Aggregation of growing crystals in suspension: III. Accounting for adhesion and repulsion

In this paper we modify our previously published model describing the aggregation behaviour of crystals growing in suspensions. That model considered aggregation only in terms of cementing, i.e. the growth of a neck between colliding particles.

In this current work, we introduce into our model the case of aggregation of growing crystals in the presence of inter-particle forces. Here we postulate that aggregation may be augmented by adhesion between the colliding particles, or diminished by repulsion of particles. In the new model we characterise the adhesive or repulsive force by a surface energy acting along a linear contact and subsequently define an adhesion Mumtaz number, MA. In the case of simultaneous cementing and adhesion the Mumtaz number is simply the sum of the separate cementing and adhesion numbers.

In this work, two crystallographic forms of calcium oxalate monohydrate (COM) and one form of calcite under both low and high ionic strength conditions were studied. Calcite crystals were found to evidence adhesion between the colliding particles, both a high and low ionic strength. Furthermore, the ionic strength also influenced the cementing parameter. The different forms of COM exhibited both different cementing and inter-particle behaviour. These effects were attributed to differences between the systems at the nano-scale and that aggregation behaviour depends on the anisotropic nature of growing crystals.