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The edges of torn plastic sheets and growing leaves often display hierarchical buckling patterns. We show that this complex morphology (i) emerges even in zero strain configurations, and (ii) is driven by a competition between the two principal curvatures, rather than between bending and stretching. We identify the key role of branch-point (or "monkey-saddle") singularities in generating complex wrinkling patterns in isometric immersions, and show how they arise naturally from minimizing the elastic energy., Comment: 6 pages, 6 figures. This article supersedes arXiv:1504.00738

The edge of torn elastic sheets and growing leaves often form a hierarchical buckling pattern. Within non-Euclidean plate theory this complex morphology can be understood as low bending energy isometric immersions of hyperbolic Riemannian metrics. With this motivation we study the isometric immersion problem in strip and disk geometries. By finding explicit piecewise smooth solutions of hyperbolic Monge-Ampere equations on a strip, we show there exist periodic isometric immersions of hyperbolic surfaces in the small slope regime. We extend these solutions to exact isometric immersions through resummation of a formal asymptotic expansion. In the disc geometry we construct self-similar fractal-like isometric immersions for disks with constant negative curvature. The solutions in both the strip and disc geometry qualitatively resemble the patterns observed experimentally and numerically in torn elastic sheets, leaves and swelling hydrogels. For hyperbolic non-Euclidean sheets, complex wrinkling patterns are thus possible within the class of finite bending energy isometric immersions. Further, our results identify the key role of the degree of differentiability (regularity) of the isometric immersion in determining the global structure of a non-Euclidean elastic sheet in 3-space., Comment: 5 Pages, 7 figures