Ion drag and thermophoretic forces acting on free falling charged particles in an rf-driven complex plasma

Monodisperse plastic microspheres have been dropped through a long radio-frequency discharge column. The trajectories of the falling particles have been measured. It was observed that the particles are driven out of the plasma. From the trajectory analysis and plasma measurements the forces on the particles have been derived. Special attention has been paid to the thermophoresis and ion drag forces which are also considered to be responsible for the void formation in microgravity experiments. Two experimental situations have been considered here: first a plasma characterized by its natural symmetric electric potential and temperature distribution and second, a plasma with an asymmetric temperature and electric potential profile. For both cases a good agreement has been found between the measured “trajectory force” obtained from the particle trajectory analysis and the sum of the ion drag, thermophoretic and electric field force.