Studies On Adhesion And Relevant Mechanical Behaviors Of Biological And Bio-inspired Composite Structures

Geckos,beetles and tree frogs show superior adhesion and detachment properties in their natural habitats.The adhesive footpads of these creatures also show multifunctional features,such as environmental adaptability,reusability,and self cleaning.Currently,a big challenge for the artificial bio-inspired adhesives is how to realize strong adhesion and easy detachment on target substrates.Moreover,the artificial bio-inspired adhesives still lack multifunctional properties,such as the environmental adaptability,stability,durability and the tolerance to contamination.In this study,in order to deeply understand the adhesion enhancement and multifunctional properties of biological adhesives,the adhesion and relevant mechanical behaviors of three representative biological and bio-inspired composite structures were studied.The following are the main research contents and results.The first part of this thesis is about the wet adhesion of composite microstructured surfaces inspired by newt footpads.In this part,the attachment and climbing abilities of newts were firstly examined and evaluated under three different wetting conditions.Then the micro and nanostructures of newts’ footpads were characterized.Surfaces with composite microstructures inspired by newt footpads were designed and fabricated.The wet static friction and adhesion properties of microstructured surfaces were investigated.Effects of the pillar area ratio,micropattern on static friction and effects of the preload,retraction speed,amount of liquid,approach-retraction cycle on adhesion were investigated experimentally.It was found that the composite micropatterns with a larger pillar area ratio and a smaller pillar diameter show the potential to fast increase the static friction,and that there exists an optimum amount of liquid(about 0.1(μL)that can enhance the adhesion.The results indicate that the dense nanopillar arrays on newts’ footpads may function to keep footpads wet to retain significant adhesion and friction.The second part of the thesis is about the finite element modeling of the viscoelastic contact for a composite micropillar.In this part,the viscoelastic contact behavior between a spherical asperity and a soft polymer was firstly studied by the analytical and finite element methods.The numerical results for both methods were compared to verify the reliability of the finite element one in modeling the viscoelastic contact.Then,the finite element method was used to systematically investigate the viscoelastic contact between a spherical asperity and a composite micropillar that consists of a stiff base stalk and a soft top layer.Effects of the thickness of the soft top layer,stiffness of the base stalk,radius of the spherical asperity,and plateau load on contact properties were evaluated.The tensile stress distribution along the contact interface between a flat rigid substrate surface and the composite micropillar was also investigated.The results show that the viscoelastic properties of the soft top layer of the composite micropillar can facilitate the adaptation to rough surfaces.It was found that there exists a critical thickness for the soft top layer of the composite micropillar.The existence of the critical thickness likely correlates with the stiffness of the composite micropillar and the hardening behavior of the soft top layer.It is suggested that the thickness of the soft top layer should not be less than the critical value.The third part of the thesis is about the molecular dynamic simulation of the nanocomposite structures of Gecko adhesive setae.In this part,the 3D molecular structural models of the conserved 34-residue β-rich central domain and the whole monomer containing 117 amino acid residues in the representative corneous βprotein(Ge-cprp-9)of Gecko adhesive setae were firstly built.Then the molecular dynamic behaviors for molecular structures of the central β-sheet domain and the whole monomer were investigated.Effects of the predefined temperature and initial rate on molecular dynamic behaviors were evaluated.The stable molecular structures of the central β-sheet domain and the whole monomer were also forecasted.The results show that under different predefined temperature and initial rate conditions,the total energy tends to be stable and minimum at the end of the timestep,which indicates that the molecular structures of the central β-sheet domain and the whole monomer of Gecko corneous β protein tend to be stable.It was found that both the central β-sheet domain and the whole monomer of Gecko corneous β protein only contain 3 β strands,which are more stable during the whole simulation timestep scale.In addition,mutual effects were observed between the central β-sheet domain and the random coils of N and C terminals of the whole monomer.The above research results can help to better understand the adhesion enhancement and environmental adaptability of biological adhesives,which may provide insights and guidelines for the design and development of new generation bio-inspired adhesives.