Specific AFM analytical methods were tested to the surface evaluation of wheat powders at micrometric scale in dry conditions. The objective was to evaluate the adhesion properties of particles based on the main wheat endosperm components (starch, protein, and arabinoxylan) that are characterized by large diversity in terms of chemical composition and particle characteristics. Experiments were conducted by using the force spectroscopy mode to determine interactions generated by the surfaces of particles that were glued on AFM tips, and two reference flat and smooth surfaces, a glass and a polysine slide. Particular care has been devoted to characterize the surfaces in contact since local contact radius is a key parameter in the interpretation of the interactions. The statistical analysis of the normalized adhesion forces have shown the sensitivity of the AFM technique for hydrophobic wheat components and their correlation with the surface tension of the particles.

An original method based on atomic force microscopy (AFM) in contact mode was developed to abrade progressively the surface of tablets made of starch or gluten polymers isolated from wheat. The volume of the material removed by the tip was estimated from the analysis of successive topographic images of the surface, and the shear force was measured by keeping a constant normal force. Our data together with a simple tribological model provide clear evidence for a higher hardness and shear strength of starch compared to gluten. Gluten appears to have mechanical properties close to soft materials, such as talc, whereas starch displays higher hardness close to calcite. Our results are in a better agreement with structural properties of gluten (complex protein network) and starch (granular and semi-cristalline structure) than earlier studies by micro-indentation. This work shows that the AFM scratching method is relevant for the characterization of any polymer surface, in particular in application to materials made of different polymers at the nano-scale.

By investigating more than six decades of length scales (from nm to mm), we have studied how linear elastic well-known solutions hold water at the close neighbourhood of the tip of a crack propagating in oxide glass under stress-corrosion regime. The existence of dissipative mechanisms at small scale was especially targeted. Subcritical crack propagation was performed by a loading cell on Double Cleavage Drilled Compression samples under controlled atmosphere. Post-mortem and in-situ observations were performed by optical techniques and atomic force microscopy (AFM). A 2D/3D analysis of this sample was realized according to linear elastic fracture mechanics in order to discuss the experimental results and to ensure the mechanical test control at all scales. The mechanical effect of capillary condensation observed by AFM at the crack tip was modelled according to a cohesive zone model. This allowed notably to evaluate the negative Laplace pressure in the liquid and to explain the crack closure mechanism in glass. A digital image correlation technique was used on series of consecutive AFM in-situ images. We showed that the elastic solution for the surface displacement field is valid up to a distance of 10 nm from the crack tip. The height correlation functions along the AFM images of fracture surfaces were also analyzed. We showed that the cut-off length, found close to few tens of nanometres and previously interpreted as the process zone size, is most probably due to the finite size of the AFM scanning probe and in agreement with the DIC, no process zone larger than 20 nm is observable.

The presence of a liquid condensed phase inside the nanometrically sharp cracks in glasses plays an important role in the physics and chemistry of crack propagation, as well as in many industrial problems related to the strength and life time of glass products. The slow crack propagation in glasses is commonly explained by the stress-corrosion theory: water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and thus enhance the sub-critical crack propagation velocity, which is ruled by the rate of a thermally activated chemical reaction. However, the details of the local environmental condition at the crack tip in moist air are very complex and still need careful investigation. In this talk, I will present direct evidence of the presence of a submicrometric liquid condensate at the crack tip of a pure silica glass during very slow propagation. These observations are based on in-situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Several macroscopic measurements of anomalous crack propagation in glasses had hinted at the presence of such a condensate, yet the direct observation of this phenomenon was not possible until recently due to the difficulty of real-time measurements at such small scale. The mechanical and chemical consequences of such a condensate will be discussed in the frame of stress-corrosion theory.