Research On Instability Mechanical Behavior And Actuating Mechanism Of Bilayer/Curved-Surface Structure Of Soft Materials

Micro-electronic devices,actuators and soft robots which based on soft materials are emerging areas for future engineering.Development of bionic actuators with multifunctional and autonomous deformation characteristics by adopting the deformation characteristics of soft materials has attracted more and more researchers.The principle of the autonomous deforming actuator is to respond to external stimuli by changing its shape.Therefore,the key to develop actuators with high performance is to explore actuation mechanisms that can drive autonomous shape transformation effectively and quickly.In recent years,surface instabilities such as wrinkling,folding,creasing,ridging and structural instabilities such as bending and torsion have become new actuation mechanisms which attracted much attentions.Investagation on these new actuation mechanisms will contribute to design actuators with specific actuate function and excellent bionic performance.This thesis aims to introduce bioinspired mechanics which based on elastic instability into the structure of soft materials to study the surface instabilities of bilayer structure and the structure instabilities of hydrogel structure with curved surface.Then the surface and structure instabilities are applied to the research of autonomous actuation mechanisms and the design of autonomous deforming actuator.The main researches in this thesis are as follows:(1)Bilayer structures with polymethyl methacrylate or polystyrene shape memory polymer as substrates and Zn O or Gold as films were prepared.Influences of the film thicknesses,prestrains,shape memory effect and thermal expansion mismatch on the evolutions of surface topographies of bilayer structure were investigated by experiments.The results revealed that the isotropic wrinkles without any preferential orientation were induced on the surface of bilayer by the thermal expansion mismatch between film and substrate,while the striped wrinkles perpendicular to the direction of prestrain were resulted from the prestrain and shape memory effect.Based on the thin plate buckling model for hard films,the unique mechanical behaviors of the bilayer structures were effectively characterized,and the evolution law of its instability behavior was revealed.(2)Micro-fabrication process and surface plasma treatment were introduced to fabricate patterned and unpatterned film/polydimethylsiloxane one-layer/bilayer structure.The surface instabilities of these structures under uniaxial compressing were studied by experiments and finite element simulations.According to the evolutions of the droplet shape on the surface of these structures under uniaxial compressing/stretching,the anisotropic wettability and the influence mechanism of lattice hole and wrinkling on the droplet shape were presented.It can be found that the deformation of the droplet from circular shape to ellipsoidal shape was programed by uniaxial compressing/stretching.The results elucidated that a significant increase in pinning effect of contact line were attributed to the coupling of surface instability and lattice hole.Therefore,the dependence of the anisotropy of surface wettability on the surface topography was verified.(3)The linear poroelastic theory and free swelling experiments were used to research the swelling kinetics of hydrogel sphere,hemisphere and hemispherical shell.The shape-morphing actuation mechanism based on mechanical instability was studied by regulating the structural(inner diameter and outer diameter)and material(the ratio of crosslinker and sodium acrylate)parameters.The results presented that creases were formed on the surface of hydrogel sphere during the free swelling.The crease spacing was linearly related to the square root of swelling time,while the crease depth first increased and then decreased to zero.The similar creasing formed on spherical surfaces of hydrogel hemisphere and hemispherical shell.Concomitantly to the subsequent developments of edge buckling and surface creasing,the transformation of the hydrogel structure "opening" to "closed" to "opening" were achieved.It demonstrated that shape transformations of hydrogel hemispherical shell could be controlled by tailoring the structure design and material parameters.And the hydrogel gripper/bionic-flower were designed based on this research.(4)The coupling behavior of surface instability and structure instability of semicylindrical-shell-shaped hydrogel actuator subjected to nonuniform swelling was studied by free swelling experiments.The investigation considered the effect of geometrical factors(thicknesses,lengths and angles),crosslinker contents and environmental solutions on the mechanical behavior of surface,edge and structure instabilities during free swelling.It demonstrated that the reticulated creases were formed on the outer cylindrical surface in the early stage of free swelling.The subsequent transition from the reticulated creases to the striated patterns was attributed to the coupling of surface instability and structural bending.The crease disappeared completely when the final homogeneous swelling was reached.Simultaneously,the semicylindrical shell structure undergone shape transformation from "opening" to "closed" to "opening".The reproducible actuation of a hydrogel gripper was realized based on the above deformation characteristics.