Drying-Induced Pressure Rise and Fracture Mechanics Modeling of the Sphagnum Capsule

Abstract

The Sphagnum capsule can disperse spores at an extraordinarily high velocity and acceleration during drying. Briefly, the pressure rise induced by the decrease in the environmental humidity inside the spore chamber causes crack growth between the lid and the capsule wall. At a critical condition, the lid of the capsule suddenly fractures, and the top spores are propelled by the high pressure. Motivated by this phenomenon, we develop a similar mechanics model to study the drying-induced pressure rise and the fracture mechanism of the Sphagnum capsule in this paper. We investigate the drying-induced pressure rise and obtain the deformation configuration for various stiffness ratios of different parts. We also establish a fracture mechanics model and calculate the energy release rate to study the lid separation during the ejection of spores. We find that the energy release rate increases with crack growth when the crack is short, maximizes at an intermediate central crack angle of around [Formula: see text], and gradually decreases with further increase in the central crack depending on the loading type. Such a nonmonotonic relationship between the energy release rate and the crack length can be readily used to explain the spontaneously fast unsteady crack growth and the following potential crack arrest reported in the literature. The results and the modeling method obtained in this paper can be used to explain similar fracture-related spore launching of plants and design bioinspired structures to realize the drying-induced fast movement.