Dust collisions in protoplanetary disks: From monodisperse to bidisperse grain aggregates

Context. The coagulation of dust particles into pebbles is the first step of planet formation in protoplanetary disks. However, dust growth from micrometre-sized particles to pebble sizes has been linked to barriers that stand in the way of dust growth. Aims. We investigate the roles of grain size, porosity, and impact velocity in collisions between equal-sized aggregates composed of monodisperse and bidisperse grains to determine the conditions that promote dust growth or fragmentation in protoplanetary disks. Methods. We used discrete-element method (DEM) simulations to recreate collisions between granular aggregates with a given porosity, where the size of the constituent monomers can vary. Various in-house software tools were used for the sample generation and analysis of the results. Results. Collisions between granular aggregates reveal that monomer size distribution strongly affects fragmentation outcomes. For monodisperse grain aggregates, smaller monomers favour adhesion and a sharp transition to fragmentation with steep fragment-size distributions, while larger monomers promote partial adhesion and shallower distributions. Bidisperse aggregates exhibit intermediate behaviour. Fragmentation velocity increases when monomers are smaller and porosity is higher. Coordination analysis shows that smaller grains allow more efficient reorganisation and compaction. Conclusions. Monomer-size distribution plays a key role in determining the collisional evolution of aggregates. Together with porosity, these parameters strongly influence aggregate fragmentation and growth, with direct implications for dust evolution and pebble formation in protoplanetary disks. Our results support the inclusion of mixed-monomer aggregates in future collision models to better represent early planet formation processes.