  In dead zones of protoplanetary discs, it is assumed that micrometre-sized
particles grow Brownian, sediment to the midplane and drift radially inward.
When collisional compaction sets in, the growing aggregates collect slower and
therefore dynamically smaller particles. This sedimentation and growth phase of
highly porous ice and dust aggregates is simulated with laboratory experiments
in which we obtained mm- to cm-sized ice aggregates with a porosity of 90\% as
well as cm-sized dust agglomerates with a porosity of 85\%. We modelled the
growth process during sedimentation in an analytical calculation to compute the
agglomerate sizes when they reach the midplane of the protoplanetary disc. In
the midplane, the dust particles form a thin dense layer and gain relative
velocities by, e.g., the streaming instability or the onset of shear
turbulence. To investigate also these collisions, we performed additional
laboratory drop tower experiments with the high-porosity aggregates formed in
the sedimentary-growth experiments and determined their mechanical parameters,
including their sticking threshold velocity, which is important for their
further collisional evolution on their way to form planetesimals. Finally, we
developed a method to calculate the packing-density-dependent fundamental
properties of our dust and ice agglomerates, the Young's modulus, the Poisson
ratio, the shear viscosity and the bulk viscosity from compression
measurements. With these parameters, it was possible to derive the coefficient
of restitution which fits our measurements. In order to physically describe
these outcomes, we applied a collision model. With this model, predictions
about general dust-aggregate collisions are possible.
