Experiments on the quantitative determination of the weaker forces ~ion drag, thermophoresis, and electric field force! on free-falling dust particles in a rf discharge tube are presented. The strongest force, gravity, is balanced by gas friction and the weaker forces are investigated in the radial ~horizontal! plane. Under most discharge conditions, the particles are found to be expelled from the central plasma region. A transition to a situation where the falling particles are focused into the plasma center is observed at low gas pressures using small particles. These investigations allow a quantitative understanding of the mechanism of unwanted dust-free areas ~so-called voids! in dusty plasmas under microgravity. Good quantitative agreement with standard models of the ion drag is found.
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