Abstract
A primitive LDV system without Bragg cells and analog demodulator is applied to measure the size and the charge of indivisual particles. Charged particles suspended in a steady air flow are fed into an ac electric field, where two laser beams intersect to form a stationary fringe. The light signal emitted from the particle by diffraction is recorded, and its wave form is directly analysed by a personal computer to give information on the amplitude and the phase of the particle oscillation. The information can be converted into the relaxation time and the mobility of the particle, and therefore the equivalent size and the charge of the particle can be obtained. An oblique air flow to the fringe planes may be used to distinguish the polarity of the charge.
The experimental results show that the sizes of DOP particles from a conventional aerosol generator of the vibrating orifice type, which generates mono-disparsed particles in the size range between 1 to 2μm in radius, can be measured very accurately. Further, the results show also that the particles possess a constant and high positive charge (1.5×10-15C/particle) regardless of the particle sizes. To reveal possible limitations in the proposed method, the measuring system is simulated. The results indicate feasibility of the present method if the amplitude of the particle is greater than the fringe spacing at an adequate frequency to give reasonable phase lag.
