Liquid migration during extrusion of shear thickening suspensions

In many industrial applications dense suspensions are forced from a reservoir through a constriction into a smaller pipe or channel. Dilution of suspensions is often encountered as they travel through a constriction. This phenomenon, commonly referred to as liquid migration, LM, can lead to catastrophic consequences. 

We investigate LM during the extrusion of well-characterised shear-thickening suspensions. We find a critical concentration exists. Below this concentration suspensions can be extruded homogeneously, above it LM occurs. We show this critical concentration depends on a characteristic shear rate in the die and can be predicted by coupling the suspension shear rheology to a simplified 'suspension-balance' model describing the relative solid-liquid flow in response to local stress gradients. In addition, we demonstrate how x-ray imaging can be utilised to visualise LM as a suspension travels through a constriction and the impact this has on the suspension upstream of the constriction.