Spontaneous Formation of Oriented Lateral Wrinkles Assisted by Shrinkage Controlled Cracking

Abstract

tunable optical devices, [8][9] and as structured templates for surface arrays, [10][11] etc. Therefore, the fabrication of oriented wrinkles is of great importance.Wrinkling occurs to release the compressive stresses caused by the lateral stresses generated between the two layers of a rigid capping layer/soft substrate. [12][13] The lateral stresses (e.g., thermal stresses) can induce changes in the surface morphology of common bilayer or multilayer composites (e.g., aluminum (Al)/poly dimethylsiloxane (PDMS), Al/polystyrene (PS)/silicon (Si)) which can be considered as infinite-planar 2D systems compared to the wavelength of the surface wrinkles. In such systems, surface wrinkles do not have any preferred orientation (i.e., disordered wrinkles), since the direction of the lateral stresses between the layers is on average, isotropic. However, when anisotropy of the lateral stresses between layers is introduced, the lateral stresses are first concentrated and accumulate in a certain region or along edges with topographic features, the detailed process being related to how anisotropy is introduced. When these accumulated lateral stresses are released anisotropically, making a change in the wrinkled orientation. [12][13][14] Therefore, the introduction of anisotropy of the lateral stresses is essential for the formation of oriented wrinkles.Currently, there are three main approaches to introduce the anisotropy of the lateral stresses for the generation of oriented wrinkles, one is to use patterned templates. [12,[15][16][17] Wang et al. prepared oriented wrinkles on a polydopamine (PDA)/PS bilayer film patterned with microgrooves by thermal annealing where the lateral stresses were preferentially concentrated and released at the edges of the groove patterns. [12] Qian et al. placed PDMS molds with striped relief structures as a confine on the PS/polyvinilpyrrolidone (PVP)/Si surface. The PVP layer subjected to water vapor changes to a viscous state, resulting in a mismatch in the Young's modulus of PS/PVP, when the diffusion forces accumulated along the edges of the mold are dedicated to releasing the compressive stress in a specified direction and the highly oriented wrinkles will be generated. [16] The second approach is by laser or plasma treatment. [13,[18][19][20] Okayasu et al. introduced a series of parallel striped cracks with different spacing dimensions on a silicon substrate through a standard photolithography process. During the heat treatment process, the lateral stresses perpendicular to the stripe direction could be anisotropically released in strain concentrations at the Oriented wrinkled surfaces play a vital role in the preparation of functional components. However, existing methods for the fabrication of oriented wrinkle patterns are often limited by specific equipment (e.g., high vacuum systems) and cumbersome processes. In this study, an approach utilizing shrinkage controlled cracking method is proposed to introduce prefabricated topographic patterns on flat semi-crosslinked polydimethylsiloxa...

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

“…[138] Internal Force Stimulus: Internal force stimulus is mainly provided by a crosslinking-induced volumetric contraction of precursors for elastic substrates (e.g., PDMS). [84,86,139,140] Typically, the liquid precursors are first cast onto a stiff skin, subsequently wrinkles on the elastomer substrate are formed after crosslinking-contraction of the precursors and releasing of watersoluble polymers as sacrificial layers, where the crosslinkinginduced volumetric contraction of the precursors provides the driving force for developing and stabilizing surface wrinkle. [139] This internal force stimulus simplifies the wrinkling procedure since no additional external forces have to be introduced.…”