Adhesion and aggregation of fine particles

The influence of interparticle adhesion on the formation and properties of aggregates is reviewed. Increasing the adhesive forces between particles in an aggregate should raise the strength of the aggregate because each particle contact then requires more force for fracture. However, it is well known experimentally that strongly adhesive fine particles lead to fluffy structures which contain fewer contacts and which are therefore weak, even though each individual particle contact may be stronger. Thus, adhesion can both increase and decrease the strength of aggregates, since the process of aggregation is inhibited by adhesion, whereas, the strength of the final aggregate is proportional to adhesion. This paper reviews the background to this problem and then gives two well-defined examples of opposing behaviours: one where there is an exact correlation between adhesion and aggregates in the case of red blood cells; the other where adhesion reduces aggregation in a computer simulation of adhering spheres.