Typical wall cluster during the experiment from the large size distribution of aggregates composed of monodisperse dust. a) Original greyscale image. b) Contrast enhanced image. The red line in b) indicates the delimitation of the total cluster area. The packing density φ_p = 0.91 is determined by dividing the dark area by the area enclosed by the red line (see Sect. 4.2 for more details on the cluster morphology).