On Shear Adhesion of Gecko-inspired Adhesive Fibrils with Elastomers
and Normal Adhesion of Snail-inspired Adhesives with Shape Memory Polymers
Dr. Changhong LINGHU
School of Mechanical & Aerospace Engineering
Nanyang Technological University
Date & Time
Wednesday, 29 November 2023
Room 7-34 and 35, Haking Wong Building
Natural fibrillar adhesives, widely recognized for their strong and reversible adhesion, have inspired the development of numerous synthetic adhesive systems applied in diverse fields. These biological and bioinspired adhesive fibrils exhibit a wide range of aspect ratios spanning four orders of magnitude. Accurate prediction of their adhesion performance is crucial for their practical applications. Prior investigations have primarily focused on adhesion under normal loading, although shear loading is also commonly encountered in various scenarios. To date, only a few shear adhesion models exist, and they are limited to either high or low aspect ratios. Rigorous mechanics analysis of adhesive fibrils’ shear adhesion strength across a wide range of aspect ratios is lacking. Here, we developed a mechanics model based on the compliance method using the Timoshenko beam theory for evaluating the energy-controlled interfacial failure of the adhesive fibrils across a wide range of fibrillar aspect ratios. Our model captures the shear adhesion strength across a wide aspect ratio range from 10-4 to 102accurately and is in excellent agreement with the FEA results.
However, these smart adhesives made of conventional elastomers showed a relatively weak adhesion strength on the order of 100 kPa. Snails can lock on walls when its secreted mucus is dried, generating stronger adhesion on the order of MPa. Similar to the snail mucus, shape memory polymers (SMPs), with unique properties such as tunable elastic modulus, temporary shape-locking, and shape-recovery upon external stimulations, are emerging as a new class of smart materials with stronger adhesion strength and switchable adhesion capabilities. A prominent feature of the adhesion between SMP and a spherical indenter is the so-called R2G adhesion, defined as making contact in the rubbery state to a certain indentation depth followed by detachment in the glassy state. While it has been demonstrated that the R2G adhesion with SMPs can achieve orders of magnitude higher adhesive strength compared to conventional elastic adhesive systems, the fundamental mechanics of R2G adhesion and why it leads to such tremendous adhesion enhancement remain poorly understood. Here, combined experimental testing, theoretical analysis, and finite element analysis (FEA) based on a thermomechanical constitutive model of the SMP are carried out to investigate the mechanics of R2G adhesion with a rigid spherical indenter. The study shows that the orders of magnitude enhancement of R2G adhesion over conventional elastic adhesion systems is governed by the shape locking effect during the transition from the rubbery to glassy states. The experimental measurements and FEA analysis demonstrate that the net effect of shape locking leads to a pull-off force of a sphere nearly the same as that of a flat punch on an elastic half-space with the same contact radius. An explicit expression of the pull-off force for R2G adhesion is proposed based on flat-punch adhesion. Our results revealed the fundamental mechanics of R2G adhesion and provided guidance for the design of SMP smart adhesives.
令狐昌鸿，新加坡南洋理工大学博士后研究员。浙江大学工程力学学士（导师王高峰教授）、浙江大学固体力学硕士（导师宋吉舟教授）、新加坡南洋理工大学机械与宇航学院固体力学博士（导师夏焜教授和高华健教授）。主要从事智能界面粘附力学及其应用研究。研究工作主要基于界面力学/物理/化学，利用智能材料可调特性，揭示不同条件下界面粘附强度强弱控制的机理，探索界面粘附强度调控规律，并基于此研发自适应界面粘附系统，集成并应用于机器人、机械手、无人机、可穿戴柔性电子器件、微纳组装、3D打印、医疗、超材料等领域。拥有中国国家发明专利10项，实用新型专利8项；于Elsevier发表专著章节一章；在JMPS、EML、IJSS、Science Advances、Nature Communications、PNAS、National Science Review、npj Flexible Electronics、Soft Matter等期刊上发表SCI论文20多篇，H 因子14，引用830
Changhong LINGHU is currently a Postdoctoral Research Fellow working under the guidance of Prof. Huajian Gao and Prof. K Jimmy Hsia in the School of Mechanical and Aerospace Engineering at Nanyang Technological University, Singapore. He received his Ph.D. degree in Mechanical & Aerospace Engineering from Nanyang Technological University, Singapore in 2023, and his M.S. and B.E. in Solid Mechanics and Engineering Mechanics from Zhejiang University, China in 2020 and 2017, respectively.
He works in the interdisciplinary frontiers of mechanics, materials, and engineering. He is dedicated to exploring the Mechanics and Applications of Adaptable Interfacial Adhesion for Industry 5.0 across multiple domains. He has led and collaborated with multiple research teams of undergraduate and postgraduate students to conduct research in smart adhesives. His work has resulted in publications of 22 peer-reviewed papers (with 830 citations, H-index =14), including two ESI highly cited papers (Top 1‰) in journals such as Nature Communications, Science Advances, PNAS, npj Flexible Electronics, Soft Matter, Journal of the Mechanics and Physics of Solids, Extreme Mechanics Letters, International Journal of Solids and Structures, and one chapter in an Elsevier book. He has also served as a reviewer for SCI journals including Extreme Mechanics Letters and Chemical Engineering Journal.
He has 18 patents with over ten of them being successfully commercialized. Moreover, he has impact by implementing his research findings. His accomplishments have been recognized by approximately 50 innovation and entrepreneurship awards.
To date, he has mentored 36 undergraduates, with 30 of them pursuing postgraduate studies at prestigious universities such as Stanford University, Massachusetts Institute of Technology, Zhejiang University.