Surface Instabilities Of Elastic Graded Material And System

Surface instability has attracted much attention due to its valuable mechanical,optical and thermal performances,which enable wide applications in stretchable electronics,tunable optics,micro-and nano-metrology,etc.Extensive studies have been focused on surface creasing of a homogeneous compliant material or surface wrinkling of a stiff thin film on a homogeneous compliant substrate.However,many practical systems,such as poly(dimethylsiloxane)(PDMS)exposed to UV/Ozone treatment and human skins,involve elastic graded materials with elastic properties varying in the thickness direction.In this thesis.surface instabilities of an elastic graded material and a film/elastic graded substrate system are studied theoretically,numerically and experimentally.The influences of material properties and geometric parameters on the critical wrinkling condition are systematically investigated as well as the morphology evolution.These results establish the scientific foundation to understand the underlying physics of surface instabilities of system involving an elastic graded material and provide guidelines to precisely control the surface morphology.The main research contents and conclusions are as follows:(1)The elastic graded soft material was fabricated by oxidizing the surface of Polydimethylsiloxane(PDMS)with UV light.Nano-indentation experiments were conducted on the UV/Ozone treated PDMS by atomic force microscope(AFM).It is found that the UV/Ozone treated PDMS is a typical elastic graded material with the Young’s modulus exponentially decaying in the thickness direction.The thin film/elastic graded substrate system was obtained by the integration of the Si nanoribbon with the UV/Ozone treated PDMS via the tape transfer printing technique.(2)For surface instabilities of an elastic graded material,a theoretical model was developed to obtain the critical wrinkling condition for the onset of surface wrinkling for the sinusoidal mode including the critical wrinkling strain and wavelength.A plane-strain finite element model was also established to study the onset of surface instabilities as well as the postbuckling behavior of the system by introducing the depth-dependent Young’s modulus in the elastic graded material.A custom-made apparatus combining with a stylus profiler was used to realize the in situ measurement of surface morphology evolution.The UV/Ozone treated PDMS with the exponential decaying modulus was then taken as an example in the theoretical analysis and numerical simulations.The morphology evolution of surface wrinkling of an elastic graded layer from finite element analysis indicates the transitions of sinusoidal mode to arch mode and then to the period-doubling mode with co-existing crease mode and folding mode,which agree quantitatively well with experimental observations(3)For a film/elastic graded substrate system,a theoretical model was developed to obtain the critical wrinkling condition for the onset of surface wrinkling for the sinusoidal mode.A finite element model was also established to study the surface instability of the film/elastic graded substrate system.With the custom-made apparatus combining with a stylus profiler,the in situ measurement of surface morphology evolution was realized.The Si/elastic graded PDMS was then taken as an example to illustrate the surface instabilities of the film/elastic graded substrate system.It is shown that the morphology evolution of surface wrinkling of a film/elastic graded substrate system transits from sinusoidal mode to the period-doubling mode directly as the compressive increasesThese results of an elastic graded material and a film/elastic graded substrate system are helpful to provide physical insights on the influence of material inhomogeneity on surface instabilities.