Measurement of Electrostatic Dipoles and Net Charge on Air Dispersed Particles [article]

Payel Bagga, University Of Canterbury
Dipoles are expected to often dramatically enhance the mutual collision rate of diffusing particles (above the effects of Brownian or turbulent motion). However, this spreading awareness of the possible influence of discrete dipoles on particles is still based largely on theory, and some qualitative experience of particle behaviour from microgravity experiments. Individual particle dipoles have not yet been definitely isolated in experiments, nor measured in practical situations. In this
more » ... , it was intended to measure, for the first time, distributions of dipole strength (as well as net charge and particle size) on particulates dispersed into air by typical industrial and pharmaceutical processing methods. The instruments designed to do this were built around a sampling head which allowed examination of a flow of dust dispersed into an air stream. During dispersal, the particles suffered tribocharging by mutual separation and collision on walls. Examination of the particles involved recording the path of particles as they moved through a non-uniform electric field around a central electrode, which was supplied with high voltage. Particles were attracted towards the central electrode (of 0.5 mm diameter in this study) if they contained dipoles, independent of the polarity of the field or their net charge. Particles to be examined were illuminated by a laser sheet as they moved past, and a high speed video captured their trajectories (over a field of view of around 5 mm). The equation of motion of a particle which involved the forces of both particle net charge and dipole strength was applied to the particle path to evaluate both these parameters. The particle trajectories were modelled, and checked against the observed experimental trajectories. The voltage applied to the probe varied from 4 kV to 18 kV but for most of the runs 6 kV voltage was used. The electric field around the probe tip was assumed to be same as that for a spherical electrode of the same size as the probe. The flow field axially towards an [...]
doi:10.26021/2271 fatcat:6maalovwlrhifcyyflbqtsmte4