自然界中的蓮花葉面具有超疏水之特性，其接觸角約150°、滑動角約5°。其表面具能防水、自潔和流體滑動於界面之能力，可應用於玻璃清潔、太陽能電池、浴室及交通工具之清潔。基於仿生學建立超疏水表面之結構，在近幾年引發極廣泛的基礎應用研究。然而該結構之結構強度低、大量複製困難，並易受外力受損失去自潔性的缺點。
本研究在於設計與分析創新微米級微結構，其與自然仿生者大不相同。相對於凸柱狀之蓮花結構，提出一種多層次半封閉微結構，其封閉處可困住空氣，產生超疏水性之機制包括回復力、基材彈性與封閉性之空氣彈簧。實驗所用之材料為聚二甲基矽氧烷(polydimethylsiloxane, PDMS)。此本半封閉性之反蓮花結構，其接觸角可達168°、滑動角約於5°，水滴可於微結構表面反彈。實驗主要探討此結構之靜動態接觸角與液滴撞擊超疏水性結構的動態行為。此超疏水表面不但有可大量生產之可行性，且具有極佳的自潔效果，此研究設計結果有應用於具沙塵遮蔽問題之沙漠地區太陽能板的潛力，可利用其效果達到省水且自潔的功能。

In nature, Leaves of Lotus are superhydrophobic surfaces. The contact angle is about 150° and sliding angle is below 5°. The surfaces have the abilities of water proofing, self-cleaning and slipping at the fluid-surface interface. Dominant applications can be found in the self-cleaning of glasses and surfaces of solar cell, vehicles and sanitary systems. Based on the biomimicry, the study on creating super hydrophobic structures has triggered intense basic and applied research over the past several years. However, this kind of structures have some disadvantages, such as low structure strength and hard to replicate. It is easy to loss the ability of self-cleaning by the damage result from the external force.
In this study, we show an innovative novel super hydrophobic micro-structure. Opposite to the pillar lotus structure, the structure is multi-leveled concaved with trapped air. The mechanisms for waterproof include the restoring force, the elastic deformation of the structure and the air spring force resulting from the concaved structure with trapped air. The material of micro-structures is PDMS (polydimethylsiloxane). This study demonstrates that semi-mural micro-structure of negative lotus structure has the following characteristics which the contact angle is 168°, sliding angle is about 5°. Droplets will rebound on the micro-structure of the surface. The experiment is composed of two parts: static and dynamic contact angle and the dynamic behavior of droplets impacting. Superhydrophobic surface has the possibility of mass-fabricating and great self-cleaning effect. The result of this research will have great potential of application in the dust-cleaning problem of solar cell.in the desert to achieve the water-saving and self-cleaning function.

Key words: Lotus effect, Contact angle, PDMS, Superhydrophobicity, Air spring.

[1] Aikifa R., Yang S., Xianfeng W., Tao R., Bin D., Jianyong Y., and Salem S. A., Novel fluorinated polybenzoxazine–silica films: chemical synthesis and superhydrophobicity, RSC Advances. 2,12804-12811 (2012).
[2] Bartolo D., Josserand C. and Bonn D. Retraction dynamics of aqueous drops upon impact on non-wetting surfaces, J. Fluid Mech., 545, 329-338 (2005).
[3] Bhushan B., Jung Y. C., and Koch K., Micro-, Nano- and Hierarchical Structures for Superhydrophobicity, Self-Cleaning and Low Adhesion, Phil. Trans. R. Soc. 367(1894),1631-1672 (2009).
[4] Bico J.,Thiele U. and Quéré D., Wetting of textured surfaces. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 206(9), 41–46 (2002).
[5] Bar-Cohen Y., Biomimetics: biologically inspired technologies, Boca Raton, Taylor and Francis. ,(2006).
[6] Cassie A.B.D. and Baxter S., Wettability of Porous Surfaces. Trans. Faraday Soc. 40, 546-551 (1944).
[7] Cossal G. E., Coghe A. and Marengo M., The impact of a single drop on a wetted solid surface, Experiments in Fluids. 22, 463-472 (1997).
[8] Daewoo Han and Andrew J. Steckl, Superhydrophobic and Oleophobic Fibers by Coaxial Electrospinning, Langmuir. 25, 9454-9462 (2009).
[9] Furmidge C. G., Studies at phase interfaces: Sliding of liquid drops on solid surfaces and a theory for spray retention, Journal of Colloid Science. 17(4), 309 (1962).
[10] Gao H., Wang X., Yao H., Gorb S. and Arzt E., Mechanics of hierarchical adhesion structures of geckos, Mechanics of Materials. 37, 275–285 (2005).
[11] Gao L. and McCarthy T. J., Contact Angle Hysteresis Explained, Langmuir. 22, 6234-6237 (2006).
[12] Gao X. F. and Jiang L., Biophysics: Water-repellent Legs of Water Striders, Nature. 432, 36 (2004).
[13] de Gennes P. G., Brochard-Wyart F. and Quéré D., Capillarity and Wetting Phenomena, Drops, Bubbles, Pearls, Waves, Springer. New York. (2004).
[14] Genzer J. and Efimenko K., Recent developments in superhydrophobic surfaces and their relevance to marine fouling: a review, Biofouling. 22, 339-360 (2006).
[15] Jung Y. C. and Bhushan B. Dynamic Effects of Bouncing Water Droplets on Superhydrophobic Surfaces, Nanotechnology. 17, 4970–4980 (2006).
[16] Koch K., Bhushan B., and Barthlott W., Multifunctional Surface Structures of Plants: An Inspiration for Biomimetics, Progress in Materials Science. 54, 137-178 (2009).
[17] Liangliang Cao, Andrew K. Jones, Vinod K. Sikka, Jianzhong Wu and Di Gao, Anti-Icing Superhydrophobic Coatings, Langmuir. 25(21), 12444-12448 (2009).
[18] LIU Jian Lin, XIA R., and ZHOU X. H., A new look on wetting models: continuum analysis, SCIENCE CHINA Physics,Mechanics & Astronomy. 55, 2158-2166 (2012).
[19] Mao T., Kuhn D.C.S. and Tran H., Spread and Rebound of Liquid Droplets upon Impact on Flat Surfaces, AIChE J. 43(9), 2169-2179 (1997).
[20] Mirjami Kiuru and Esa Alakoski, Low sliding angles in hydrophobic and oleophobic coatings prepared with plasma discharge method, Materials Letters. 58, 2213-2216 (2004).
[21] Mundo Chr., Sommerfeld M. and Tropea C., Droplet-wall collisions: Experimental studies of the deformation and breakup process, Int. J. Multiphase Flow. 21, 151-173 (1995).
[22] Neinhuis C. and Barthlott W., Characterization and Distribution of Water-repellent, Self-cleaning Plant Surfaces, Annals of Botany. 79, 667-677 (1997).
[23] Pasandideh-Fard M., Qiao Y. M., Chandra S., and Mostaghimi J., Capillary effects during droplet impact on a solid surface. Phys. Fluids. 5, 650-659 (1996).
[24] Quéré D., Non-sticking dropsm, INSTITUTE OF PHYSICS PUBLISHING. 68, 2495–2532 (2005).
[25] Quéré D., Bouncing transitions on microtextured materials, Europhys. Lett. 74, 306 (2006).
[26] Richard D., Clanet C. and Quéré D. Contact time of a bouncing drop, Nature. 417, 811 (2002).
[27] Richard D. and Quéré D. Bouncing water drops, Europhys. Lett. 50, 769 (2000).
[28] Rioboo R., Marengo M. and Tropea C., Outcomes from a drop impact on solid surfaces, Atomization and Sprays. 11, 155-165 (2001).
[29] Rioboo R., Marengo M. and Tropea C., Time evolution of liquid drop impact onto solid, dry surfaces, Experiments in Fluids. 33, 112-124 (2002).
[30] Rothstein J. P. Slip on Superhydrophoboic Surface. Annual Review of Fluid Mechanics. 42, 89–109 (2010).
[31] Shigeaki Inada and Wen-Jei Yang, Solidification of molten metal droplets impinging on a cold surface, Experimental Heat Transfer. 7, 93-100 (1994).
[32] Šikalo Š., Tropea C. and Ganić E.N. Dynamic wetting angle of a spreading droplet, Experimental Thermal and Fluid Science. 29(7), 795-802 (2005).
[33] Šikalo Š., Marengo M., Tropea C. and Ganić E.N. Analysis of impact of droplets on horizontal surface, Experimental Thermal and Fluid Science. 25, 503-510 (2002).
[34] Tsai P., Pacheco S., Pirat C., Lefferts L. and Lohse D. Drop Impact upon Micro- and Nanostructured Superhydrophobic Surfaces, Langmuir. 25(20), 12293-12298 (2009).
[35] W. E., Squamation and ecology of sharks, Courier Forschungsinstitut Senckenberg. 78, 1–255 (1985).
[36] Wenzel, R. N. Resistance of solid surface to wetting by water. Ind. Eng.Chem. 28, 988 (1936).
[37] Xiying Li, Xuehu Ma and Zhong Lan, Dynamic Behavior of the Water Droplet Impact on a Textured Hydrophobic/Superhydrophobic Surface: The Effect of the Remaining Liquid Film Arising on the Pillars’ Tops on the Contact Time, Langmuir. 26(7), 4831–4838 (2010).
[38] Yong-Bum Park, Hwon Im, Maesoon Im and Yang-Kyu Choi, Self-cleaning effect of highly water-repellent microshell structures for solar cell applications, J. Mater. Chem. 21, 633-636 (2011).
[39] Young, T. An Essay on the Cohesion of Fluids. Phil. Trans. R. Soc. Lond. 95, 65–87 (1805).
[40] Zheng, Q. S., Yu, Y., and Zhao Z. H. Effects of Hydraulic Pressure on the Stability and Transition of Wetting Modes of Superhydrophobic Surfaces, Langmuir., 21(26), 12207–12212 (2005).
[41] 廖士貴, 國立成功大學機械工程學系碩士論文 (2010).
[42] 陳彥君, 國立成功大學機械工程學系碩士論文 (2011).
[43] 石嘉嘉, 國立成功大學機械工程學系碩士論文 (2012).
[44] 翁敏航, 太陽能電池-原理、元件、材料、製程與檢測技術，東華書局 (2010).