  The force on a microparticle in a complex plasma by streaming ions, the
so-called ion drag force, is not well known. However, it is important for the
understanding of interesting phenomena in complex plasmas such as the void
formation under microgravity conditions. The PK-4 experiment, which is
developed for a later use on board of the International Space Station, is
ideally suited for investigating this problem. In this experiment a complex
DC-plasma is created in a glass tube in which the microparticles flow from the
cathode to the anode. Measuring the microparticle velocities, the forces on the
particles for different particle sizes, pressures, and DC-currents can be
extracted by assuming force balance. Experiments have been performed in the
laboratory as well as under microgravity using parabolic flights. The results
of these experiments will be presented and compared to theoretical predictions.
