时间：2007年4月12日10:00am-11:am
地点：玉泉校区邵科馆211室
题目：Mechanics of robust and reversible adhesion: bottom–up designed hierarchical adhesion structures in biology
报告人：美国布朗大学高华健教授
高华健教授简介：
美国布朗大学Walter H. Annenberg教授，德国马普金属所所长, 力学、材料学专家。1982年于西安交通大学获学士学位，1984、1988年在哈佛大学分别获得工学硕士、博士学位。1988年受聘于斯坦福大学，任助理教授；1994年晋升为副教授并获终身职位；2000年晋升为教授；2001年加入马普学会，任马普金属所教授、所长；2006年受聘担任布朗大学Walter H. Annenberg教授。他在薄膜及其它材料体系的微观力学、计算模拟等领域做出了重要贡献。曾获得多项学术奖励；发表论文200余篇；是三种国际学术期刊的编委，Journal of the Mechanics and Physics of Solids主编。是材料微观力学、生物材料、材料计算等方面国际著名学者。目前的研究重点是生物系统中的纳米力学。
报告内容简介：
Gecko and many insects have evolved specialized adhesive tissues with bottom–up designed (from nanoscale and up) hierarchical structures that allow them to maneuver on vertical walls and ceilings. The adhesion mechanisms of gecko must be robust enough to function on unknown rough surfaces and also easily releasable upon animal movement. How does nature design such macroscopic sized robust and releasable adhesion devices? How can an adhesion system designed for robust attachment simultaneously allow easy detachment? These questions have motivated the present investigation on mechanics of robust and releasable adhesion in biology. On the question of robust adhesion, we introduced a hierarchical gecko hairs model, which assumes self-similar fibrillar structures at multiple hierarchical levels mimicking gecko’s spatula ultrastructure, to show that structural hierarchy plays a key role in robust adhesion: it allows the work of adhesion to be exponentially enhanced with each added level of hierarchy. We demonstrate that, barring fiber fracture, the hierarchical gecko hairs can be designed from nanoscale and up to achieve flaw tolerant adhesion at any length scales. However, consideration of crack-like flaws in the hairs themselves results in an upper size limit for flaw tolerant design. On the question of releasable adhesion, we hypothesize that the asymmetrically aligned seta hairs of gecko form a strongly anisotropic material with adhesion strength strongly varying with the direction of pulling. We use analytical solutions to show that a strongly anisotropic elastic solid indeed exhibits a strongly anisotropic adhesion strength when sticking on a rough surface. Furthermore, we perform finite element calculations to show that the adhesion strength of a strongly anisotropic attachment pad exhibits essentially two levels of adhesion strength depending on the direction of pulling, resulting in an orientation-controlled switch between attachment and detachment. These findings not only provide a theoretical foundation to understand adhesion mechanisms in biology but also suggest possible strategies to develop novel adhesive materials for engineering applications.
Selected References:
[1] H. Gao, X. Wang, H. Yao, S. Gorb and E. Arzt, "Mechanics of hierarchical adhesion structure of gecko," 2005, Mechanics of Materials, Vol. 37, pp. 275-285.
[2] H. Gao and H. Yao, "Shape insensitive optimal adhesion of nanoscale fibrillar structures," 2004, Proceedings of the National Academy of Sciences of USA, Vol. 101, pp. 7851-7856.
[3] H. Yao and H. Gao, “Mechanics of robust and releasable adhesion in biology: Bottom–up designed hierarchical structures of gecko Journal of the Mechanics and Physics of Solids,” 2006, Vol. 54, pp. 1120–1146
