The Physics of Protoplanetesimal Dust Agglomerates. III. Compaction in Multiple Collisions

TL;DR

This study investigates how repeated collisions compact porous dust aggregates, increasing their density and altering their physical properties, which impacts their behavior in protoplanetary environments.

Contribution

It provides experimental data on the compaction process of dust aggregates through multiple collisions and its effects on their physical and collisional properties.

Findings

Volume filling factor doubled after 1000 collisions

Aggregates reached an equilibrium filling factor of ~0.36 after ~2000 collisions

Fragmentation occurred below the typical velocity threshold

Abstract

To study the evolution of protoplanetary dust aggregates, we performed experiments with up to 2600 collisions between single, highly-porous dust aggregates and a solid plate. The dust aggregates consisted of spherical SiO$_2$ grains with 1.5$\mu$m diameter and had an initial volume filling factor (the volume fraction of material) of $\phi_0=0.15$. The aggregates were put onto a vibrating baseplate and, thus, performed multiple collisions with the plate at a mean velocity of 0.2 m s$^{-1}$. The dust aggregates were observed by a high-speed camera to measure their size which apparently decreased over time as a measure for their compaction. After 1000 collisions the volume filling factor was increased by a factor of two, while after $\sim2000$ collisions it converged to an equilibrium of $\phi\approx0.36$. In few experiments the aggregate fragmented, although the collision velocity was well below the canonical fragmentation threshold of $\sim1$ m s$^{-1}$. The compaction of the aggregate has an influence on the surface-to-mass ratio and thereby the dynamic behavior and relative velocities of dust aggregates in the protoplanetary nebula. Moreover, macroscopic material parameters, namely the tensile strength, shear strength, and compressive strength, are altered by the compaction of the aggregates, which has an influence on their further collisional behavior. The occurrence of fragmentation requires a reassessment of the fragmentation threshold velocity.