This work deals with the common atomic force microscopy observation that small particles weakly adhering to a substrate tend to form aggregates at least when imaged under air. We show that it is possible to measure the coverage of mica surfaces with alpha-Al2O3 colloids in situ by means of static light scattering. The dynamic properties of the speckled pattern obtained from the layer are also used to probe the dynamic of the particles. It is then concluded that even the weakest adhesion of the particles does not allow in situ aggregation and surface mobility (at least, on the time scale of the laboratory experiment). The adhering particles aggregate only during the drying stage, due to the action of lateral capillary forces. A model for the aggregation mechanism is introduced taking into account the balance of forces acting on the particles during the drying stage.