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 1.5Â \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 \sim 2000 collisions it converged to an equilibrium of \phi \approx 0.36 .
In few experiments the aggregate fragmented , although the collision velocity was well below the canonical fragmentation threshold of \sim 1  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 .