A measuring system for the time variation of size and charge of a single spherical particle and its applications

A single charged particle is trapped in a simple quadrupole electrode assembly and oscillated by means of a controlled electric field. A laser Doppler velocimeter (LDV) of the fringe mode is used for detecting the amplitude and phase lag of the particle oscillation with respect to the driving ac field. A unique method for the LDV signal processing is presented that takes the advantage of the sinusoidal oscillation of the particle at a known frequency. Superimposed to the ac drive, a dc drive is added for highly accurate measurements of particle size and charge. This enables us also to discriminate the polarity of charge without Bragg cells.In this paper, the basic principle of the method of the size and the charge measurements is explained and the accuracy of the measurements is demonstrated experimentally. The errors in the size and charge measurements, respectively, are less than 1% and 3% with a confidence coefficient of 99%. Since this apparatus repeats the measurements every 0.3 s for a single particle, the errors can be reduced to 0.1% and 0.3% when the measured values over a period of 30 s, or over 100 data are averaged.As some areas of its applications, experimental data are presented on the Rayleigh instability of evaporating charged droplets. It is shown that there are three types of Rayleigh fission. One of the types seemed to show occurrence of air breakdown around a micron sized spherical particle, which has not been reported so far. However experiments in highly insulating gas (SF6)(SF6) revealed that it was not the case but a type of Rayleigh fission. Nevertheless the experimental results gave important information on the charge limit of spherical solid particle due to electric breakdown in air at normal room conditions.Some cares to ensure the advantages of the present method are presented, and possible improvements of the apparatus are also suggested for future use.