Converging flow to characterise the stability of dispersons

 
During manufacturing, processing and application dispersions are often exposed to complex flow fields including strong extensional components (diverging or converging stream lines). An extremely important aspect of the behaviour of flowing suspensions is their colloidal stability which is strongly affected by the kinematics of the bulk flow and by the forces acting between particles. Although much is known about Brownian and shear-induced aggregation the process of agglomeration in a suspension subjected to an extensional flow is poorly understood. Trajectory analysis for dilute suspensions with attractive interactions revealed that the collision rate is dependent on the bulk flow kinematics (flow type) and stability is significantly higher in shear flow compared to extensional flows under otherwise comparable conditions [1]. Our study focuses on the aggregation process of concentrated dispersions exposed to extensional flow-fields. In order to study this phenomenon experimentally we use a ring-slit device, which has already been established to characterize flow-induced aggregation for industrial quality control or product development purposes [2]. This method allows for a variation of flow conditions in a wide range, but the basic principles controlling aggregation in this test are still not very well understood. The dynamics of the aggregation process are to be investigated in terms of the dependence on the flow kinematics, colloidal interaction forces, dispersion properties and the particle-wall interactions. In this work we report experimental results that show the influence of various colloidal properties on the stability of concentrated dispersions exposed to converging flow.