Indentation-triggered pattern transformation in hyperelastic soft cellular solids

This paper explores indentation-triggered microstructural instability in hyperelastic cellular solids through combined experimental, numerical, and theoretical efforts. The results demonstrate that when the indentation depth is greater than a critical value, local instability occurs and further propagates into a rectangular region beneath the indenter. The width of the rectangular region scales with the contact width, and we propose a simple scaling relation to estimate the maximum depth to which the instability can propagate based on the elastic contact theory. The results reported here may find such applications as in the integrity evaluation of soft cellular materials and structures and the development of advanced functional materials with unique optical, acoustic and wetting properties.     

Wrinkling of a bilayer resting on a soft substrate under in-plane compression

The wrinkling problem of a bilayer film resting on a soft substrate under in-plane compression is investigated via theoretical analysis and numerical simulations. Such a system may undergo two different wrinkling modes, namely surface wrinkling of the top single layer on a composite substrate or bilayer wrinkling on a homogeneous substrate, depending on the geometric and material parameters. In the present paper, a theoretical solution is first derived for the former case, while a solution available in the literature is applied for the latter. The solution derived here can degenerate both to the classical solution for a stiff layer resting on a semi-infinite soft substrate and that for a stiff layer lying on a compliant substrate of finite thickness. A series of finite element simulations are performed to demonstrate the accuracy of the theoretical solution. In addition, it is found that, at a given compressive strain, a certain change in the elastic modulus of the intermediate layer may lead to a dramatic surface pattern transformation between the two wrinkling modes, indicating a novel approach to realise surface pattern switching.     

An electromechanical liquid crystal model of vesicles

An electromechanical liquid crystal model is developed for characterizing the equilibrium morphology of a lipid vesicle under coupled mechanical and electrical fields. A general equation that governs the vesicle shape is established, which incorporates the effects of elastic bending, osmotic pressure, surface tension, Maxwell pressure, as well as flexoelectric and dielectric properties of the lipid membrane. As an illustration of the model, the problem of an axisymmetric vesicle (e.g., a sphere or a cylinder) in a uniform electric field is considered in some detail, with results in agreement with relevant experimental results. The model provides an efficient tool for studying morphological evolution of dielectric vesicles under mechanical and electrical fields.     

Buckling and post-buckling of a stiff film resting on an elastic graded substrate

Systems consisting of a hard layer resting on an elastic graded soft substrate are frequently encountered both in nature and industry. In this paper, we study the surface wrinkling problem of such a composite system subjected to in-plane compression. The Young’s modulus of the elastic substrate is assumed to vary along its depth direction. In particular, we investigate two typical variations in the modulus, described by a power function and an exponential function, respectively. Analytical solutions which permit to determine the critical compressive strain for the onset of wrinkling and the wrinkling wavelength are derived. A series of finite element simulations are performed to validate the theoretical solutions and demonstrate the prominent features of the postbuckling evolution of the system. The results may not only find applications in thin-film metrology and surface patterning but also provide insight into the wrinkling phenomena of various living tissues.     

Elastic Analysis of Physisorption-Induced Substrate Deformation

Physisorption may cause a dimple on a deformable solid surface due to adsorbat-substrate interaction. The interactive force between the adsorbate and the crystal atoms depends on their distances, which may change with substrate deformation. This feature of displacement-dependence indicates that the equilibrium problem is a force-deformation coupled nonlinear procedure. In the present study, a continuum mechanics model, in which the force is considered as a function of the displacement field of the medium, is presented to calculate the physisorption-induced deformation in a semi-infinite elastic medium. It is found that the nonlinear effect due to force-deformation coupling should be taken in consideration in the adsorbate-substrate interaction analysis.     

Design methods of rhombic tensegrity structures

As a special type of novel flexible structures, tensegrity holds promise for many potential applications in such fields as materials science, biomechanics, civil and aerospace engineering. Rhombic systems are an important class of tensegrity structures, in which each bar constitutes the longest diagonal of a rhombus of four strings. In this paper, we address the design methods of rhombic structures based on the idea that many tensegrity structures can be constructed by assembling one-bar elementary cells. By analyzing the properties of rhombic cells, we first develop two novel schemes, namely, direct enumeration scheme and cell-substitution scheme. In addition, a facile and efficient method is presented to integrate several rhombic systems into a larger tensegrity structure. To illustrate the applications of these methods, some novel rhombic tensegrity structures are constructed.     

Buckling and postbuckling of a compressed thin film bonded on a soft elastic layer: a three-dimensional analysis

The wrinkling of a stiff thin film bonded on a soft elastic layer and subjected to an applied or residual compressive stress is investigated in the present paper. A three-dimensional theoretical model is presented to predict the buckling and postbuckling behavior of the film. We obtained the analytical solutions for the critical buckling condition and the postbuckling morphology of the film. The effects of the thicknesses and elastic properties of the film and the soft layer on the characteristic wrinkling wavelength are examined. It is found that the critical wrinkling condition of the thin film is sensitive to the compressibility and thickness of the soft layer, and its wrinkling amplitude depends on the magnitude of the applied or residual in-plane stress. The bonding condition between the soft layer and the rigid substrate has a considerable influence on the buckling of the thin film, and the relative sliding at the interface tends to destabilize the system.     

Capillary Adhesion of Microbeams： Finite Deformation Analysis

Capillary force may cause adhesion of devices at micro- and nano-scales. Considering the fact that large deformation is often involved in adhesion of microbeams, we analysed the capillary adhesion of two beams using finite deformation elasticity theory. The critical adhesion condition can be obtained from the present method as a function of the bending stiffness, Young＇s contact angle, the spacing of the two beams as well as the surface tensions of the solid and liquid phases. The solution for the capillary adhesion of a beam with a rigid substrate is also given. The results from the finite deformation analysis are compared with that of infinitesimal deformation method in order to show the necessity of accounting for the nonlinear effect associated with large deflection. The method adopted in this study can also be used to solve other adhesion problems associated with van der Waals force or electrostatic force.     

Directional Motion of Droplets in a Conical Tube or on a Conical Fibre

Manipulating the directional movement of liquid droplets is of significance for design and fabrication of some microfluidic devices, An energy-based method is adopted to analyse the directional movement of a droplet deposited in a conical tube or on a conical fibre. We perform an experiment to investigate the directional motion of a droplet in an open conical tube. Our theoretical analysis and experimental observations both demonstrate that surface tension can drive the droplet to move in the conical tube. The critical condition of the liquid moving in the conical tube is presented. We also analyse a droplet on a conical hydrophilic fibre, which can move from the thinner to the thicker end.