When the depositing particles attract each other, they will deposit and aggregate at the same time. This situation will result in a porous layer made of particle aggregates at the surface, and is referred to as ripening. The porosity of this layer will depend whether the particle aggregation process is fast or slow. Slow aggregation will lead to a more compact layer, while fast aggregation to a more porous one. The structure of the layer will resemble the structure of the aggregates formed in the later stages of the aggregation process.