  One of the most important questions in the field of planet formation is how
mm-cm sized dust particles overcome the radial drift and fragmentation barriers
to form kilometer-sized planetesimals. ALMA observations of protoplanetary
disks, in particular transition disks or disks with clear signs of
substructures, can provide new constraints on theories of grain growth and
planetesimal formation and therefore represent one possibility to progress on
this issue. We here present ALMA band 4 (2.1 mm) observations of the transition
disk system Sz 91 and combine them with previously obtained band 6 (1.3 mm) and
7 (0.9 mm) observations. Sz 91 with its well defined mm-ring, more extended gas
disk, and evidence of smaller dust particles close to the star, is a clear case
of dust filtering and the accumulation of mm sized particles in a gas pressure
bump. We computed the spectral index (nearly constant at $\sim$3.34), optical
depth (marginally optically thick), and maximum grain size ($\sim\,0.61$ mm) in
the dust ring from the multi-wavelength ALMA observations and compared the
results with recently published simulations of grain growth in disk
substructures. Our observational results are in very good agreement with the
predictions of models for grain growth in dust rings that include fragmentation
and planetesimal formation through the streaming instability.
