Credit: (a),(b) E. Hohlfield and L. Mahadevan [7]

Figure 1: Localized folds, called sulci, induced on soft materials due to compressive stresses are ubiquitous in nature: (a) the arm of an infant, (b) a primate brain. (c) Schematic illustration of a bifurcation diagram showing the scaled height $h$ of a sulcus plotted against the applied strain. A sulcus nucleates at a critical strain $ϵc$ due to a spontaneous breaking of scale symmetry.