Cohesion of a bird nest.

Anyone who has tried to pick up pine needles knows that the more we compress the aggregate the easiest it is shaped into a complex geometrical form. One striking difference between aggregates of flexible frictional fibers and other granular materials like rigid spheres is the effective cohesion of their assembly. We need to add glue or water capillary bridges between grains to shape aggregates of spherical particles and build sandcastles. For fibers, no need for glue to build a nest. Here we study an assembly of monodisperse flexible fibers. We first use X-ray microtomography to characterize the geometry of the initial assembly, the number of frictional points and bending curvatures of the fibers. Using force-displacement measurements, we characterize the variation of the macroscopic cohesive strength of the aggregate with the geometry of the fibers, the fibers mechanical properties and the packing preparation. Finally, we relate the macroscopic mechanical behavior of the assembly with the filament reorganization at the microscopic scale.