tokens: ; Chiang Youdin 2010 ; Klahr et al. 2018 ). In Schneider et al. ( 2019 ), we studied the transition from test particle to collective behavior in a levitation experiment, analyzing the free-fall velocity of grains in a cloud. We empirically found that the sedimentation velocity depends on what we call sensitivity factor F S and the closeness C of the individual particle asThe closeness C of a particle is constructed from the interparticle distances between the grain and all other particles j as N is the total number of particles. The sensitivity factor F S in Equation ( 5 ) depends on the average solid-to-gas ratio of the system:The solid-to-gas ratio is defined as the ratio between the total dust mass and the average gas mass,where m p is the mass of a single particle, V is the total volume covered by particles, ρ g is the gas density in the chamber, ρ p is the bulk density of the individual grains, and s is the particle diameter. As seen in Equation ( 5 ), Schneider et al. ( 2019 ) also empirically found that particles are only influenced by the other particles in a cloud if the average solid-to-gas ratio is above a threshold value crit ; otherwise, F s = 0 and particles sediment with v 0 . The sensitivity α connecting the solid-to-gas ratio to the sensitivity factor in Equation ( 5 ) was just a constant in Schneider et al. ( 2019 ). This description is purely empirical and was deduced from a single experiment so far. Here, we present a systematic analysis, where we varied the gas pressure, particle size, and rotation frequency of the chamber and improved the setup and data acquisition. 2. Levitation Experiment 2.1. Setup The setup of the experiment (see Figure 1 ) follows the principle used in aggregation experiments by Poppe Blum ( 1997 ) and Blum et al. ( 1998 ), but is especially based on earlier experiments on dense clouds by Schneider et al. ( 2019 ). Figure 1. Experimental setup without auxiliary parts. The vacuum chamber is evacuated to a preset pressure. Two cameras observe the particles from the front. Illumination is provided by LED modules. Particles—in this study, hollow glass spheres of different sizes and densities—are dispersed once at the beginning of the experiment, within a rotating vacuum chamber with low ambient pressure. Particles are injected using a vibrating sieve in an extension of the vacuum chamber. The gas inside follows the rigid rotation of the chamber. The vacuum chamber has a diameter of 320 mm. Inside the chamber, a ring of LEDs is used to illuminate the particles. The scattered light of the particles is detected by two non-rotating cameras. These cameras observe the particles from the front at a distance of 40 cm on a 1′′, 5 megapixel sensor with a spatial resolution in the order of 10 μ m. The frame rate is 40 fps with an exposure time of 6 ms. The field of view is 19 × 15 cm. Imaging and synchronization of both cameras are controlled by a machine vision computer. Spatial calibration was realized with a
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