State Diagram of fine cohesive powders at zero shear

We present measurements of the tensile strength for a set of model toners, with a typical diameter of 10 microns, as a function of the consolidation s tress and void fraction. The technique used to change the consolidation sta te is to centrifuge the powder. For each consolidation state the tensile st rength is measured by subjecting the powder to a tensile stress by means of a direct gas flow. The average free volume is calculated by measuring the powder bed height by means of an ultrasonic device. All these processes all ow us to draw the State Diagram of the powder at zero shear. The influence of the hardness, and size of the asperities, both controlled by flow contro l additives, as well as the influence of the radius of the particle, is det ermined. We find that, contrary to the case of low consolidations where the tensile strength was a linear function of the consolidation stress, at hig h consolidation stresses the tensile strength is a sublinear function of th e compressive stresses.