Spinodal surface instability of soft elastic thin films

When the thicknesses of thin films reduce to microns or even nanometers, surface energy and surface interaction often play a significant role in their deformation behavior and surface morphology. The spinodal surface instability induced by the van der Waals force in a soft elastic thin film perfectly bonded to a rigid substrate is investigated theoretically using the bifurcation theory of elastic structures. The analytical solution is derived for the critical condition of spinodal surface morphology instability by accounting for the competition of the van der Waals interaction energy, elastic strain energy and surface energy. Detailed examinations on the effect of surface energy, thickness and elastic properties of the film show that the characteristic wavelength of the deformation bifurcation mode depends on the film thickness via an exponential relation, with the power index in the range from 0.749 to 1.0. The theoretical solution has a good agreement with relevant experiment results.

Stress distribution and surface instability of an inclined granular layer

Static granular materials may avalanche suddenly under continuous quasi-static drives. This phenomenon, which is important in many engineering applications, can be explained by analyzing the stability of the elastic solutions. We show this for a granular layer driven by its inclination angle in gravity, where the elastic problem can be solved generally and analytically. It is found that a loss of stability may occur only at the free surface of the layer. This result is considered to be relevant for understanding surface avalanches and the flows observed experimentally.

The effect of G?rtler instability on hypersonic boundary layer transition

The evolution of Gortler vortices and its interaction with other instabilities are investigated in this paper. Both the Mack mode and the Gortler mode exist in hypersonic boundary-layer flows over concave surfaces, and their interactions are crucially important in boundary layer transition. We carry out a direct numerical simulation to explore the interaction between the GOrtler and the oblique Mack mode. The results indicate that the interaction between the forced Gortler mode and the oblique Mack mode promotes the onset of the transition. The forced oblique Mack mode is susceptible to nonlinear interaction. Because of the development of the GOrtler mode, the forced Mack mode and other harmonic modes are excited.

Bioinspired Multiscale Wrinkling Patterns on Curved Substrates:An Overview

The surface wrinkling of biological tissues is ubiquitous in nature.Accumulating evidence suggests that the mechanical force plays a significant role in shaping the biological morphologies.Controlled wrinkling has been demonstrated to be able to spontaneously form rich multiscale patterns,on either planar or curved surfaces.The surface wrinkling on planar substrates has been investigated thoroughly during the past decades.However,most wrinkling morphologies in nature are based on the curved biological surfaces and the research of controllable patterning on curved substrates still remains weak.The study of wrinkling on curved substrates is critical for understanding the biological growth,developing threedimensional(3D)or four-dimensional(4D)fabrication techniques,and creating novel topographic patterns.In this review,fundamental wrinkling mechanics and recent advances in both fabrications and applications of the wrinkling patterns on curved substrates are summarized.The mechanics behind the wrinkles is compared between the planar and the curved cases.Beyond the film thickness,modulus ratio,and mismatch strain,the substrate curvature is one more significant parameter controlling the surface wrinkling.Curved substrates can be both solid and hollow with various 3D geometries across multiple length scales.Up to date,the wrinkling morphologies on solid/hollow core-shell spheres and cylinders have been simulated and selectively produced.Emerging applications of the curved topographic patterns have been found in smart wetting surfaces,cell culture interfaces,healthcare materials,and actuators,which may accelerate the development of artificial organs,stimuli-responsive devices,and micro/nano fabrications with higher dimensions.

BIOMECHANICAL MODELING OF SURFACE WRINKLING OF SOFT TISSUES WITH GROWTH-DEPENDENT MECHANICAL PROPERTIES

This paper explores growth induced morphological instabilities in biological soft materials. In view of that the growth of a living tissue not only changes its geometry but also can alter its mechanical properties, we suggest a refined volumetric growth model incorporating the effects of growth on the mechanical properties of materials. Analogy between this volumetric growth model and the conventional thermal stress model is addressed for both small and finite de- formation problems, which brings great ease for the finite element analysis based on the suggested model. Examples of growth induced surface wrinkling behavior in soft composites, including core- shell soft cylinders and three-layered soft tissues, are explored. The results and discussions foresee possible applications of the model in understanding the correlation between the morphogenesis and growth of soft biological tissues （e.g. skins and tumors）, as well as in evaluating the defor- mation and surface instability behavior of soft artificial materials induced by swelling/shrinkage.

Prediction and Regulation of Delamination at Flexible Film/Finite-Thickness-Substrate Structure Interfaces

Regulating the surface instability of thin film/substrate structures has been successfully applied to prepare new ductile electronic devices.However,such electronic devices need to be subjected to external loads during operation,which can easily induce delamination of the thin-film electronic device from the substrate.This study aims to investigate the instability characteristics of hard films on flexible substrate surfaces from theoretical analysis and numerical simulation perspectives.Considering finite-thickness substrates,this paper establishes theoretical models for pure bending,bent wrinkle,partial delamination,and total delamination buckling of film/substrate structures based on the nonlinear Euler–Bernoulli beam theory and the principle of minimum energy;then the effects of material and geometric parameters of the structure,interfacial adhesion strength,and pre-strain on the evolutionary path of the four patterns are discussed.The study results show that:the greater Young’s modulus of the substrate is,the larger the parameter region where partial delamination of the film/substrate structure occurs,and the smaller the parameter region where bent wrinkle occurs.By varying Young’s modulus,thickness of the film and substrate,interfacial adhesion coefficient,and pre-strain,the buckling pattern of the structure can be predicted and regulated.The parametric design intervals for each pattern are summarized in the phase diagram.The results of this paper provide theoretical support for the design and reliability evaluation of flexible electronic devices.