# Micromechanical Characterization of Diatom Frustules of Multiple Origin
## 2 State-of-the-Art
### Mechanical Strength of Diatom Frustules and Atomic Force Microscopy AFM
Another element of interest was the identification of pores between the silica spheres, i.e., the packing of the spheres inside the frustules is not fully dense, which opposes the assumption of high solidification calculated from deflection tests. The tests by [48] were performed on diatoms in natural conditions, i.e., without removing or cleaning the external mucilaginous layer. Therefore, the results obtained involve the whole system (silica frustule + organic coating),which is why the AFM experimentation could not distinguish the mechanical contributions of each layer. Different sample preparation processes must be applied to visualize diatoms and assess frustules in the AFM. These cells can be analyzed in dry or wet environments. Sample preparation techniques may include oxidation procedures of the organic material covering the cell or its coating prior to observation. If the organic components are not cleaned, many details can be darkened. However, it is essential to consider that some diatom species with low silica content may collapse during sample preparation. AFM resolution is reduced because of scattering generated by mucilaginous organic coverage, mainly when sensitive tip methods are applied in liquid environments [22]. Ref. [52] have implemented AFM microscopy to observe living diatoms and follow their growth stages, to recognize the topography of their frustules, to determine the thickness of the organic coatings (ranging in the range of 10 nm), to observe biomineralization processes and to record the movements of these organisms, as well as the formation of chains. The production of binders that allow cells to adhere to substrates or other diatoms has been recognized as efficient in permanent submersion conditions in fresh and salty environments. Most manufactured adhesives lose their properties upon contact with humid environments as they undergo chemical changes that detach them from the substrate or the environment that supports them [23]. Ref. [53] evaluated the topography and mechanical properties of adhesive substances secreted by two species of diatoms through their pores or \"Raphe\". The observations determined that, in both species, the secretion covers almost the totality of the frustules. From the force curves, it was possible to calculate the adhesive strength, around 3.8 nN. For the particular case of secretions through the \"Raphe\" in the form of strands or fibers, breaking strengths as high as 60 nN were reported. Diatoms may have multiple mechanisms for producing organic coatings on the cell surface [49]. Ref. [23] studied natural adhesives from diatoms of the _Eunotia sudetica_ species. This species was selected considering its size (tens of microns) and ease of micrometric manipulation. These organisms were cultured in controlled fluid environments and subsequently self-adhered to glass slides. The analysis focused on the force-distance curves obtained from the adhesion zone of the cells to the glass substrate. From the loading and unloading application cycles, it was concluded that the adhesive force increases with extension. Suppose the load is removed after an adhesive breakage (separation of the cell from its substrate), and contact is regained. In that case, the adhesive properties are recovered, provided that a continuous interaction is allowed for more than 30 s.  
Ref. [24] reports that the C\({}_{\text{S}}\) recovery rates obtained in a one-dimensional consolidation test on diatomaceous soil are up to twice as high as those recorded for kaolin-type soil. This phenomenon can be associated with the elastic properties of diatom frustules. Ref. [24] apud [11] points out that diatoms are not entirely rigid and show considerable elastic deformation before breaking. This conclusion results from multiple tests performed on frustules subjected to loads through AFM.