Finite Element Modeling of the Cyclic Wetting Mechanism in the Active Part of Wheat Awns

Abstract

Many plant tissues and organs are capable of moving due to changes in the humidity of the environment, such as the opening of the seed capsule of the ice plant and the opening of the pine cone. These are fascinating examples for the materials engineer, as these tissues are non-living and move solely through the differential swelling of anisotropic tissues and in principle may serve as examples for the bio-inspired design of artificial actuators. In this paper, we model the microstructure of the wild wheat awn (Triticum turgidum ssp. dicoccoides) by finite elements, especially focusing on the specific microscopic features of the active part of the awn. Based on earlier experimental findings, cell walls are modeled as multilayered cylindrical tubes with alternating cellulose fiber orientation in successive layers. It is shown that swelling upon hydration of this system leads to the formation of gaps between the layers, which could act as valves, thus enabling the entry of water into the cell wall. This supports the hypothesis that this plywood-like arrangement of cellulose fibrils enhances the effect of ambient humidity by accelerated water or vapor diffusion along the gaps. The finite element model shows that a certain distribution of axially and tangentially oriented fibers is necessary to generate sufficient tensile stresses within the cell wall to open nanometer-sized gaps between cell wall layers.

Cellulose: A Review of Water Interactions, Applications in Composites, and Water Treatment

“…This whole subject of hydrogels that respond to external stimuli is an area of research in its own right, and the readers are referred to further reviews. 150 Returning to the natural systems that display changes in shape upon actuation in water, a finite element modeling study by Zickler et al 151 showed that the probable mechanism for the swelling and shape changes in wheat awns is the formation of gaps between laminations, which act as valves, allowing moisture into the cell walls. This interplay of structure of laminations, and the molecular level of cellulose and its interactions with water demonstrate that there are most likely hierarchical features that lead to macroscopic behavior.…”

Soft matter in hard confinement: phase transition thermodynamics, structure, texture, diffusion and flow in nanoporous media

“…Sorption-induced pressures or deformations are of obvious interest for applications of nanoporous media as sensors or mechanical actuators. In the case of soft porous materials, e.g., aerogels [175], porous polymers [176] and biomaterials [156], the deformations are particularly large and can therefore be used for large-scale, reversible locomotion [177,178,179] induced via fluid sorption. And this in a remarkably energyefficient manner, as has been documented in a recent study on water sorption-induced deformation of wood by Bertinetti, Fischer and Fratzl [180].…”

Three-dimensional functional gradients direct stem curling in the resurrection plant Selaginella lepidophylla

“…self-opening windows) [9,64]. In terms of implementation, computational models would help decipher the interaction between morphological and compositional gradients, and how these features could be an asset for the design of synthetic systems [65–67]. This would be a starting point for the realization of proof-of-concept prototypes for novel multi-objective, programmable origami composites and metamaterials with uses in a variety of applications [11,68–70].…”

“…self-opening windows) [9,64]. In terms of implementation, computational models would help decipher the interaction between morphological and compositional gradients, and how these features could be an asset for the design of synthetic systems [65][66][67].…”