Response of an Isolated Particulate Subjected to Lateral Electric Fields in a DC Glow Discharge

Isolated spheres of borosilicate glass are suspended in a dc glow discharge in neon. The isolated sphere is laterally displaced by applying a transient voltage across a pair of electrodes mounted on the wall of the discharge tube. By monitoring the response of the particulate when subjected to a step change in the voltage and to sinusoidally varying voltages of different frequencies, the charge on the suspended sphere is inferred. Unlike most previous studies, this paper considers heavier (from 16 to 42 times heavier) particulates and heavily damped conditions. Measurements are reported for 20.3- and 4.9-mum -diameter borosilicate glass spheres. It is found that lateral excitation of an isolated particulate in a dc glow discharge cannot drive the motion to classical resonance as has been observed in previous work involving axial displacements in RF plasmas. The classical forced damped oscillator model used successfully in previous work to explain experimental observations is found to be inadequate for the conditions of this paper. Rather, the base excitation (BE) model is found to exhibit good agreement with the present experimental results at frequencies exceeding the frequency at which the displacement is a maximum. Sheath polarization, neglected in previous work, is included in this paper. It is found that drag due to collisions with neutrals is insufficient to account for the total drag on the particulate under heavily damped conditions. A new means of estimating the charge from the frequency response of the particulate motion using the BE model is described. Based on this method, charge numbers of Z = 9.2 times10⁵ and Z = 1.5 times10⁴ are inferred for the 20.3- and 4.9-mum spheres, respectively.