A 3D micromechanical study of hygroscopic coiling deformation in Pelargonium seed: from material and mechanics perspective

The awned seeds of the family Geraniaceae are special seed-dispersal units. The awns can generate periodic coiling and uncoiling movement through interaction with the diurnal humidity cycle. To investigate how this natural actuator acquires the remarkable property, we determined the three-dimensional morphological features of the coiling awns of Pelargonium peltatum through X-ray microtomography (micro-CT). Many streaks with sharp corners were found distributed in the surface of the inner layer cells, indicating there are two different microfibril angles (MFAs) in the special tilted helix structure of each one cellulose fiber within the cell wall. A simplified mechanical model of the cell wall was developed. Moreover, finite element method was conducted based on the proposed model to analyze the basic mechanism of the coiling deformation. The results showed that stiff cellulose fibers with special tilted helix structures could direct the shrinkage forces resulted from the matrix, so as to generate torsional and bending movement simultaneously. Therefore, the inner layer cell could generate an anti-clockwise coiling deformation macroscopically. In addition, effects of other structural factors including fiber frequency of the cellulose fiber skeletons, modulus ratios between the fibers and matrix, and macroscopical combination among the cells on the degree of the coiling deformation were also studied. The results determined in this work may benefit the development of new kind of intelligent composites.