Folds on the surface of soft materials are shown to be a consequence of a nonlinear instability.
"Even as we probe physics on ever-smaller scales, materials that can be held and manipulated with our hands often still resist our understanding. Elastic materials, in particular, still confound because of the nonlinear relationship between strain and the displacement of the material needed to maintain the rotational invariance of the elastic energy. The effects of these nonlinearities are often more pronounced at free surfaces, where strain can be alleviated by a large rotation of the surface. When a slab of an elastic material such as rubber is compressed, it develops a sulcus—a sharp furrow in its surface that plunges into the material. First reported for photographic gelatin films over one hundred years ago, they are not just a laboratory curiosity. Sulci create large strains that can lead to material failure. They are also a common motif in the morphogenesis of many organs, most famously in the characteristic folds on the surface of the human brain or, say, the arm of an infant ..."Physics - Folding furrows, Physics 4, 19 (2011), DOI: 10.1103/Physics.4.19, Folding furrows, Christian D. Santangelo, a viewpoint on: Unfolding the Sulcus, by Evan Hohlfeld and L. Mahadevan, Phys. Rev. Lett. 106, 105702 (2011) – Published March 07, 2011, Download PDF
