Dust Collisions in Protoplanetary Disks: Atomic Simulations of the Surface Free Energy

TL;DR

This study uses molecular dynamics simulations to investigate how temperature, composition, and vacuum conditions affect the surface free energy of dust particles in protoplanetary disks, impacting their coagulation and planet formation.

Contribution

It provides new quantitative insights into how dust surface free energy varies with temperature and composition in vacuum, highlighting the importance of surface cleanliness.

Findings

SFE decreases with increasing temperature and compositional complexity.

Vacuum SFE values are two orders of magnitude higher than terrestrial measurements.

Surface hydroxylation reduces SFE, affecting dust coagulation.

Abstract

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. However, protoplanetary dust grains have a wide variety of mineralogical compositions, temperatures, and a low-pressure environment lacking in water vapor. We perform molecular dynamics simulations using a ReaxFF-type potential of the SFE of silica, albite, and anorthite at temperatures ranging from 30 to 700 K in a true vacuum. 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 two orders of magnitude higher than those usually measured in terrestrial laboratories. Our results confirm previous work that suggests that hydroxylation by monolayers of water produces this reduction in SFE in experiments. The coagulation of dust grains thus appears to depend critically on the cleanliness of their surfaces, as well as their temperature and composition.