Nat Mater. 2011 Oct 23;10(12):952-7. doi: 10.1038/nmat3144.
Mechanical instabilities that cause periodic wrinkling during compression of layered materials find applications in stretchable electronics and microfabrication, but can also limit an application's performance owing to delamination or cracking under loading and surface inhomogeneities during swelling. In particular, because of curvature localization, finite deformations can cause wrinkles to evolve into folds. The wrinkle-to-fold transition has been documented in several systems, mostly under uniaxial stress. However, the nucleation, the spatial structure and the dynamics of the invasion of folds in two-dimensional stress configurations remain elusive. Here, using a two-layer polymeric system under biaxial compressive stress, we show that a repetitive wrinkle-to-fold transition generates a hierarchical network of folds during reorganization of the stress field. The folds delineate individual domains, and each domain subdivides into smaller ones over multiple generations. By modifying the boundary conditions and geometry, we demonstrate control over the final network morphology. The ideas introduced here should find application in the many situations where stress impacts two-dimensional pattern formation.
机械不稳定性会导致层状材料在压缩过程中周期性起皱，这在可拉伸电子产品和微制造中有应用，但也会因分层或在加载下开裂以及在肿胀过程中表面不均匀而限制应用的性能。特别是，由于曲率局部化，有限变形会导致皱纹演变成褶皱。在几个系统中已经记录了从皱纹到褶皱的转变，主要是在单轴应力下。然而，在二维应力配置下，褶皱的成核、空间结构和侵入动力学仍然难以捉摸。在这里，我们使用双轴压缩应力下的两层聚合物系统，表明在应力场重新组织过程中，重复的皱纹到褶皱的转变会产生褶皱的层次网络。褶皱描绘出各个区域，并且每个区域在多个世代中进一步划分为更小的区域。通过修改边界条件和几何形状，我们证明了对最终网络形态的控制。这里介绍的思想应该在许多情况下得到应用，其中应力会影响二维图案形成。
