Directional Adhesion Mechanism Of Biomimetic Adhesive Structures

The hierarchical micro/nano adhesive system of geckos provides a biomimetic model for the researches and developments of dry adhesives with biomimetic structures. This kind of biomimetic adhesives has wide applications in the vast fields of adhesion based climbing robots, micromanipulations, micro/nano transfer printing techniques and biomedical adhesives, etc. Researches on adhesion mechanics, micro/nano fabrication methods and test techniques of biomimetic adhesive structures will form the basis for the developments of high-performance biomimetic adhesive materials and have become a hot research field at home and abroad. However, very few jobs on analysis and optimization have been done with respect to practically applicable biomimetic fibrillar adhesive structures with directional adhesion property.This dissertation focuses on clarifying the directional adhesion mechanism of typical biomimetic adhesive structures. To be specific, the adhesion problems of several typical fibrillar adhesive structures on a rigid flat surface or on a rigid curved surface, the design and optimization of both single-level and two-level adhesive fiber arrays, and the femtosecond laser two-photon polymerization based fabrication and the atomic force microscopy (AFM) platform based test of fibrillar adhesive structures are systematically investigated in this dissertation.Firstly, in order to study the directional adhesion behavior of an inextensible straight fiber on a rigid flat surface, an inextensible side contact model (ISCM) is proposed via a variational method based on the inextensible Euler elastica theory and the surface energy concept. The ISCM can be used to directly predict the detachment mode and the adhesion law of the fiber. It is found that increasing the applied shear force, the slanted angle or the bending compliance will effectively enhance the normal pull-off force. Moreover, all the adhesion equilibrium states are found stable by using the stability criterion of energy minimization.Secondly, by taking the axial deformation energy into account in the energy functional of the system, the ISEM model is generalized to the extensible case, thus forming an extensible side contact model (ESCM) that is applicable to extensible straight fiber. Moreover, the ESCM can be slightly modified so as to be applicable to study the pretension effect on the adhesion behavior. It is found that, due to the extensibility of the fiber, there exists a maximum normal pull-off force (MNPF) when an optimal shear force is applied. The MNPF increases with the fiber’s axial stiffness when its bending stiffness is constant. Studies on the pretension effect demonstrate that when an optimal pretension is applied, the MNPF is maximized.Thirdly, the ESCM is generalized to the case of two-level fibers. The generalized model predicts that for a typical two-level fiber, there exists a structural parameters-dependent critical shear force, below and above which the bending deformation dominates or the axial deformation dominates. In the bending deformation dominated region, the adhesion force increases almost linearly with the applied shear force and the adhesion law can be tuned effectively by varying the structural parameters; while in the axial deformation dominated region, the adhesion law is almost the same with that predicted by the classical Kendall model. Additionally, an optimum structure behaving consistently with geckos’setal arrays is obtained by designing the structural parameters properly, especially the slanted angles.The side contact model is also generalized to the case of a straight fiber in side contact with a curved surface, especially a sinusoidal surface. Detailed studies show that, for fibers adhered on a sinusoidal surface, there exists a benefit position so that the normal pull-off force is maximized. More importantly, if a proper shear force is applied, the normal pull-off force increases with the shear force; however, if the shear force exceeds a critical value, the fiber will slide along the surface, which would benefit for matching the position between the fiber and the curved surface, thus promoting the fiber’s adhesion force to a certain degree. Based on these studies and further analysis, the size effect of roughness found in the geckos’ adhesion on surfaces with different roughness is qualitatively explained.Based on the derived adhesion law of a single straight fiber, the design and optimization of both single-level and two-level adhesive fiber arrays are systematically studied. For each kind of fiber array, the structural parameters are defined, the condition that adjacent fibers do not adhere with each other (anti-bunching condition) and the rough surface adaptable condition are analyzed, and the adhesion strength is formulated. On these bases, the optimization problem is analyzed and discussed. The derived knowledge and analysis method should contribute to rational design of biomimetic adhesive fiber arrays. On the aspect of experiment, the femtosecond laser two-photon polymerization technique is adopted to fabricate a single fibrillar adhesive structure. The adhesive fiber array is also fabricated parallelly with the aid of computed hologram loaded spatial light modulator (SLM). In order to test the directional adhesion property of the fabricated adhesive structure, a micro cantilever with a micro pillar at its free end is designed and fabricated. It can be used as a sensing element and is integratable to the AFM measurement platform so as to measure the two axial forces, the normal adhesion force and the shear force of the adhesive structure.It’s concluded from the above researches that the following contributions have been made to the fields of biomimetic adhesion:1) Two models (ISCM and ESCM) are proposed to predict the directional adhesion laws for inextensible and extensible straight fibers adhered on a rigid flat surface respectively;2) The ESCM model is generalized to the case of two-level fibers adhered on a rigid flat surface and the case of single-level fibers adhered on a rigid curved surface;3) A method for characterizing the directional adhesion property of a single biomimetic adhesive structure is proposed by using a specially designed micro-cantilever as the sensing element and integrating it into the two-axial force measurement platform of AFM.