Particle interactions in flowing suspensions

Abstract

For a fully destabilized suspension of non-Brownian praticles in laminar tube flow, the extent of orthokinetic flocculation can be calculated by classical Smoluchowski theory, using the average shear rate G and the average residence time t . It can be shown very simply that the dimensionless quantity Gt (and hence the degree of flocculation) depends only on the tube dimension and not on the flow rate. However, calculations based on this approach predict far more flocculation than is observed experimentally. There are two major reasons for the discrepancy: 1) the Smoluchowski treatment of orthokinetic flocculation neglects hydrodynamic interaction between particles, which can be introduced by a semi-empirical method due to van de Ven and Mason and this step leads collision efficiencies which are considerably less than unity and depend both on shear rate and on interparticle forces; 2) the shear rate is not uniform in the tube but varies from zero at the tube axis to a maximum value at the wall. Since the major contribution to the flow comes from regions close to the tube axis, where the shear rate is low, the simple averaging procedure considerably overestimates the degree of flocculation. From experimental measurements on the degree of flocculation of dispersions achieved by laminar flow through narrow tubes at different flow rates it is possible to draw semi-quantitative conclusions concerning particle interaction and the strength of flocs. The effect of helical winding of the tube is briefly considered and shown to give more flocculation than in a straight tube. Some experimental results for latex particles destabilized by cationic polymers flowing through straight and coiled tubes are mentioned.