gecko foot-hair. Nature, 2000, 405 (6787): 681-685
[2] Johnson K, Kendall K, Roberts A. Surface energy and the contact of elastic solids. Proceedings of the Royal Society of London A, 1971, 324: 301-313
[3] Autumn K, Sitti M, Liang Y A, et al. Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of the United States of America, 2002, 99(19): 12252-12256
[4] Autumn K, Peattie A M. Mechanisms of adhesion in geckos. Integrative and Comparative Biology, 2002, 42(6): 1081-1090
[5] Huber G, Gorb S N, Spolenak R, et al. Resolving the nanoscale adhesion of individual gecko spatulae by atomic force microscopy. Biology Letters, 2005, 1(1): 2-4
[6] Huber G, Mantz H, Spolenak R, et al. Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements. Proceedings of the National Academy of the United States of America, 2005, 102(45): 16293-16296
[7] Hansen W R, Autumn K. Evidence for self-cleaning in gecko setae. Proceedings of the National Academy of the United States of America, 2005, 102(2): 385-389
[8] Autumn K, Majidi C, Groff R E, et al. Effective elastic modulus of isolated gecko setal arrays. Journal of Experimental Biology, 2006, 209(18): 3558-3568
[9] Autumn K, Hansen W. Ultrahydrophobicity indicates a non-adhesive default state in gecko setae. Journal of Comparative Physiology A, 2006, 192(11): 1205-1212
[10] Gao H J, Yao H M. Shape insensitive optimal adhesion of nanoscale fibrillar structures. Proceedings of the National Academy of the United States of America, 2004, 101(21): 7851-7856
[11] Geim A K, Dubonos S V, Grigorieva I V, et al. Microfabricated adhesive mimicking gecko foot-hair. Nature Materials, 2003, 2(7): 461-463
[12] Sitti M, Fearing R S. Synthetic gecko foot-hair micro/nano-structures as dry adhesives. Journal of Adhesion Science and Technology, 2003, 17(8): 1055-1073
[13] Jin M H, Feng X J, Feng L, et al. Superhydrophobic aligned polystyrene nanotube films with high adhesive force. Advanced Materials, 2005, 17(16): 1977-1981
[14] Yurdumakan B, Raravikar N R, Ajayan P M, et al. Synthetic gecko foot-hairs from multiwalled carbon nanotubes. Chemical Communications, 2005, (30): 3799-3801
[15] Tsai Y C, Shih W P, Wang Y M, et al. E-beam photoresist and carbon nanotubes as biomimetic dry adhesives. Proceedings of the 19th IEEE International Conference on MEMS, 2006, 926-929
[16] Zhao Y, Tong T, Delzeit L, et al. Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive. Journal of Vacuum Science and Technology B, 2006, 24(1): 331-335
[17] Yoona E S, Singha R A, Konga H, et al. Tribological properties of bio-mimetic nano-patterned polymeric surfaces on silicon wafer. Tribology Letters, 2006, 21(1): 31-37
[18] Jeong H E, Lee S H, Kim J K, et al. Nanoengineered multiscale hierarchical structures with tailored wetting properties. Langmuir, 2006, 22(4): 1640-1645
[19] Northen M T, Turner K L. A batch fabricated biomimetic dry adhesive. Nanotechnology, 2005, 16(8): 1159-1166
[20] Northen M T, Turner K L. Meso-scale adhesion testing of integrated micro- and nano-scale structures. Sensors and Actuators A, 2006, 130: 583-587
[21] Kustandi T S, Samper V D, Sun W, et al. Self-assembled nanoparticles based fabrication of gecko foot-hair-inspired polymer nanofibers. Advanced Functional Materials, 2007, 17: 2211-2218.
[22] Kim D S, Lee H S, Kwon T H, et al. Replication of high-aspect-ratio nanopillar array for biomimetic gecko foot-hair prototype by UV nano embossing with anodic aluminum oxide mold. Microsyst Technol, 2007, 13: 601–606.
[23] Aksak B, Murphy M P, Sitti M. Adhesion of biologically inspired vertical and angled polymer microfiber arrays. Langmuir, 2007, 23(6): 3322-3332.
[24] Greiner C, Arzt E, Campo A D. Hierarchical gecko-like adhesives. Adv. Mater. 2009, 21: 479–482
[25] Jeong H E, Lee S H, Suh K Y, et al. High aspect-ratio polymer nanostructures by tailored capillarity and adhesive force. Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314, 2008: 359–364.
[26] Qu L, Dai L, Wang Z L, et al. Carbon Nanotube arrays with strong shear binding-on and easy normal lifting-off. Science, 2008, 322:238-242.
[27] Jeong H E, Lee J K, Kim H N, et al. A Nontransferring Dry Adhesive with Hierarchical Polymer Nanohairs. The Proceedings of the National Academy of Sciences of the United States of America, 2009, 106(14): 5639-5644
[28] Suh K Y, Kim Y S, Lee H H. Capillary force lithography. Advanced Materials, 2001, 13(18): 1386-1389
[29] Xia Y, Whitesides G M. Soft lithography. Angewandte Chemie International Edition, 1998, 37(5): 550-575
[30] Odom T W, Love J C, Wolfe D B, et al. Improved pattern transfer in soft lithography using composite stamps. Langmuir, 2002, 18(13): 5314-5320
[31] Choi S J, Yoo P J, Baek S J, et al. An ultraviolet-curable mold for sub-100-nm lithography. Journal of the American Chemical Society, 2004, 126(25): 7744-1745
[32] Suh D, Choi S J, Lee H H. Rigiflex lithography for nanostructure transfer. Advanced Materials, 2005, 17(12): 1554-1560
[33]戴振东,孙久荣.壁虎的运动及仿生研究进展.自然科学进展, 2006, 16(5): 519-523
[34]郭策,戴振东,吉爱红等.壁虎脚趾运动调控的研究.中国生物医学工程学报, 2006, 25(1): 110-113
[35] Dai Z D, Tong J, Ren L Q. Researches and developments of biomimetics in tribology. Chinese Science国家自然科学基金申请书第13页版本1.004.756 Bulletin, 2006, 51(22): 2681-2689
[36] Dai Z D, Yu M, Gorb S. Adhesion characteristics of polyurethane for bionic hairy foot. Journal of Intelligent Material Systems and Structures, 2006, 17(8-9): 737-741
[37] Yu M, Ji A H, Dai Z D. Effect of microscale contact state of polyurethane surface on adhesion and friction. Journal of Intelligent Material Systems and Structures, 2006, 17(8-9): 819-822
[38]王辉静,梅涛.仿壁虎粘附阵列与粗糙表面间粘附仿真分析.中国科学技术大学学报, 2006, 36(8): 845-850
[39]王辉静,梅涛,汪小华.一种新型仿壁虎爬行机器人的粘附阵列设计.机器人, 2006, 28(2): 191-194
[40] Wang H, Mei T, Wang X, et al. Interaction and simulation analysis between the biomimetic gecko adhesion array and rough surface. Proceedings of the 2005 IEEE International Conference on Mechatronics and Automation, 2005, 1063-1068
[41] Wang H, Chen S, Wang X, et al. Design of a biomimetic gecko adhesion array for microrobots. Proceedings of the 2005 IEEE International Conference on Robotics and Biomimetics, 2005, 495-498
[42] Ren N, Wang X, Mei T, et al. Detachment control analysis for nanofabricated adhesive mimicking gecko foot-hairs. Proceedings of the 2006 IEEE International Conference on Mechatronics and Automation, 2006, 267-271
[43]金邦坤,何平笙.微接触印刷法制造聚合物多层次准三维立体微结构. 2005, 18(3): 439-442
[44]吴福贵,杨海洋,王政等.毛细微模塑和转移微模塑制备PVP/Ru(dpphen)3Cl2微图形.高分子材料科学与工程, 2006, 22(6): 188-191
[45]刘建平,石锦霞,杨小敏等.薄层软刻蚀法制备PMMA微图案结构.高分子学报, 2007, (1): 42-45
[46] Campo A del and Greiner. SU-8: a photoresist for high-aspect-ratio and 3D submicronlithography. Journal of Micromechanics and Microengineering, 2007, 17: R81-R95;
[47] Aksak B, Murphy M P, and Sitti M. Adhesion of biologically inspired vertical and angled polymer microfiber arrays. Langmuir, vol. 23, pp. 3322-3332, 2007.
[48] Chen W, Fadeev A Y, M. C. Hsieh, D. ?ner, J. Youngblood, and T. J. McCarthy. Ultrahydrophobic and ultralyophobic surfaces: some comments and examples. Langmuir, vol. 15, pp. 3395-3399, 1999.
[49] Kim T W and Bhushan B. Adhesion analysis of multi-level hierarchical attachment system contacting with a rough surface. Journal Of Adhesion Science and Technology, 2007, 21(1): 1-20
[2] Johnson K, Kendall K, Roberts A. Surface energy and the contact of elastic solids. Proceedings of the Royal Society of London A, 1971, 324: 301-313
[3] Autumn K, Sitti M, Liang Y A, et al. Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of the United States of America, 2002, 99(19): 12252-12256
[4] Autumn K, Peattie A M. Mechanisms of adhesion in geckos. Integrative and Comparative Biology, 2002, 42(6): 1081-1090
[5] Huber G, Gorb S N, Spolenak R, et al. Resolving the nanoscale adhesion of individual gecko spatulae by atomic force microscopy. Biology Letters, 2005, 1(1): 2-4
[6] Huber G, Mantz H, Spolenak R, et al. Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements. Proceedings of the National Academy of the United States of America, 2005, 102(45): 16293-16296
[7] Hansen W R, Autumn K. Evidence for self-cleaning in gecko setae. Proceedings of the National Academy of the United States of America, 2005, 102(2): 385-389
[8] Autumn K, Majidi C, Groff R E, et al. Effective elastic modulus of isolated gecko setal arrays. Journal of Experimental Biology, 2006, 209(18): 3558-3568
[9] Autumn K, Hansen W. Ultrahydrophobicity indicates a non-adhesive default state in gecko setae. Journal of Comparative Physiology A, 2006, 192(11): 1205-1212
[10] Gao H J, Yao H M. Shape insensitive optimal adhesion of nanoscale fibrillar structures. Proceedings of the National Academy of the United States of America, 2004, 101(21): 7851-7856
[11] Geim A K, Dubonos S V, Grigorieva I V, et al. Microfabricated adhesive mimicking gecko foot-hair. Nature Materials, 2003, 2(7): 461-463
[12] Sitti M, Fearing R S. Synthetic gecko foot-hair micro/nano-structures as dry adhesives. Journal of Adhesion Science and Technology, 2003, 17(8): 1055-1073
[13] Jin M H, Feng X J, Feng L, et al. Superhydrophobic aligned polystyrene nanotube films with high adhesive force. Advanced Materials, 2005, 17(16): 1977-1981
[14] Yurdumakan B, Raravikar N R, Ajayan P M, et al. Synthetic gecko foot-hairs from multiwalled carbon nanotubes. Chemical Communications, 2005, (30): 3799-3801
[15] Tsai Y C, Shih W P, Wang Y M, et al. E-beam photoresist and carbon nanotubes as biomimetic dry adhesives. Proceedings of the 19th IEEE International Conference on MEMS, 2006, 926-929
[16] Zhao Y, Tong T, Delzeit L, et al. Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive. Journal of Vacuum Science and Technology B, 2006, 24(1): 331-335
[17] Yoona E S, Singha R A, Konga H, et al. Tribological properties of bio-mimetic nano-patterned polymeric surfaces on silicon wafer. Tribology Letters, 2006, 21(1): 31-37
[18] Jeong H E, Lee S H, Kim J K, et al. Nanoengineered multiscale hierarchical structures with tailored wetting properties. Langmuir, 2006, 22(4): 1640-1645
[19] Northen M T, Turner K L. A batch fabricated biomimetic dry adhesive. Nanotechnology, 2005, 16(8): 1159-1166
[20] Northen M T, Turner K L. Meso-scale adhesion testing of integrated micro- and nano-scale structures. Sensors and Actuators A, 2006, 130: 583-587
[21] Kustandi T S, Samper V D, Sun W, et al. Self-assembled nanoparticles based fabrication of gecko foot-hair-inspired polymer nanofibers. Advanced Functional Materials, 2007, 17: 2211-2218.
[22] Kim D S, Lee H S, Kwon T H, et al. Replication of high-aspect-ratio nanopillar array for biomimetic gecko foot-hair prototype by UV nano embossing with anodic aluminum oxide mold. Microsyst Technol, 2007, 13: 601–606.
[23] Aksak B, Murphy M P, Sitti M. Adhesion of biologically inspired vertical and angled polymer microfiber arrays. Langmuir, 2007, 23(6): 3322-3332.
[24] Greiner C, Arzt E, Campo A D. Hierarchical gecko-like adhesives. Adv. Mater. 2009, 21: 479–482
[25] Jeong H E, Lee S H, Suh K Y, et al. High aspect-ratio polymer nanostructures by tailored capillarity and adhesive force. Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314, 2008: 359–364.
[26] Qu L, Dai L, Wang Z L, et al. Carbon Nanotube arrays with strong shear binding-on and easy normal lifting-off. Science, 2008, 322:238-242.
[27] Jeong H E, Lee J K, Kim H N, et al. A Nontransferring Dry Adhesive with Hierarchical Polymer Nanohairs. The Proceedings of the National Academy of Sciences of the United States of America, 2009, 106(14): 5639-5644
[28] Suh K Y, Kim Y S, Lee H H. Capillary force lithography. Advanced Materials, 2001, 13(18): 1386-1389
[29] Xia Y, Whitesides G M. Soft lithography. Angewandte Chemie International Edition, 1998, 37(5): 550-575
[30] Odom T W, Love J C, Wolfe D B, et al. Improved pattern transfer in soft lithography using composite stamps. Langmuir, 2002, 18(13): 5314-5320
[31] Choi S J, Yoo P J, Baek S J, et al. An ultraviolet-curable mold for sub-100-nm lithography. Journal of the American Chemical Society, 2004, 126(25): 7744-1745
[32] Suh D, Choi S J, Lee H H. Rigiflex lithography for nanostructure transfer. Advanced Materials, 2005, 17(12): 1554-1560
[33]戴振东,孙久荣.壁虎的运动及仿生研究进展.自然科学进展, 2006, 16(5): 519-523
[34]郭策,戴振东,吉爱红等.壁虎脚趾运动调控的研究.中国生物医学工程学报, 2006, 25(1): 110-113
[35] Dai Z D, Tong J, Ren L Q. Researches and developments of biomimetics in tribology. Chinese Science国家自然科学基金申请书第13页版本1.004.756 Bulletin, 2006, 51(22): 2681-2689
[36] Dai Z D, Yu M, Gorb S. Adhesion characteristics of polyurethane for bionic hairy foot. Journal of Intelligent Material Systems and Structures, 2006, 17(8-9): 737-741
[37] Yu M, Ji A H, Dai Z D. Effect of microscale contact state of polyurethane surface on adhesion and friction. Journal of Intelligent Material Systems and Structures, 2006, 17(8-9): 819-822
[38]王辉静,梅涛.仿壁虎粘附阵列与粗糙表面间粘附仿真分析.中国科学技术大学学报, 2006, 36(8): 845-850
[39]王辉静,梅涛,汪小华.一种新型仿壁虎爬行机器人的粘附阵列设计.机器人, 2006, 28(2): 191-194
[40] Wang H, Mei T, Wang X, et al. Interaction and simulation analysis between the biomimetic gecko adhesion array and rough surface. Proceedings of the 2005 IEEE International Conference on Mechatronics and Automation, 2005, 1063-1068
[41] Wang H, Chen S, Wang X, et al. Design of a biomimetic gecko adhesion array for microrobots. Proceedings of the 2005 IEEE International Conference on Robotics and Biomimetics, 2005, 495-498
[42] Ren N, Wang X, Mei T, et al. Detachment control analysis for nanofabricated adhesive mimicking gecko foot-hairs. Proceedings of the 2006 IEEE International Conference on Mechatronics and Automation, 2006, 267-271
[43]金邦坤,何平笙.微接触印刷法制造聚合物多层次准三维立体微结构. 2005, 18(3): 439-442
[44]吴福贵,杨海洋,王政等.毛细微模塑和转移微模塑制备PVP/Ru(dpphen)3Cl2微图形.高分子材料科学与工程, 2006, 22(6): 188-191
[45]刘建平,石锦霞,杨小敏等.薄层软刻蚀法制备PMMA微图案结构.高分子学报, 2007, (1): 42-45
[46] Campo A del and Greiner. SU-8: a photoresist for high-aspect-ratio and 3D submicronlithography. Journal of Micromechanics and Microengineering, 2007, 17: R81-R95;
[47] Aksak B, Murphy M P, and Sitti M. Adhesion of biologically inspired vertical and angled polymer microfiber arrays. Langmuir, vol. 23, pp. 3322-3332, 2007.
[48] Chen W, Fadeev A Y, M. C. Hsieh, D. ?ner, J. Youngblood, and T. J. McCarthy. Ultrahydrophobic and ultralyophobic surfaces: some comments and examples. Langmuir, vol. 15, pp. 3395-3399, 1999.
[49] Kim T W and Bhushan B. Adhesion analysis of multi-level hierarchical attachment system contacting with a rough surface. Journal Of Adhesion Science and Technology, 2007, 21(1): 1-20