Cake collapse in frontal filtration of colloidal aggregates: mechanisms and consequences

Microstructures of cakes formed on ultrafiltration membrane as a result of packing of colloidal aggregates generated in rapid coagulation have been investigated though small angle neutron scattering experiments. Scattering curves show an extensive collapse of fractal structure of colloidal aggregates due to their packing in the cake. Accordingly experimental values of solid fraction in the cake are higher than expected from gentle packing of fractal aggregates. Different densification features have been detected both at small and large scale depending on the friction forces between the particles and the strength of inter-particles bonds.

There is no doubt that collapse or some kind of densification may occur when filtering preaggregated spherical particles, such as latex or silica particles. The macroscopic collapse of cakes originates from structural arrangements that take place at shorter spatial scales. With SANS measurements we have proveded evidence that these spatial scales correspond to large groups of particles. The mechanism by which such cakes collapse involves very small motions of particles that are in contact with each other. In some cases these motions leave the small-scale coordination unchanged and in other cases, the organization of particle–particle contact within the aggregates collected in the cake can be slightly changed. However in all cases the continuous deformations cause some interparticle bonds break. Indeed simple rotation of one particle around another may be enough to bend a branch of an aggregate. Only friction forces that tend to maintain established particle contacts oppose. In the pre-aggregated suspensions we have examined, the bonds originate from Ca ++ o r K + i ons that bind the surfaces together. Since we observe extensive collapse we conclude the yield stress of these bonds must be low compared to the forces applied during the packing of aggregates in the filtration cake. Enhancing the strength of these bonds should limit the collapse of initial fractal structure. Flocculation of particles with other type of coagulants such as polycations or polymer for aggregation of primary particles would rather proceed from a bridging mechanism more than a charge neutralization is consider for further investigations. Acknowledgments We thank P. Lindner and J. Zipfel for help and useful suggestions with he neutron scattering experiments and CNRS for providing financial support to this work through two ACI programs (Young Team and Water and Interfaces).