Dipartimento di Fisica "Enrico Fermi", Università di Pisa, Largo B.Pontecorvo 3, I-56127 Pisa, Italy.
Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Pisa, Largo B.Pontecorvo 3, I-56127 Pisa, Italy.
Int J Mol Sci. 2021 Oct 29;22(21):11732. doi: 10.3390/ijms222111732.
Ultrathin molecular films deposited on a substrate are ubiquitously used in electronics, photonics, and additive manufacturing methods. The nanoscale surface instability of these systems under uniaxial compression is investigated here by molecular dynamics simulations. We focus on deviations from the homogeneous macroscopic behavior due to the discrete, disordered nature of the deformed system, which might have critical importance for applications. The instability, which develops in the elastoplastic regime above a finite critical strain, leads to the growth of unidimensional wrinkling up to strains as large as 0.5. We highlight both the dominant wavelength and the amplitude of the wavy structure. The wavelength is found to scale geometrically with the film length, λ∝L, up to a compressive strain of ε≃0.4 at least, depending on the film length. The onset and growth of the wrinkling under compression are quite well described by an extended version of the familiar square-root law in the strain ε observed in macroscopic systems. Under compression (ε≳0.25), we find that the wrinkling amplitude increases while leaving the cross section nearly constant, offering a novel interpretation of the instability with a large amplitude. The contour length of the film topography is not constant under compression, which is in disagreement with the simple accordion model. These findings might be highly relevant for the design of novel and effective wrinkling and buckling patterns and architectures in flexible platforms for electronics and photonics.
在衬底上沉积的超薄分子膜在电子学、光子学和增材制造方法中被广泛应用。本文通过分子动力学模拟研究了这些系统在单轴压缩下的纳米级表面不稳定性。我们关注的是由于变形系统的离散、无序性质而导致的与均匀宏观行为的偏离，这对于应用可能具有至关重要的意义。在有限的临界应变以上的弹塑性范围内，不稳定性会导致一维起皱，直到应变达到 0.5 左右。我们强调了波状结构的主导波长和振幅。发现波长与膜长呈几何比例缩放，在至少 ε≃0.4 的压缩应变下，至少在膜长范围内，λ∝L。在压缩下，起皱的起始和增长通过在宏观系统中观察到的应变 ε 中的熟悉平方根定律的扩展版本得到了很好的描述。在压缩下（ε≳0.25），我们发现褶皱的振幅增加，而横截面几乎保持不变，为大振幅不稳定性提供了一种新的解释。在压缩下，膜形貌的轮廓长度不是恒定的，这与简单的手风琴模型不一致。这些发现可能与电子学和光子学中柔性平台中新型有效褶皱和屈曲模式和结构的设计高度相关。
