Collisions between centimeter-to decimeter-sized dusty bodies are important to understand the mechanisms leading to the formation of planetesimals. We thus performed laboratory experiments to study the collisional behavior of dust aggregates in this size range at velocities below and around the fragmentation threshold. We developed two independent experimental setups with the same goal to study the effects of bouncing, fragmentation, and mass transfer in free particle-particle collisions. The first setup is an evacuated drop tower with a free-fall height of 1.5 m, providing us with 0.56 s of microgravity time so that we observed collisions with velocities between 8 mm s −1 and 2 m s −1 . The second setup is designed to study the effect of partial fragmentation (when only one of the two aggregates is destroyed) and mass transfer in more detail. It allows for the measurement of the accretion efficiency as the samples are safely recovered after the encounter. Our results are that for very low velocities we found bouncing as could be expected while the fragmentation velocity of 20 cm s −1 was significantly lower than expected. We present the critical energy for disruptive collisions Q , which showed up to be at least two orders of magnitude lower than previous experiments in the literature. In the wide range between bouncing and disruptive collisions, only one of the samples fragmented in the encounter while the other gained mass. The accretion efficiency in the order of a few percent of the particle's mass is depending on the impact velocity and the sample porosity. Our results will have consequences for dust evolution models in protoplanetary disks as well as for the strength of large, porous planetesimal bodies.

In laboratory experiments we determine the mass gain and loss in central collisions between centimetre‐ to decimetre‐size SiO2 dust targets and submillimetre‐ to centimetre‐size SiO2 dust projectiles of varying mass, size, shape and at different collision velocities up to ∼56.5 m s−1. Dust projectiles much larger than 1 mm lead to a small amount of erosion of the target but decimetre targets do not break up. Collisions produce ejecta, which are smaller than the incoming projectile. Projectiles smaller than 1 mm are accreted by a target even at the highest collision velocities. This implies that net accretion of decimetre and larger bodies is possible. Independent of the original size of a considered projectile, after several collisions, all fragments will be of submillimetre size which might then be (re)accreted in the next collision with a larger body. The experimental data suggest that collisional growth through fragmentation and reaccretion is a viable mechanism to form planetesimals.

In the past, laboratory experiments and theoretical calculations showed a mismatch in derived sticking properties of silicates in the context of planetesimal formation. It has been proposed by Kimura et al. (2015) that this mismatch is due to the value of the surface energy assumed, supposedly correlated to the presence or lack of water layers of different thickness on a grain's surface. We present tensile strength measurements of dust aggregates with different water content here. The results are in support of the suggestion by Kimura et al. (2015). Dry samples show increased strengths by a factor of up to 10 over wet samples. A high value of γ = 0.2 J/m 2 likely applies to the dry low pressure conditions of protoplanetary disks and should be used in the future.

In protoplanetary disks, CO 2 is solid ice beyond its snow line at ∼ 10AU. Due to its high abundance, it contributes heavily to the collisional evolution in this region of the disk. For the first time, we carried out laboratory collision experiments with CO 2 ice particles and a CO 2 -covered wall at a temperature of 80 K. Collision velocities varied between 0 -2.5 m/s. Particle sizes were on the order of ∼ 100 µm. We find a threshold velocity between the sticking and the bouncing regime at 0.04 m/s. Particles with greater velocities but below 1 m/s bounce off the wall. For yet greater velocities, fragmentation occurs. We give analytical models for the coefficients of restitution and fragmentation strength consistent with the experimental data. Set in context, our data show that CO 2 ice and silicate dust resemble each other in the collisional behavior. Compared to water ice the sticking velocity is an order of magnitude smaller. One immediate consequence as example is that water ice particles mantled by CO 2 ice lose any "sticking advantage." In this case, preferential planetesimal growth attributed to the sticking properties of water ice will be limited to the region between the H 2 O ice line and the CO 2 ice line.

For the investigation of collisions among protoplanetesimal dust aggregates, we performed microgravity experiments in which the impacts of high-porosity millimeter-sized dust aggregates into 2.5 cm high-porosity dust aggregates can be studied. The dust aggregates consisted either of monodisperse spherical, quasi-monodisperse irregular, or polydisperse irregular micrometer-sized dust grains and were produced by random ballistic deposition with porosities between 85% and 93%. Impact velocities ranged from $0.1 to $3 m s À1 , and impact angles were almost randomly distributed. In addition to the smooth surfaces of the target aggregates formed in our experiments, we ''molded'' target aggregates such that the radii of the local surface curvatures corresponded to the projectile radii, decreasing the targets' porosities to 80%Y85%. The experiments showed that impacts into the highest porosity targets almost always led to sticking, whereas for the less porous dust aggregates, consisting of monodisperse spherical dust grains, the collisions with intermediate velocities and high impact angles resulted in the bouncing of the projectile with a mass transfer from the target to the projectile aggregate. Sticking probabilities for the impacts into the ''molded'' target aggregates were considerably decreased. For the impacts into smooth targets, we measured the depth of intrusion and the crater volume, and were able to derive some interesting dynamical properties which can help to derive a collision model for protoplanetesimal dust aggregates. Future models of the aggregate growth in protoplanetary disks should take into account noncentral impacts, impact compression, the influence of the local radius of curvature on the collisional outcome, and the possible mass transfer between the target and projectile agglomerates in nonsticking collisions.