  Dust particles are the building blocks from which planetary bodies are made.
A major goal of the studies of planet-forming disks is to constrain the
properties of dust particles and aggregates in order to trace their origin,
structure, and the associated growth and mixing processes in the disk.
Observations of scattering and/or emission of dust in a location of the disk
often lead to degenerate information about the kind of particles, such as size,
porosity, or fractal dimension of aggregates. Progress can be made by deriving
the full (polarizing) scattering phase function of such particles at multiple
wavelengths. This has now become possible by careful extraction from scattered
light images. Such an extraction requires knowledge about the shape of the
scattering surface in the disk and we discuss how to obtain such knowledge as
well as the associated uncertainties. We use a sample of disk images from
observations with VLT/SPHERE to, for the first time, extract the phase
functions of a whole sample of disks with broad phase angle coverage. We find
that polarized phase functions come in two categories. Comparing the extracted
functions with theoretical predictions from rigorous T-Matrix computations of
aggregates, we show that one category can be linked back to fractal, porous
aggregates, while the other is consistent with more compact, less porous
aggregates. We speculate that the more compact particles become visible in
disks where embedded planets trigger enhanced vertical mixing.
