【作者】 张鹏；

【导师】 刘世元；

【作者基本信息】 华中科技大学 ， 机械电子工程， 2013， 博士

【摘要】 以微机电系统(MEMS, Micro Electro Mechanical System)器件可拆卸安装和固定为例的多种潜在应用(如爬壁机器人、微操作、微装配等),迫切需求一种可牢固粘附又能反复使用的粘合方式。目前广泛使用的湿性粘合剂多存在易污染、易脱落、不可重复使用等缺点,因此研制一种克服上述缺点的干性粘合剂将具有十分重要的意义和广阔的应用前景。自然界中壁虎具有非常优秀的爬行能力,无论是水平的天花板还是竖直的墙壁,无论是粗糙的树皮还是光滑的玻璃,壁虎都能快速爬行或纹丝不动,即便是潮湿的环境也能行动自如,这种粘附特性为干性粘合剂的仿生设计与制作提供了极好的范例和启示。壁虎脚毛的分级层次结构,特别是其微米级刚毛/纳米级绒毛的高深宽比微纳层次结构,是保证壁虎既能产生巨大粘附力又能适应不同表面形貌的关键。为此,本学位论文开展高深宽比微纳层次结构的仿壁虎毛优化设计与制作工艺的研究,从壁虎毛粘附机理出发,提出一种基于多约束条件的自底向上的优化设计方法,以及一种基于感应耦合等离子体(ICP, Inductively Coupled Plasma)刻蚀与厚胶光刻工艺的简单低成本的双层阵列制作方法。本文首先对壁虎毛粘附机理进行了研究。通过JKR模型和最小势能原理分析了单根绒毛在顶接触状态与边接触状态下的受力情况,研究了单根绒毛的粘附力与脱附角,解释壁虎如何产生巨大粘附力又能快速脱附；通过分别建立粗糙表面模型和多级阵列模型,模拟了壁虎毛在粗糙表面的粘附与脱附过程,研究了结构粘附力与接触面粗糙度、阵列层级的关系,解释壁虎为何能适应不同形貌的粗糙表面；通过Cassie模型分析了壁虎绒毛的疏水性,通过JKR模型分析了灰尘与绒毛阵列的吸附力,解释壁虎如何同时在潮湿和干燥的环境下保持自清洁特性。其次对仿壁虎毛结构进行了优化设计。对仿壁虎毛结构的层数进行了分析与选择,从增强粘附、保持稳定与适应环境等角度出发,提出单根纤维的最优粘附约束、单根纤维的稳定性约束、纤维之间的防纠结约束、纤维阵列的表面适应性约束、纤维阵列的疏水性约束、纤维阵列的自清洁约束、多级结构的最大粘附能约束,多数能与实际壁虎结构参数相符合,并在此基础上提出一种基于多约束条件的自底向上的优化设计方法,对仿壁虎毛结构进行了优化设计与分析。接下来对仿壁虎毛结构制作工艺进行了探索。研究了基于SU-8胶的厚胶光刻工艺,制作出了最大长径比为6的单层SU-8纤维阵列；研究了基于Bosch技术的ICP刻蚀工艺,制作出了最大长径比为5的单层聚二甲基硅氧烷(PDMS, Polydimethylsiloxane)纤维阵列；在前两个工艺基础上,提出一种结合ICP刻蚀与厚胶光刻的Si/SU-8复合模具模塑成型工艺,制作出了顶层长径比为5、底层长径比为2的双层PDMS纤维阵列。最后对仿壁虎毛结构进行了性能测试。对无结构、单层结构、双层结构的多种样品进行了实验检测,包括疏水性能、法向粘附性能和切向粘附性能,并结合前文理论解释实验现象。单层阵列的平均水接触角为143.6°,双层阵列的平均水接触角为150.4°,有结构样品中测得最大法向粘附强度为1.52N/cm2(单层直径5μm间距5μm长度20μm),最大切向粘附强度为0.79N/cm2(单层直径3μm间距3μm长度15μm),通过对比发现双层阵列相较于单层阵列和平面结构具有更好的疏水性和粗糙表面适应性。更多还原

【Abstract】 It is very urgent to develop a kind of powerful and repeatable adhesive, due to its wide applications in deployment and disassembly of micro electro mechanical system (MEMS) devices such as wall-climbing robots, micro-manipulation, micro-assembly, etc. The commonly used wet adhesives are easy to be contaminated and fall off, and can not be reused, which have unfavorable effect on adhesion. As a result, the developing of a kind of dry adhesive without the above shortcomings would have important significance and wide application prospect. In nature, geckos have developed complex foot-hair structures capable of smart adhesion. They can crawl quickly or keep sill on horizontal cell, vertical wall, rough bark and smooth glass, even in a wet environment. The powerful adhesive characteristics of geckos inspire human for design and fabrication of a new kind of biomimetic dry adhesive. The multiscale 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 gecko’s ability to adopt and stick to any different surfaces with powerful adhesion force. Accordingly, the design and fabrication method mimicking gecko foot-hairs will be investigated in this paper. Starting from the adhesion mechanism of gecko foot-hairs, a bottom-up optimal design method based on various restraining conditions is proposed. Subsequently, a simple and low-cost way to make dual-level microfiber arrays based on inductively coupled plasma (ICP) etching and thick film photolithography is developed. The main contens of this paper are shown in the following.Firstly, the adhesion mechanism of gecko foot-hairs has been studied. We analyze the force of a single spatula in top contact and side contact through JKR model and minimum potential energy principle. The adhesion force and the detachment angle are calculated, which can explain why geckos both have stable attachment and easy detachment. By establishing the models of rough surfaces and hierarchical structures, we simulate the processes of attachment and detachment of gecko foot-hairs. The relationships among adhesion force, surface roughness and layer of structures are discussed, which can explain why geckos have excellent adaptability on different surfaces. We analyze the hydrophobicity of spatulae through Cassie model, and compare the adhesion between dirt particle and spatulae through JKR model, which can explain how geckos manage to keep their feet clean while walking both in watery and dry environment.Secondly, an optimal design mimicking gecko foot-hairs has been finished. We discuss and select the layer of hierarchical structure. In order to achieve goals of robust adhesion, well stability and excellent adaptability, we propose constraints such as the optimal adhesion and stability of a single fiber, the anti-bunching between adjacent fibers, the adaptability on rough surface, the hydrophobicity and self-cleaning of fiber arrays, and the maximal adhesion work of hierarchical structures. A bottom-up optimal design method for geometrical parameters is proposed based on these constraints, and a procedure is also developed here to find out optimum parameters mimicking gecko foot-hairs with a discussion of the results.Thirdly, investigations have been made on the fabrication methods of structures mimicking gecko foot-hairs. The thick film photolithography based on SU-8photoresist is studied, and single-level fiber arrays of SU-8with an aspect ratio of6are fabricated. The ICP etching process based on Bosch technology is also studied, and single-level fiber arrays of Polydimethylsiloxane (PDMS) with an aspect ratio of5are fabricated. Then we propose a process of micromodeling from a composite mold of SU-8photoresist and silicon by using ICP etching and thick film photolithography, and fabricate the dual-level fiber arrays of PDMS with a top aspect-ratio of5and a bottom aspect-ratio of2.Finally, the properties of structures mimicking gecko foot hairs have been tested. We compare the hydrophobic properties, the normal and shear stresses among unpatterned, single-level and dual-level structures, and explaine the experimental results with previous theories. The average contact angle of single-level arrays is143.6°, while the dual-level arrays’ is150.4°. The maximum normal stress of patterned structures is1.52N/cm2(single-level array with5μm diameter,5μm spacing and20μm height), while the maximum shear stress is0.79N/cm2(single-level array with3μm diameter,3μm spacing and15μm height). It is found that dual-level structures have better hydrophobicity and adaptability on rough surface than single-level and unpatterned ones.更多还原