3, Wageningen University & Research, Wageningen, , Netherlands
4, Amsterdam University, Amsterdam, , Netherlands
2, CEA-Saclay, Gif-sur-Yvette, , France
Because of the disordered nature of amorphous systems, their fracture involves local structural rearrangements as well as micro-fracturing processes, and is not fully understood. Several experiments performed on silicate glasses have shown that, indeed, a crack progresses in these materials by creating first a series of nano-cracks around it tip. This "damaged" region was shown to extend over ~10 nanometers. Because of the smallness of this size, direct observation is impossible, and the scope of our work is to build, and fracture in a controlled way amorphous structures made from basic bricks much larger than the silica tetrahedron (the size of which is ~5A). We have worked on an agar gel, for which the basic entity is the junction between chains (~15nm), on a so-called Casimir gel where microgels play the role of "atoms" (diameter~1µm) and on a gel made from an emulsion with droplet size ~50µm.
These materials being very soft gels (Young's modulus E<100kPa), we had to imagine new microfluidic devices in order to ensure their controlled fracture. Cracks were observed using conventional or confocal microscopy. By studying both the shape of the crack and the displacement field in the vicinity of the crack tip, we could evaluate the distance to linear elasticity, and disclose the relevant time and length scales.
In the case of the agar gel, it is still impossible to observe directly dissipative processes, but their large scale consequences can be fully characterized. In the case of the Casimir gel, micro-cracking ahead of the propagating crack tip is observed directly, and the process zone size is measured as a function of temperature, which rules the interaction potential between microgels. Finally, in the emulsion-based gel, local rearrangements which do not involve cracks are also observed.