Mechanics in dehydration-induced leaf morphing behaviors

• Main Author: Guo, Kexin
• Other Authors: K Jimmy Hsia
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Physics
• Online Access: https://hdl.handle.net/10356/178603
• Institution: Nanyang Technological University

Plant leaf morphogenesis gives rise to a fascinating variety of shapes, with leaves exhibiting intriguing adaptive responses to changes in their environment. The remarkable ability of plant leaves to undergo shape-morphing in response to environmental cues has inspired the development of engineering machines and devices. While some typical shape changes in plants and their mechanical driving forces have been extensively studied, numerous unexplored behaviors and underlying mechanical principles still exist in nature. Understanding such phenomena not only offers insights into how plants overcome environmental challenges but also paves the way for innovative approaches in developing biomimetic structures and devices. This thesis focuses on three shape-morphing phenomena observed in plant leaves as passive responses to the strain mismatch caused by dehydration. The research findings indicate the significant influence of mechanics on the formation of leaf shapes. The first behavior we investigate is the corrugated folding behavior of Rhapis excelsa leaves upon dehydration. R. excelsa displays a unique and intriguing leaf shape, with slightly zig-zag shaped cross-sections which further shows significant corrugated folding after dehydration. Through an integrated approach involving experimental measurements, numerical simulations, and theoretical modeling, we uncover the mechanisms underlying the leaf’s response to water stress, specifically focusing on the differential deformation between the 'hinge cells' and major veins. By these means, we elucidate the relation between structure, behavior, and function in plant leaves that is mostly dominated by mechanics. Drawing inspiration from these plant morphing mechanisms, we demonstrate the development of biomimetic soft machines capable of morphing in response to water evaporation or rehydration. The second study in this thesis concerns the folding of dried leaves about the midrib. Typically, folding in is induced by the differential strain between two layers at a folding hinge. Here we discover that in-plane strain mismatch can also drive the folding of a lamina. Many plant leaves have a midrib that is substantially thicker and more rigid than the laminae and other vasculature, providing mechanical support to the leaf. As the leaf naturally detaches and dries, the leaf lamina shrinks more than the stiff midrib, resulting in in-plane strain mismatch between the mesophyll tissues and the midrib. For thin-sheet materials like plant leaves, uniform uniaxial stretching of the sheet leads to wrinkling, a phenomenon that has been extensively studied analytically and numerically. We found that locally stretched sheets demonstrate bending perpendicular to the loading direction so that a local tensile loading at the midline (similar to midrib of leaves) leads to symmetric folding of the lamina. The folding is characterized by a folding angle that increases with the applied strain. By numerical simulation, we analyzed the relationship between the folding angle and the strain. We also systematically studied the effects of various geometric properties on the folding behavior. Lastly, we investigate the rolling of Costus spicatus leaves upon dehydration. While rolling and its mechanism have been well-studied in some typical Gramineae species such as rice and maize leaves, rolling is a prevalent phenomenon in monocotyledonous leaves, and its mechanism in other monocot families remains to be demonstrated. Through histological analysis, we characterize the cellular structure of the leaf and quantify the changes in cell size and geometry. We demonstrate that the nonuniform cellular structure, specifically the distribution of hypodermal cells, drives leaf folding through differential shrinking between the adaxial and abaxial layers of the leaf tissue. We identify several mechanics-induced phenomena in the rolled leaves, including mechanical anisotropy, and kink formation on the rolled adaxial epidermis.