Bouncing and sticking collisions of organic nanoparticles: Atomistic study

Abstract

Context. Whether nanoparticles bounce or stick during collisions determines whether particles grow or fragment, hence shaping collision-induced agglomeration processes. The collision behavior of organic matter may strongly differ from that of silica or ice grains. Aims. We explore the microscopic processes underlying the bouncing behavior of organic nanoparticles. Methods. Molecular dynamics simulations based on a reactive potential, which follow molecular motion on an atomistic scale, are used. Results. For the exemplary case of glycolic acid molecules, warm nanoparticles (250 K) always show sticking, while at low velocities (2.5 m s−1) cold nanoparticles (100 K) exhibit a considerable probability for bouncing. This behavior can be traced back to the distant electrostatic repulsion of the nanoparticles at certain orientations; this prevents a closer approach, during which van der Waals and H-bonded interactions would lead to sticking. At higher temperatures, molecular vibrations and conformational flexibility average over the nanoparticle interaction, such that attraction dominates and bouncing is prevented. Our results are in qualitative agreement with laboratory experiments. Conclusions. Organic matter distinctly influences the collision behavior of nanoparticles.