Simulating collisional dust growth in protoplanetary disks: Combining discrete element and kinetic Monte Carlo methods

After I give a short review on the protoplanetary disk as an environment for dust growth, I present my contributions to the research, focusing on two aspects: tangential contact interactions between dust particles, and methods to simulate collisional dust growth. Discrete element method simulations have been used to study granular systems, including agglomerate collisions and agglomeration in granular gases, for a long time. However, the prominent model for tangential contact interactions introduces some unphysical behaviour. The presented research proposes an improved model and its non-linear extension, both of which are easier to use and do not share this problematic behaviour. In addition, I helped to demonstrate how readily the important distinction between static and dynamic friction is implemented into this model. Traditionally, simulating agglomeration in a dilute granular gas involved significant computational effort or drastic assumptions, using the discrete element method or probabilistic methods like the kinetic Monte Carlo technique, respectively. In this work, I discuss a hybrid simulation method that combines the discrete element and the kinetic Monte Carlo methods in a highly beneficial manner: Determining the agglomerates from a population that are colliding next using the kinetic Monte Carlo theory, the discrete element method is only used to resolve the collision dynamics. In addition to steady-state size distributions of charged agglomerates, binding energies and fractal dimensions of charged and of adhesive agglomerates are presented as first results of the hybrid simulation method.