The charge on falling dust particles in a RF plasma with DC negative bias

Dust particles acquire a negative charge when introduced into a low pressure RF argon plasma and levitate in the sheath of the lower RF driven electrode due to a balance between the upward electrostatic force and the downward forces of ion drag and gravity. An external negative DC voltage greater than the negative self-bias is applied to the driven electrode, which further repels the plasma electrons so increasing the sheath size and moving the equilibrium height of the dust particles upwards. Increasing the negative DC voltage is found to cause the lowest dust particles to fall through the sheath to the driven electrode. The trajectories of these falling particles have been recorded with a high speed camera and analysed. The charge on the dust particles is deduced from the equation of motion. It is found that the dust particles always remain negatively charged even deep in the sheath.