A closer look at individual collisions of dust aggregates: material mixing and exchange on microscopic scales

ABSTRACT Collisions between aggregates with different histories and compositions are expected to be commonplace in dynamically active protoplanetary discs. None the less, relatively little is known about how collisions themselves may contribute to the resulting mixing of material. Here, we use state-of-the-art granular dynamics simulations to investigate mixing between target/projectile material in a variety of individual aggregate-aggregate collisions, and use the results to discuss the efficiency of collisional mixing in protoplanetary environments. We consider sticking collisions (up to 10–20 m s–1 for our set-up) and disruptive collisions (40 m s–1) of ballistic particle–cluster aggregation (BPCA) and ballistic cluster–cluster aggregation (BCCA) clusters, and quantify mixing in the resulting fragments on both individual fragment and sub-aggregate levels. We find that the mass fraction of material that can be considered to be ‘well-mixed’ (i.e. locally made up of a mix of target and projectile material) to be limited, typically between 3 and 6 per cent for compact BPCA precursors, and increasing to 20–30 per cent for more porous BCCA clusters. The larger fragments produced in disruptive collisions are equally heterogeneous, suggesting aggregate–aggregate collisions are a relatively inefficient way of mixing material with different origins on small scales.