![]() Finally, the closing stage makes the entire structure recover its original configuration. The fast bouncing back stage is caused by the sudden and dramatic expansion of cavities inside cells as shown in Fig. ![]() 30 During the opening stage, a sporangium opens slowly and stores bending energy in the annulus wall, accompanied by the shrinkage of cells and loss of water, as shown in Fig. Briefly, triggered by the decrease of environmental humidity, the entire spore dispersal process of fern sporangium can be roughly divided into three stages, i.e., opening, fast bouncing back, and closing. 29 The term, cavitation catapult, is a vivid analogy that the fern sporangium exhibits fast closure motion to propel the spores like a catapult by cavitation instability as shown in Fig. Some recent experimental studies have uncovered very interesting phenomena that fern sporangium can eject spores with an initial speed of up to 10 m/s and an acceleration of 106 g using the cavitation catapult mechanism. We hope that our study in this article can provide useful insights into the bio-inspired design of structures which can take advantage of cavitation instability in soft materials. When the environmental humidity is lower than a critical value, through energetic analyses, we can predict the cavitation catapult phenomenon using the model. To simplify the problem, in our model, the mechanics of cells in the sporangium are described by the polymer gel model, while the sporangium wall is modelled as a hyperelastic material. Motivated by the observation, in this article, we study cavitation instability in a similar structure as the sporangium. As a result, like a catapult, the sporangium snaps back and ejects the seeds at a high speed. When the humidity is lower than a critical value, the cavities suddenly expand dramatically inside the cells, causing a quick release of the elastic energy stored in the annular structure. Briefly, the decrease of environmental humidity causes continuous bending of the sporangium and growth of cavities inside the annulus cells, with the elastic energy accumulated in sporangium walls. An intriguing phenomenon has been recently discovered that fern sporangium can take advantage of drying-induced cavitation instability in annulus cells to disperse spores at an extraordinarily high acceleration. Cavitation is often regarded as a failure mode in soft materials.
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