The size distribution of dust aggregates as function of time as calculated by Ormel et al. (2009) for bare grains (left panel) and ice-coated grains (right panel). The gas density is n_g = 10⁵  cm^-3. For bare grains the size distribution evolves towards a steady state due to the emergence of fragmentation, while aggregates with ice-coated grains keep growing. The gray shading denotes the spread obtained from independent runs of the Monte Carlo code. The lower panels show the filling factor of the dust aggregates. Decreasing filling factors indicate a fractal structure. But for larger aggregates compaction halts the fractal growth.