Knowing the morphology of deposit onto collectors is a key objective for predicting the performance of a granular bed during clogging. In this study, deposits of nanoparticles onto spherical collectors were visualized with digital microscopes, in predominant Brownian diffusion conditions. An original device using axially magnetized beads was developed to guarantee that the flow rate around the collectors and thus the structure of the nanoparticle deposits was not disrupted by any support. Visualizations of the clogging of a collector line exhibited a homogeneous ellipsoidal deposit all around the beads. The deposit thickness increased linearly irrespective of the bead point, and was maximal at the top and minimal at the contact points. Moreover, microscope viewings of a collector layer during its clogging by nanoparticles were carried out. Results highlighted that the pressure drop increased as a function of the pore closure degree, similarly for all generated agglomerates, and consequently independently of the porosity and the localization of the deposit. These observations were confirmed by theoretical modelling, and suggested that the airflow rate passes mainly through open zones, and not through particle deposits. (C) 2016 Elsevier B.V. All rights reserved.