ADHESION OF FINE PARTICLES IN DISPERSIONS

When a dispersion of fine particles is concentrated, the product can c ontain clumps which arise from the aggregation of the particles. There are several drivers: sedimentating, drying, filtration, forcing the p articles together to produce agglomerated structures which are much la rger than the primary particles. The problem of understanding this phe nomenon is twofold: on the one hand, it is difficult to measure aggreg ates in a concentrated slurry; on the other, there is no theory to pre dict when aggregates should form in an apparently-stable dispersion. T his paper describes a new experimental method for measuring aggregates in concentrated suspensions, showing that the aggregation phenomenon can be followed over a wide range of experimental conditions. In parti cular, the results show that the aggregates exist at small concentrati ons in ostensibly stable dispersions even before concentrating takes p lace. Colloids based on polymers, ceramics, biological cells and emuls ions all showed this aggregation effect. We have called these aggregat ed structures ''multiplets'' to distinguish them from the more normal flocs produced by destabilising the colloid. A theory of aggregation i s proposed to fit the experimental results. This theory is based on th e idea that multiplets form as a consequence of small adhesion forces between particles immersed in liquid; a molecular dynamics simulation using this concert of adhesion forces is used to demonstrate the forma tion of multiplet material at low concentrations. The theory seeks to show how the size of multiplets should vary with adhesion and with par ticle concentration.