Morphology and mechanical behavior of diatoms in wet and dry states studied using nano-XCT

Abstract

Background: Diatoms are widely studied biological objects because of their large variety of geometric shapes and their unique physical and chemical properties. They survive widely in nature within moisture. Imaging the diatoms three dimensionally in moisture and correlating their mechanical behavior is an interesting and challenging topic.

Results: Here, the morphology and mechanical properties of diatoms were studied in wet state and then in dry state. A customized sample holder was integrated into a laboratory transmission X-ray microscope to image the morphology changes and volume shrinkage of the diatom while transitioning from the wet to the dry state. The measured volume shrinkage of a single diatom cell of Actinocyclus sp. is about 0.16. By performing an in-situ micromechanical experiment in both states, the maximal loading force of a single Actinocyclus sp. was determined until cracking appeared and compared in both states. This value is in the range of several hundred µN in the wet state and single-digit mN in the dry state. The normalized stiffness of the studied diatoms is significantly higher in the dry state than in the wet state. 2D radiograph and 3D tomography imaging of the diatoms reveal the different locations for crack propagation in both states.

Conclusions: Our study supplies the important imaging method, the structure and functional information of the diatoms for future studies on diatoms in moisture but also in dry state. This information can help design bio-inspired materials and even in the development of bio-sustainable materials.

Keywords: 3D visualization; Diatom; In situ micromechanical behavior; X-ray tomography.

Fig. 1

Virtual cross-sections of the Actinocyclus sp. in dry state, based on 3D nano-XCT data. a Valve surface of Actinocyclus sp. with radiate areolae. b, c Valve view of rimoportulae (labiate process) presenting at margin of the valve face with the simple, round aperture (b) and the shape of the lips on the internal valve face (c); this type of diatom has about 14–16 rimoportulae. d Girdle view of Actinocyclus sp. Red arrows: the opening of labiate processes, blue arrows: the rimoportulae (valve view of internal part of the labiate processes), green arrow: pseudonodulus

Fig. 2

Virtual cross-sections of the same Actinocyclus sp. in wet and dry state based on 3D nano-XCT data. a, b Valve view (a) and girdle view (b) of Actinocyclus sp. in wet state with intracellular structures distribution in a diatom cell. c 3D rendering of the intracellular structures (grey-yellow) and part of the frustule wall (blue) from the Actinocyclus sp. cell in the wet state. d, e Valve view (d) and (e) girdle view of the same Actinocyclus sp. cell in the dry state with the intracellular structures sticking to the frustule wall. f 3D rendering of the intracellular structures (grey-yellow) and part of the frustule wall (blue) from the Actinocyclus sp. cell in dry state. Red and green arrows: the intracellular structures

Fig. 3

Typical in-situ compression test on Actinocyclus sp. in wet state. a to d Radiographs recorded during the micromechanical test. e Load–displacement curve. The indicated data points (a, b, c, d) in e correspond to the radiographs a to d. Red arrow: the horizontal crack initiates from the connection regions between valve face and the girdle band and propagates perpendicular to the compression direction with a size about 1 µm wide and 7 µm long. Green arrow: the diatom cell collapsed and delaminated. Blue arrow: the maximal force (F_max) before the first crack

Fig. 4

Typical in-situ compression test on Actinocyclus sp. in dry state. a to d Radiographs recorded during the micromechanical test. e Load–displacement curve. The indicated data points (a, b, c, d) in e correspond to the radiographs a to d. Red arrows: the micro-crack within the compression test. Red right bracket: the crack opening and size. Green arrow: the upper valve face collapsed until it is supported again by the girdle band. Blue arrow: the maximal force (F_max) before the first crack

Fig. 5

The images of Actinocyclus sp. after the in-situ compression test. a, b Virtual cross-section of the Actinocyclus sp. after the micromechanical test in the wet state (a) and in the dry state (b). Red arrows: positions of the microcracks