Dust Collisions in Protoplanetary Disks: Atomic Modelling to Better Understand Surface Free Energies in Unique Environments

Coagulation of dust particles in protoplanetary disks is the first step on the journey to the formation of planets. The surface free energy (SFE) of the dust particles determines the effectiveness of particles sticking to each other after collision, as well as the critical collision velocity above which fragmentation will occur. Studies of SFE have focused on the simplest silicate, silica, usually at standard temperature and pressure. Previously reported values show orders of magnitude in difference, creating significant uncertainty in how planetesimals are forming. However, protoplanetary dust grains have a wide variety of compositions and temperatures, and a low-pressure environment lacking in water vapor. We perform molecular dynamics simulations using a ReaxFF-type potential to quantify the SFE minerals and ice at temperatures ranging from 30 to 700 K in both true vacuum along with water vapor. We find that the SFE drops by tens of percent with increasing temperature or shifting to more complex silicate compositions. More dramatically, we find that the values of the SFE in a vacuum are 2 orders of magnitude higher than those usually measured in terrestrial laboratories. We then quantify the unique role surface coverage of water vapor plays in drastically reducing the SFE, even at low coverages. Results confirm previous work that suggests that hydroxylation by monolayers of water produces this reduction in SFE in experiments. We suggest that SFEs for minerals and ices should be sampled as a function of the specific environment along the disk and provide these values for future models.