Granulation is the process of mixing a dry solid powder and a liquid phase in order to produce agglomerates. This method of processing is used in a wide range of industrial applications from food stuffs to pharmaceuticals to produce aggregates with specific physical characteristics such as size and mechanical strength. Much work has been done to try and understand the various parameters that control granule properties; however less progress has been made on mechanistic understanding. At low volume fractions the mixing of solid and liquid produces pastes and suspensions, yet at high volume fractions granules are produced. In this work we present a model system of glass spheres and glycerol that exhibits a stress dependent transition from suspension to granules at a critical volume fraction. Using rheological data we find two divergent viscosity branches corresponding to frictional and frictionless flow. The frictional branch diverges at a volume fraction phi(m), beyond which the system cannot rearrange in response to further stress, but instead fractures and granulates. The frictionless branch diverges at a volume fraction phi(rcp), above which the system will always granulate. Thus we make contact with recent work on the role of friction in suspension shear thickening (Wyart and Cates, Physical Review Letters 112, 098302, 2014).
