Several aspects of the physical and rheological behavior of soft matter are considered in the light of some experiments. Granular or colloidal media are used as a reference system. We show that, depending on the rheological properties of the paste and the plate separation velocity and the thickness of the paste layer, two main failure modes are obtained. The first mode is the inwards shear flow of the paste with viscous fingering instabilities. Also, to what has been observed with sample fluids and with non-Newtonian colloidal suspensions or polymer solutions. The second failure mode is stemming from the formation of bubbles by cavitation similarly to what has been observed in soft adhesive polymer layers. The crossover between the two failure modes is determined by the conditions required to generate a pressure drop able to trigger the growth of a bubble size of the inter-granular distance.

adhesion, cohesion, interfacial flow, pastes, granular matter, rheology.