Measurement of the ion drag force on falling dust particles and its relation to the void formation in complex (dusty) plasmas

Abstract

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.

Comment on “Measurement of the ion drag force on falling dust particles and its relation to the void formation in complex (dusty) plasmas” [Phys. Plasmas 10, 1278 (2003)]

“…9,10 There were, however, considerable problems with the quantitative description of the void formation: The value of the ion drag force calculated from the standard model by Barnes et al 11 was systematically smaller ͑by about an order of magnitude͒ than the electric force. 4,12 Nevertheless, in recent papers Zafiu et al 13,14 claim that the ion drag force calculated from the Barnes model is quite sufficient to explain the void formation-in contrast to all the previous works above. It is the purpose of this Comment to resolve this inconsistency.…”

Response to “Comment on ‘Measurement of the ion drag force on falling dust particles and its relation to the void formation in complex (dusty) plasmas’ ” [Phys. Plasmas 10, 4579 (2003)]

“…We have already discussed the applicability of collisionless models to our experiment to some extent in our previous papers. 7,8 For our experiments at lower gas pressure ͑20 Pa͒ 8 the ion mean free path is significantly larger than the characteristic impact parameter for 90 degree collisions b /2 , but only of the order of the electron Debye length De . It is therefore reasonable to apply the standard approach of Barnes et al 9 with a Coulomb logarithm cutoff at the Debye length.…”

“…In our analysis 7,8 we have used the electron Debye length, whereas Khrapak et al 10,11 argue that the linearized Debye length would be more appropriate.…”

“…1-6 Although the origin of the ion drag force is undisputed, a reliable quantitative description, especially with ion-neutral collisions present, is still lacking. Hence, our experiments on the ion drag force in an rf plasma tube 7,8 provide, to our knowledge, the first quantitative and direct measurements of this force. These measurements set a benchmark for the theoretical description of the ion drag force in the strongly and weakly collisional case.…”

Absolute measurement of the total ion-drag force on a single plasma-confined microparticle at the void edge under microgravity conditions

“…Another factor complicating the study of ion drag at the void edge is the fact that experiments on voids mean, per definition, that many dust particles are involved and that the ion drag on one particle is distorted by neighboring particles. Zafiu et al [20] cleverly solved this issue by letting microparticles vertically fall through a discharge operated between vertically aligned electrodes and study horizontal particle motion initiated by the ratio between ion-drag and electrostatic forces F i /F E not being unity. Regardless of the choice of physical model, all experiments up to now need to use assumptions on plasma parameters, such as ion velocity, local electric fields, charged particle densities, etc., to interpret the data.…”