Aggregation occurs whenever single nanoparticles cluster together forming larger objects. This happens through surface attractive interactions which despite being always present, become significative only at the nanoscale, where objects can become nanometrically close.
Unlike our macroscale experience, where aggregates of dust or powder seem easy to break down applying some mechanical or chemical force, aggregates of nanoparticles require a lot of energy to be broken apart, due to the huge number of surface-surface interactions per unit volume and the extreme proximity of the surfaces themselves. It’s so difficult, that often is easier to melt the material than to disaggregate a cluster. Basically, a nanoaggregate is for practical applications the unit one has to deal with when processing this type of materials.
The presence of aggregates introduces several drawbacks: they settle faster, reduce the accessible surface, block pores and channels, scatter light, and are in general scarcely controllable. The advantage of having high quality, monodispersed nanoparticles is almost completely lost if they form aggregates, both in terms of performance and processing.
Aggregation is a thermodynamic process that becomes a concern when some conditions are met. In fact, it depends of parameters like temperature; concentration, composition and surface charge of the nanoparticles; dielectric constant and ionic strength of the medium; presence of surfactants. Due to the irreversibility of aggregation, all these parameters have to be controlled starting from the synthesis up to the final nanoparticle application to achieve a proper control on material and process.