Current problems in dust collision physics  No approved model for single-grain collision  lab data (e.g. Poppe et al for µm particles): sticking threshold, coefficient of restitution, charging  transition from elastic to plastic particle interaction at what displacement?  new MD approach by H. Tanaka  Retention of small grains in PPDs (talk by Henning)  are we treating the dust collisions incorrectly and are we, thus, missing the fragmentation-induced release of small dust particles?  or are we missing a fundamental (transport?) process?  if meter-size aggregates exist, Schräpler & Blum (ApJ, subm.) can explain retention of µm-size grains close to the midplane (see also Paraskov et al. 2006)  Bouncing or not bouncing? (talks by Güttler, Wada, Seizinger)  Bouncing of aggregates in MD simulations by Wada only observed for unrealistically dense aggregates (C.N. > 6)  does the bouncing barrier really exist? (Wada vs Güttler/Blum/Seizinger)  what is the difference between experiments and model (e.g. particle/monomer size)?  are we missing or mis-matching a model parameter?  is (micro-) sintering important?  can new calculations on GPUs help solve this discrepancy (Seizinger talk)?  first bouncing observed! Good agreement with quasi-static compression experiments!  SPH does show bouncing (Güttler et al. 2009; Geretshauser et al. 2010)! Why? 2:1 (at half-time)

Current problems in dust collision physics (cont’d)  Growth or erosion at high impact velocities?  growth with mm-sized projectiles (on large targets) (talk by Wurm)  erosion with µm-sized particles (experiments by Schräpler & Blum, subm. to ApJ)  we need self-consistent model !  but maybe turbulence-caused collision velocities much smaller (talk by Klahr)!  Does onset of fragmentation cause new growth mode? (talks by Güttler and Wurm; work by Zsom et al., in prep.)  self-consistency of fragmentation-agglomeration hypothesis needs to be checked  earlier simple models by Dullemond, Brauer, Birnstiel, … need to be re-calculated with new laboratory data  “Stickiness” of ice  Do ice particle stick at higher velocity than silicates (talks by Wada and Arakawa)?  new experiments with ice agglomerates by Wurm and by Arakawa  sticking, bouncing, fragmentation observed; qualitatively similar to silicates  sintering is important for impact strength  no experiments for non-sintered ice  no experiments at T<150 K yet  Role of charging for dust growth?  is there a “freeze-out” of dust aggregates due to electrostatic repulsion (talk by Okuzumi) ?  how do dust aggregates charge in mutual collisions (talk by Muranushi) ?  role of lightnings in PPDs?

Current problems in dust collision physics (cont’d)  Porosity matters! (talks by Güttler and Wurm)  we are getting more and more data on the role of porosity and the porosity evolution in dust collisions  High-speed collisions (talks by Nakamura and Krivov)  detection of very small ejecta (down to ~ µm) in Aerogel  very steep ejecta size distribution  still problems for plastic-covered solid impactors (role of nylon?)  critical fragmentation energy at dust sizes unknown (talk by Krivov) Anybody interested in a coordinated Japanese-German program on Dust Collisions in Astrophysics ?  There have recently been the first three German- Japanese research groups funded through DFG/JST