  The streaming instability, as an example of instabilities driven by particle
feedback on a gas flow, has been proven to have a major role in controlling the
formation of planetesimals. Here, we present experiments to approach this
situation in the laboratory for particles in the Knudsen flow regime. In these
experiments, we observe a particle cloud trapped for about 30 s in a rotating
system under Earth's gravity. For average dust-to-gas ratios up to 0.08,
particles behave like individual test particles. Their sedimentation speed is
identical to that of a single free-falling particle, even in locally denser
regions. However, for higher dust-to-gas ratios, the motion of particles
becomes sensitive to clumping. Particles in locally denser regions now sediment
faster. Their sedimentation speed then depends linearly on the overall
dust-to-gas ratio. This clearly shows a transition from tracerlike behavior to
collective behavior. Beyond these findings, these types of experiments can now
be used as a gauge to test the particle feedback models in astrophysical
hydrocodes which are currently used for numerical simulations of streaming
instabilities.
