The Stress-driven Self-assembly And Instability Morphologies Of Low-dimensional Materials

The stress-driven self-assembly and instability morphologies of materials are very common in life.The study of these morphologies can not only uncover the formation mechanism of abundant morphologies in nature,but also have important application value in industry.On the one hand,it need to inhibit the aging failure caused by the instability morphology of devices during the service process under complex stress en-vironments.On the other hand,with the rapid development of manufacturing industry,especially the micro-nano manufacturing technology,various self-assembly and insta-bility morphologies can provide guidance for the bottom-up micro-nano manufacturing method.In this paper,we focus on the critical issues on self-assembly and instabil-ity morphologies of various low-dimensional materials.The stress-driven equilibrium morphologies and evolution processes of nanotube,monolayer colloidal crystal and thin film-substrate structures are studied.The physical mechanism of the hollow nanospheres coalescing into nanotube struc-tures during the solvent-thermal synthesis process is systematically revealed.Firstly,we obtain the products under different reaction time in solvent-thermal method by experi-ments,and find that the intermediate products are disordered hollow nanospheres.Then a phase diagram for the sphere-to-tube transition are achieved through linear stability analysis.It is found that the inverse Plateau-Rayleigh instability driven by the competi-tion between curvature elastic energy and surface energy is responsible for the nanotube formation observed in experiments.Finally,the nonlinear dynamic simulation based on the phase-field method is used to reproduce the evolution process of sphere-to-tube transition and the results of linear stability analysis are verified.The self-assembly morphologies of colloidal crystal on curved surface are ex-plored.Previous studies have shown that there exist two kinds of elastic instabilities,which are topological defect nucleation and boundary bifurcation,in curved crystal to alleviate the curvature induced elastic stress.In this work they are simultaneously con-sidered during crystal growth on a sphere.Phase diagrams with respect to substrate curvature,crystal size,lattice size,the stiffness and edge energy of the crystal for the equilibrium crystal morphologies are achieved by theoretical analysis and validated by Brownian dynamics simulations.The results show that the two kinds of elastic insta-bilities not only compete with each other,but also coexist in a range of combinations of factors.The formation mechanism and regulation method of ordered and localized wrin-kling pattern in the vicinity of cracks in metal films deposited on soft polymer substrates are clarified.Firstly,theoretical analysis shows that the localized wrinkling pattern is induced by the plastic deformation which is caused by the concentrated stress at the crack tip.The relationships between the width of localized wrinkling stripe versus film thickness and modulus ratio are obtained.Then numerical method which can consider the crack propagation,plastic deformation and film deformation are implemented to re-produce the evolution process of localized wrinkling.Finally,the relationships between the characteristic of wrinkling pattern and material properties are verified by numerical simulations and experimental results.The controlled wrinkling patterns in periodic thickness-gradient films on flexible substrates are investigated.Firstly,we propose a novel experimental strategy to pre-pare periodic thickness-gradient by masking the substrate with copper grids during film deposition.The results show that the thickness distribution as well as the surface wrin-kling morphology can be regulated by changing the mesh size and the deposition time.In combination with experimental observation and numerical simulation,the morpho-logical characteristics,evolution behavior and formation mechanism of different sur-face morphological patterns such as straight stripes,herringbone wrinkles and labyrinth wrinkles are discussed in detail with the change of film thickness.The symmetry breaking and high-order instability of the telephone cord buckles in thin film under gradient strain are discussed.The experimental results show that there exists asymmetric morphology in telephone cord blister near the film edge,and we believe that it is caused by the non-uniform stress near the film boundary.A phase diagram of telephone cord buckles with respect to strain gradient and average strain is obtained by numerical simulation.It is found that when the average strain is relatively small,the gradient strain triggers the symmetry breaking of the telephone cord buckles,while when the average strain is relatively large,the gradient strain would generate the asymmetric high-order instability morphologies of the telephone cord buckles.The morphology and formation mechanism of infrequent ring-shaped buckles in thin film are studied.During the film deposition,the silicone oil droplet on the substrate evaporates and the silicone oil residue remains on the substrate after the film deposition,which leaves a ring-shape interfacial weakening region and triggers the film to delami-nate and buckle in this region.The experimental and numerical simulated results show that the maximum height of the ring-shaped buckle is approximately proportional to the half width,which can be explained by the morphology of straight-side buckle.In ad-dition,the numerical simulation also finds that the maximum height of the ring-shaped buckle deflects into the outside of ring due to the curvature of buckle's boundary.